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Observed anomalous atmospheric patterns in summers of unusual Arctic sea ice melt
Journal of Geophysical Research: Atmospheres
Abstract
The Arctic sea ice retreat has accelerated over the last decade. The negative trend is largest in summer, but substantial inter-annual variability still remains. Here, we explore observed atmospheric conditions and feedback mechanisms during summer months of anomalous sea ice melt in the Arctic. Compositing months of anomalous low and high sea ice melt over 1979–2013, we find distinct patterns in atmospheric circulation, precipitation, radiation and temperature. Compared to summer months of anomalous low sea ice melt, high melt months are characterized by anomalous high sea level pressure in the Arctic (up to 7 hPa), with a corresponding tendency of storms to track on a more zonal path. As a result, the Arctic receives less precipitation overall and 39 % less snowfall. This lowers the albedo of the region and reduces the negative feedback the snowfall provides for the sea ice. With an anticyclonic tendency, 12 W/m2 more incoming shortwave radiation reaches the surface inthe start of the season. The melting sea ice in turn promotes cloud development in the marginal ice zones and enhances downwelling longwave radiation at the surface toward the end of the season. A positive cloud feedback emerges. In midlatitudes,the more zonally tracking cyclones give stormier, cloudier, wetter and cooler summers in most of northern Europe and around the Sea of Okhotsk. Farther south, the region from the Mediterranean Sea to East Asia experiences significant surface warming (up to 2.4°C), possibly linked to changes in the jet stream.
... The impact of cyclones is important but complex (Wernli & Papritz, 2018). While in general fewer cyclones throughout the summer favor a stronger sea ice loss (Knudsen et al., 2015;Screen et al., 2011), the occurrence of intense cyclones can cause a strong ice loss, particularly in late summer (Simmonds & Rudeva, 2012;Zhang et al., 2013). ...
... Many studies have shown that summer sea ice anomalies are statistically correlated with summer atmospheric circulation patterns (e.g., Knudsen et al., 2015;Lynch et al., 2016;Mills & Walsh, 2014;Serreze et al., 2016;Zhang et al., 2018). Ding et al. (2017) attributed 60% of the late summer sea ice retreat to the trend in the summer mean atmospheric circulation. ...
... In contrast to other studies, we analyze sea ice melt rather than sea ice extent, because the former is a more direct measure of sea ice changes resulting from, and leading to, alteration in atmospheric patterns (Knudsen et al., 2015). Our analysis focuses on the Arctic north of approximately 60°N and over the extended summer, May-August, for the 1979-2014 period. ...
Observations from 1979 to 2014 show a positive trend in the summer sea ice melt rate with an acceleration particularly in June and August. This is associated with atmospheric circulation changes such as a tendency toward a dipole pattern in the mean sea level pressure (SLP) trend with an increase over the Arctic Ocean and a decrease over Siberia. Consistent with previous studies, we here show the statistical relationship between the summer sea ice melt rate and SLP and that more than one SLP pattern is associated with anomalously high melt rates. Most high melt rates occur during high pressure over the Arctic Ocean accompanied by low pressure over Siberia, but a strong Beaufort High and advection of warm air associated with a cyclone located over the Taymyr Peninsula can also trigger anomalous high ice melt. We evaluate 10‐member ensemble simulations with the coupled atmosphere‐ice‐ocean Arctic regional climate model HIRHAM‐NAOSIM. The simulations have systematically low acceleration of sea ice melt rate in August, related to shortcomings in representing the strengthening pressure gradient from the Barents/Kara Sea toward Northern Greenland in recent decades. In general, the model shows the same classification of SLP patterns related to anomalous melt rates as the observations. However, the evolution of sea ice melt‐related cloud‐radiation feedback over the summer reveals contrary effects from low‐level clouds in the reanalysis and in the simulations.
... Tang et al. (2014) (a study of ERA-Interim data) suggest that the loss of Arctic sea ice, combined with the reduction in Northern Hemisphere snow cover, weakens the upper-level zonal winds and induces a higher-amplitude, poleward-shifted jet stream that increases the likelihood of extreme summer weather over the northern midlatitudes. The modeling studies of Balmaseda et al. (2010), Screen (2013), Knudsen et al. (2015), and Petrie et al. (2015) all suggest a link between declines in Arctic sea ice and northwestern European summer climate. Screen (2013) finds a link between the reduction in Arctic sea ice and increased summer rainfall over northern Europe. ...
... Screen (2013) finds a link between the reduction in Arctic sea ice and increased summer rainfall over northern Europe. Balmaseda et al. (2010), Knudsen et al. (2015), and Petrie et al. (2015) suggest that declines in Arctic sea ice may be associated with the negative phase of the summer NAO (Folland et al. 2009). ...
... In the recent low-ice period there have been a number of years in which the summer circulation projected onto the negative phase of the SNAO (Petrie et al. 2015); in many of these years northwestern Europe has experienced extreme flooding events (Blackburn et al. 2008;Folland et al. 2009;Screen 2013). Studies by Balmaseda et al. (2010), Screen (2013), Knudsen et al. (2015), and Petrie et al. (2015) have suggested that the decline in Arctic sea ice and associated changes in SSTs may be responsible for the observed negative SNAO. Petrie et al. (2015) suggested that the decline in sea ice in the Labrador Sea region leads to warming in the lower free troposphere; through thermal wind balance this implies that there is a weakening of the low-level jet over North America. ...
The atmospheric response to an idealized decline in Arctic sea ice is investigated in a novel fully coupled climate model experiment. In this experiment two ensembles of single-year model integrations are performed starting on 1 April, the approximate start of the ice melt season. By perturbing the initial conditions of sea ice thickness (SIT), declines in both sea ice concentration and SIT, which result in sea ice distributions that are similar to the recent sea ice minima of 2007 and 2012, are induced. In the ice loss regions there are strong (~3 K) local increases in sea surface temperature (SST); additionally, there are remote increases in SST in the central North Pacific and subpolar gyre in the North Atlantic. Over the central Arctic there are increases in surface air temperature (SAT) of ~8 K due to increases in ocean-atmosphere heat fluxes. There are increases in SAT over continental North America that are in good agreement with recent changes as seen by reanalysis data. It is estimated that up to two-thirds of the observed increase in SAT in this region could be related to Arctic sea ice loss. In early summer there is a significant but weak atmospheric circulation response that projects onto the summer North Atlantic Oscillation (NAO). In early summer and early autumn there is an equatorward shift of the eddy-driven jet over the North Atlantic as a result of a reduction in the meridional temperature gradients. In winter there is no projection onto a particular phase of the NAO.
... As in fall and winter, summer Arctic sea ice loss also has an Arctic amplification effect (Deser et al. 2010;Screen et al. 2012Screen et al. , 2013Peings and Magnusdottir 2014). In observations, anomalous NH atmospheric circulations in summer are significantly associated with unusual Arctic sea ice melt (Tang et al. 2014;Knudsen et al. 2014;Wu et al. 2016;Zou et al. 2021). Through observational analysis, Tang et al. (2014) suggest that Arctic sea ice reduction is associated with widespread upper-level height over the Arctic, weaker upper-level zonal winds at high latitudes, and frequent extreme summer heat events over midlatitude continents. ...
... Through linear regression analyses, we prove that Arctic SIE anomalies in summer are significantly correlated to summer atmospheric fields over North America and the North Atlantic, but not over the Eurasian sector. Such association is consistent with previous studies that sea ice reduction has significant impacts on summer circulations (Screen 2013;Tang et al. 2014;Lee 2014;Knudsen et al. 2014;Blackport and Kushner 2016;Zou et al. 2021). Using atmospheric model ensemble simulations covering 1979-2014, we demonstrate that summer Arctic sea ice loss contributes to Western Hemisphere Arctic warming, reduces the midlatitude-western Arctic temperature contrast, and leads to weakening zonal winds and vertical wind shear in the lower and middle troposphere from 55° to 75°N. ...
The westerly wind on the poleward side of the summer polar jet stream (PJS) over the Western Hemisphere has significantly weakened since the 1980s. A weak summer PJS causes warming surface temperatures and deficient precipitation over Alaska and western North America, favoring extreme wildfire events. This study investigates influences of Arctic sea ice loss on the summer PJS variability over the Western Hemisphere. Regression analysis first provides observational evidence that Arctic sea ice reduction is related to a weakening summer Western Hemisphere PJS at interannual time scales. Atmospheric model ensemble simulations are then used to demonstrate that Arctic sea ice loss significantly contributes to observed Western Hemisphere Arctic warming and reduced meridional temperature gradient between midlatitudes and the pole in the lower and middle troposphere, acting to weaken the troposphere zonal wind and vertical wind shear from 55° to 75°N, and about 20–30% of observed weakened summer PJS trend during 1979–2014. Observational analysis and the model-based results also indicate that a significant portion of the observed trends of the PJS and vertical wind shear during 1979–2014 might be attributed to the decadal variability of the summer North Atlantic Oscillation (NAO). In the future climate, as more and more ice melts in the summer, the weakening effect of sea ice on the PJS will continue and will be superimposed onto the natural decadal variability of the PJS.
... P < 0.0001, respectively), meaning cooler conditions occur during the negative phase of the AMV (AMV−), and this leads to higher Ti input in the lake (Fig. 1C). From May to August, higher snowfall is observed during times of cooler temperatures associated with lower atmospheric pressure and increased cloud cover (21). This is coherent with the spatial correlation between 500 hPa geopotential height and Ti at SSL which indicates a higher flux of Ti during times of lower atmospheric pressure ( Fig. 2A). ...
... This pattern is essentially the same, but reversed in sign compared to the relationship between 500 hPa and AMV (Fig. 2B). Greater summer anticyclonic activity, linked with AMV+, leads to higher arctic temperatures and stronger sea ice loss, and less snowpack in arctic watersheds (21)(22)(23)(24)(25)(26)(27). Map correlations between instrumental Atlantic SSTs (28) and Ti at SSL, and AMV versus SSTs show practically the same spatial patterns, but inversely correlated ( Fig. 2 C and D). ...
Global warming due to anthropogenic factors can be amplified or dampened by natural climate oscillations, especially those involving sea surface temperatures (SSTs) in the North Atlantic which vary on a multidecadal scale (Atlantic multidecadal variability, AMV). Because
the instrumental record of AMV is short, long-term behavior of AMV is unknown, but climatic teleconnections to regions beyond the North Atlantic offer the prospect of reconstructing AMV from high-resolution records elsewhere. Annually resolved titanium from
an annually laminated sedimentary record from Ellesmere Island, Canada, shows that the record is strongly influenced by AMV via atmospheric circulation anomalies. Significant correlations between this High-Arctic proxy and other highly resolved Atlantic SST proxies
demonstrate that it shares the multidecadal variability seen in the Atlantic. Our record provides a reconstruction of AMV for the past ∼3 millennia at an unprecedented time resolution, indicating North Atlantic SSTs were coldest from ∼1400–1800 CE, while current SSTs are the warmest in the past ∼2,900 y.
... This role can be investigated by various methods based on relatively more abundant atmospheric data sources, instead of with the ocean circulation patterns. Among the different seasons, the summer circulation pattern has received research focus because of its temporal proximity to the September sea-ice minimum [4][5][6][7][8][9]. However, preconditioning factors that occur during the winter and spring have also attracted much attention due to their importance in long-lead seasonal predictions [10][11][12]. ...
... The formation of monthly-to-seasonal atmospheric circulation patterns observed using low-frequency modes in the Arctic has been extensively studied in terms of their influences on the Arctic climate and sea ice [1][2][3][4][5][6][7][8][9]. Studies have shown that the activity of synoptic cyclones is more prevalent during summer in the Arctic Ocean [13][14][15]; furthermore, it has been suggested that they potentially contribute to seasonal-mean Arctic climate [13,14,20]. ...
Contribution of extra-tropical synoptic cyclones to the formation of mean summer atmospheric circulation patterns in the Arctic domain (≥60° N) was investigated by clustering dominant Arctic circulation patterns based on daily mean sea-level pressure using self-organizing maps (SOMs). Three SOM patterns were identified; one pattern had prevalent low-pressure anomalies in the Arctic Circle (SOM1), while two exhibited opposite dipoles with primary high-pressure anomalies covering the Arctic Ocean (SOM2 and SOM3). The time series of their occurrence frequencies demonstrated the largest inter-annual variation in SOM1, a slight decreasing trend in SOM2, and the abrupt upswing after 2007 in SOM3. Analyses of synoptic cyclone activity using the cyclone track data confirmed the vital contribution of synoptic cyclones to the formation of large-scale patterns. Arctic cyclone activity was enhanced in the SOM1, which was consistent with the meridional temperature gradient increases over the land–Arctic ocean boundaries co-located with major cyclone pathways. The composite daily synoptic evolution of each SOM revealed that all three SOMs persisted for less than five days on average. These evolutionary short-term weather patterns have substantial variability at inter-annual and longer timescales. Therefore, the synoptic-scale activity is central to forming the seasonal-mean climate of the Arctic.
... Storm activities play important roles in regulating Arctic energy and water cycles (e.g., Peixoto & Oort, 1992;Sorteberg & Walsh, 2008;Vihma et al., 2016;Zhang, He, et al., 2013) and thus influence temperatures, moisture, winds, and sea ice (Knudsen et al., 2015;Parkinson & Comiso, 2013;Simmonds & Keay, 2009;Sorteberg & Walsh, 2008;Tao et al., 2016;Zhang, He, et al., 2013;Zhang, Lindsay, et al., 2013). More intense and persistent storms have occurred in the Arctic (Aizawa et al., 2014;Simmonds & Rudeva, 2012;Tao et al., 2017;Yamazaki et al., 2015;Zhang et al., 2004). ...
... More intense and persistent storms have occurred in the Arctic (Aizawa et al., 2014;Simmonds & Rudeva, 2012;Tao et al., 2017;Yamazaki et al., 2015;Zhang et al., 2004). The strong intensity of these storms leads to greater wind stress and enhanced air-sea ice energy flux (Knudsen et al., 2015;Zhang, Lindsay, et al., 2013). The persistence of such storms extends the period of such air-ice-sea interaction and exerts cumulative influence on climate and environment. ...
Intense synoptic-scale storms have been more frequently observed over the Arctic during recent years. Specifically, a superstorm hit the Arctic Ocean in August 2012 and preceded a new record low Arctic sea ice extent. In this study, the major physical processes responsible for the storm's intensification and persistence are explored through a series of numerical modeling experiments with the Weather Research and Forecasting model. It is found that thermal anomalies in troposphere as well as lower stratosphere jointly lead to the development of this superstorm. Thermal contrast between the unusually warm Siberia and the relatively cold Arctic Ocean results in strong troposphere baroclinicity and upper level jet, which contribute to the storm intensification initially. On the other hand, Tropopause Polar Vortex (TPV) associated with the thermal anomaly in lower stratosphere further intensifies the upper level jet and accordingly contributes to a drastic intensification of the storm. Stacking with the enhanced surface low, TPV intensifies further, which sustains the storm to linger over the Arctic Ocean for an extended period.
... It has been recently argued that we are at risk of passing multiple climate tipping points and urgent action is needed to avoid dangerous futures (McKay et al., 2022). Two examples of positive feedbacks are (1) the melting of permafrost (e.g., in Alaska) which releases huge quantities of methane, leading to further warming (Elder et al., 2021); (2) melting Arctic ice leads to increased warming as open water absorbs much more sunlight and thus heat, than does ice (Knudsen et al., 2015). ...
The ability of food systems to feed the world’s population will continue to be constrained in the face of global warming and other global challenges. Often missing from the literature on future food security are different scenarios of population growth. Also, most climate models use given population projections and consider neither major increases in mortality nor rapid declines in fertility. In this paper, we present the current global food system challenge and consider both relatively high and relatively low fertility trajectories and their impacts for food policy and systems. Two futures are proposed. The first is a “stormy future” which is an extension of the “business as usual” scenario. The population would be hit hard by conflict, global warming, and/or other calamities and shocks (e.g., potentially another pandemic). These factors would strain food production and wreak havoc on both human and planetary health. Potential increases in mortality (from war, famine, and/or infectious diseases) cannot be easily modeled because the time, location, and magnitude of such events are unknowable, but a challenged future is foreseen for food security. The second trajectory considered is the “brighter future,” in which there would be increased access to education for girls and to reproductive health services and rapid adoption of the small family norm. World average fertility would decline to 1.6 births per woman by 2040, resulting in a population of 8.4 billion in 2075. This would put less pressure on increasing food production and allow greater scope for preservation of natural ecosystems. These two trajectories demonstrate why alternative population growth scenarios need to be investigated when considering future food system transitions. Demographers need to be involved in teams working on projections of climate and food security.
... A negative state of the SNAO is consistent with generally strongly positive Atlantic multidecadal oscillation conditions over the last decade (Sutton and Dong 2012). However, evidence is strengthening that reductions in summer Arctic sea ice due to warming of the Arctic may also favor a negative SNAO (e.g., Knudsen et al. 2015, Petrie et al. 2015. The July 2015 MSLP anomaly pattern strongly resembled the negative SNAO. ...
... The anomalously high Z850 over the Pacific sector of the Arctic are accompanied by anomalous lower tropospheric warming and the development of high SLP anomalies ( Figure S3 in Supporting Information S1). The strengthening of the surface anticyclone over the Beaufort Sea, which can strengthen downward shortwave radiation and the transpolar sea ice drift, has long been regarded as one of the key circulation patterns accelerating the summer sea ice loss (Knudsen et al., 2015;Wang et al., 2020;Wernli & Papritz, 2018). ...
Arctic summer sea ice decline accelerated from the mid‐2000s to 2012, with the 2012 record low remaining unbroken. While frequent La Niña events during this period have been suggested as a driver of this trend acceleration, no convincing evidence has been presented. Here, using a climate model nudged to observed pan‐tropical sea surface temperatures (SST), we show that the back‐to‐back La Niña events during 2010–2011, followed by a North Pacific cooling and a marginal El Niño, were a key contribution to the 2012 record low. Specifically, the La Niña events in 2010–2011 warmed the Arctic Pacific sector, whereas tropical SST anomalies in 2012 strengthened the Greenland high pressure, leading to an Arctic dipole‐like pressure pattern and strengthening of transpolar ice drift. These Arctic temperature and circulation anomalies led to the record low sea ice extent in 2012, highlighting the strong influence of tropical SSTs on Arctic climate.
... On the other hands, previous studies focused on monthly or seasonal mean Arctic sea ice anomalies impact the midhigh latitude atmospheric (Wu and Zhang 2010;Cohen et al. 2014;Coumou et al. 2015;Overland et al. 2016;Wu et al. 2016a, b;Zhang et al. 2018), and a few recent studies that have been undertaken on the Arctic sea ice melt on Eurasia climate variation. Knudsen et al. (2015) reported that Arctic sea ice melt could lead to a cooler summer in most of northern Europe but a hotter summer over part of Asia via modulating the jet stream, consistent with (Wu and Li 2021). Mori et al. (2019) suggest that the enhanced melting of the Barents-Kara sea ice had significantly amplified the probability of severe winter occurrence in central Eurasia. ...
This study investigates the association of spring (April–May) Arctic sea ice melt with simultaneous surface air temperature (SAT) over mid-high latitudes of Eurasia from 1979 to 2019 by using observational datasets and simulation experiments. The results show that spring SAT anomalies associated with Arctic sea ice melt display a dipole pattern over Eurasia. A high Arctic sea ice melt corresponds to positive SAT anomalies over northern Eurasia and negative SAT anomalies over most of Asia. The 500 hPa geopotential height anomalies exhibit a wave train structure, and a dominant positive center is located over the Ural Mountains with two negative centers over East Asia and western Europe. This atmospheric circulation anomaly differs from the traditional Eurasian pattern and the North Atlantic-Eurasian teleconnection pattern due to their different spatial modes. Simulation experiments forced by Arctic sea ice anomalies reproduce the major characteristics of observational associations. Observations and numerical simulations indicate that high Arctic sea ice melt years are often associated with heavy sea ice in winter-spring, which is favorable for the occurrence of Arctic anticyclonic circulation anomaly and lead to a positive SAT anomaly in the Arctic. The Arctic warming not only strengthens polar zonal westerly winds by increasing local baroclinicity, but also weakens zonal winds in mid-latitude through a reduction meridional temperature gradient. It may contribute to the Arctic anticyclonic anomalies enhancement, and then induces a wave train southeastward propagating into the mid-low latitudes. This configuration of atmospheric circulation anomalies provides favorable conditions for the SAT variations over Eurasia.
... The middle Eemian is also characterized by a further reduction in the Arctic sea ice during summers (Stein et al., 2017). In model simulations, Arctic sea ice loss during early summer is related to a negative summer NAO and equatorward shift of the eddy-driven jet because of a reduction in the meridional temperature gradients (Knudsen et al., 2015;Petrie et al., 2015). This is in agreement with the Sokli dD record as negative summer NAO would result in colder conditions in northern Europe associated with stronger northerly winds and moisture transport from the Arctic (Folland et al., 2009). ...
The Last Interglacial warm period, the Eemian (ca. 130e116 thousand years ago), serves as a reference for
projected future climate in a warmer world. However, there is a limited understanding of the seasonal
characteristics of interglacial climate dynamics, especially in high latitude regions. In this study, we aim
to provide new insights into seasonal trends in temperature and moisture source location, linked to shifts
in atmospheric circulation patterns, for northern Fennoscandia during the Eemian. Our study is based on
the distribution and stable hydrogen isotope composition (dD) of n-alkanes in a lake sediment sequence
from the Sokli paleolake in NE Finland, placed in a multi-proxy framework. The dD values of predominantly
macrophyte-derived mid-chain n-alkanes are interpreted to reflect lake water dD variability
influenced by winter precipitation dD (dDprec), ice cover duration and deuterium (D)-depleted meltwater.
The dD values of terrestrial plant-derived long-chain n-alkanes primarily reflect soil water dD variability
modulated by summer dDprec and by the evaporative enrichment of soil and leaf water. The dDprec
variability in our study area is mostly attributed to the temperature effect and the moisture source
location linked to the relative dominance between D-depleted continental and polar air masses and Denriched
North Atlantic air masses. The biomarker signal further corroborates earlier diatom-based
studies and pollen-inferred January and July temperature reconstructions from the same sediment
sequence. Three phases of climatic changes can be identified that generally follow the secular variations
in seasonal insolation: (i) an early warming trend succeeded by a period of strong seasonality (ii) a midoptimum
phase with gradually decreased seasonality and cooler summers, and (iii) a late climatic
instability with a cooling trend. Superimposed on this trend, two abrupt cooling events occur in the early
and late Eemian. The Sokli dD variability is generally in good agreement with other North Atlantic and
Siberian records, reflecting major changes in the atmospheric circulation patterns during the Eemian as a
response to orbital and oceanic forcings.
... The dependence of mBT on the SSW weakens in the summer and intensifies in the winter. In general, the Arctic Ocean has numerous robust cyclones in the winter but weaker and less frequent ones in the summer (e.g., [47][48][49]). Arctic cyclones are most dynamically intense during the winter [50]. ...
The minimum brightness temperature (mBT) of seawater in the polar region is an important parameter in algorithms for determining sea ice concentration or snow depth. To estimate the mBT of seawater at 6.925 GHz for the Arctic and Antarctic Oceans and to find their physical characteristics, we collected brightness temperature and sea ice concentration data from the Advanced Microwave Scanning Radiometer 2 (AMSR2) for eight years from 2012 to 2020. The estimated mBT shows constant annual values, but we found a significant difference in the seasonal variability between the Arctic and Antarctic Oceans. We calculated the mBT with the radiative transfer model parameterized by sea surface temperature (SST), sea surface wind speed (SSW), and integrated water vapor (IWV) and compared them with our observations. The estimated mBT represents the modeled mBT emitted from seawater under conditions of 2–5 m/s SSW and SST below 0 °C, except in the Arctic summer. The exceptional summer mBT in the Arctic Ocean was related to unusually high SST. We found evidence of Arctic amplification in the seasonal variability of Arctic mBT.
... Thus, the low sea ice cover along the Siberian coast contributes to the warming and high pressure over the region, reduces dT/dy and the zonal wind south of it, and thus favors the occurrence and maintenance of East Siberian blockings. This is in line with previous studies that suggested summer warming along Arctic coasts can modulate the subpolar jet and large-scale waves (e.g., Coumou et al., 2018;Knudsen et al., 2015;Petrie et al., 2015). ...
Plain Language Summary
Meiyu‐Baiu, a west—east oriented stationary front from central‐east China to Japan, usually persists from mid‐June to mid‐July. In June–July 2020, central‐east China and Japan encountered record‐breaking Meiyu‐Baiu rainfall. Through analyses of observational and reanalysis data, we found that excessive East Siberian atmospheric blockings—large‐scale quasi‐stationary anticyclonic circulations—during the 2020 Meiyu‐Baiu season increased cold air outbreaks into the Meiyu‐Baiu region, stopped the northward march of the Meiyu‐Baiu front, and enhanced the temperature contrast across the front, leading to record‐breaking Meiyu‐Baiu rainfall in 2020. Using atmospheric model experiments, we further show that the frequent East Siberian blockings can be largely attributed to excessive sea ice melting and the concurring warming along the Siberian coast. The lower‐atmospheric warming reduced local meridional temperature gradients and zonal winds, which favors East Siberian blockings. Our results highlight the effect of Arctic sea ice decline and the associated warming on Meiyu‐Baiu rainfall and provide a new mechanism for the Arctic to affect mid‐latitude precipitation in late spring‐early summer. This Arctic effect may change in the future as late spring‐early summer sea ice diminishes under global warming.
... On the other hand, a study by Screen et al. (2011) suggested that high cyclone activity in May, June, and July leads to higher than average September SIEs, according to the argument that increased cyclone activity is associated with stronger sea ice cyclonic circulation, divergence, and thus expansion in the ice cover. During anticyclonic conditions, clear-sky conditions enhance incoming solar radiation and thus summertime sea ice melt (Knudsen et al. 2015;Kay et al. 2008). Since the study by Serreze et al. (2003), both the lowest and third lowest record minima in SIE, in 2012 and 2016 respectively, have similarly been characterized by persistent anomalously low sea level pressure regimes, but were additionally influenced by extreme cyclones, referred to in the literature and media as great Arctic cyclone events (Simmonds and Rudeva 2012). ...
In this study the impact of extreme cyclones on Arctic sea ice in summer is investigated. Examined in particular are relative thermodynamic and dynamic contributions to sea ice volume budgets in the vicinity of Arctic summer cyclones in 2012 and 2016. Results from this investigation illustrate sea ice loss in the vicinity of the cyclone trajectories during each year were associated with different dominant processes: thermodynamic (melting) in the Pacific sector of the Arctic in 2012, and both thermodynamic and dynamic processes in the Pacific sector of the Arctic in 2016. Comparison of both years further suggests that the Arctic minimum sea ice extent is influenced by not only the strength of the cyclone, but also by the timing and location relative to the sea ice edge. Located near the sea ice edge in early August in 2012, and over the central Arctic later in August in 2016, extreme cyclones contributed to comparable sea ice area (SIA) loss, yet enhanced sea ice volume loss in 2012 relative to 2016.
Central to a characterization of extreme cyclone impacts on Arctic sea ice from the perspective of thermodynamic and dynamic processes, we present an index describing relative thermodynamic and dynamic contributions to sea ice volume changes. This index helps to quantify and improve our understanding of initial sea ice state and dynamical responses to cyclones in a rapidly warming Arctic, with implications for seasonal ice forecasting, marine navigation, coastal community infrastructure and designation of protected and ecologically sensitive marine zones.
... Indeed, the different models may exhibit different sea ice responses to the summer radiative forcing (Fig. 1) for example through different strength of feedbacks, like the local cloud response (cf. Knudsen et al. 2015;Abe et al. 2015). These differences in summer may then project on the winter responses. ...
The mid-Holocene (6000 years before present) was a warmer period than today in summer in most of the Northern Hemisphere. In winter, over Europe, pollen-based reconstructions show a dipole of temperature anomalies as compared to present-day, with warmer conditions in the north and colder in the south. It has been proposed that this pattern of temperature anomaly could be explained by a persisting positive phase of the North Atlantic Oscillation during this period, which was, however, not reproduced in general by climate models. Indeed, PMIP3 models show a large spread in their response to the mid-Holocene insolation changes, the physical origins of which are not understood. To improve the understanding of the reconstructed temperature changes and of the PMIP3 model spread, we analyze the dynamical response of these model simulations in the North Atlantic for mid-Holocene conditions as compared to pre-industrial. We focus on the European pattern of temperature in winter and compare the simulations with a pollen-based reconstruction. We find that some of the model simulations yield a similar pattern to the reconstructed one, but with far lower amplitude, although it remains within the reconstruction uncertainty. We attribute the northern warm part of the latitudinal dipole of temperature anomaly in winter to a lower sea-ice cover in the Nordic Seas. The decrease of sea ice in winter indeed reduces the local sea-ice insulation effect, allowing the released ocean heat to reach continental northern Europe. This decrease in winter sea-ice cover is related to an increase in the Atlantic meridional overturning circulation (AMOC) and its associated ocean heat transport, as well as the effect of insolation changes on sea ice in summer, which persists until winter. We only find a slight cooling signal over southern Europe, compared to reconstructions, mainly related to the insolation-induced cooling in winter over Africa. We show that the models that failed to reproduce any AMOC increase under mid-Holocene conditions are also the ones that do not reproduce the temperature pattern over Europe. The change in sea level pressure is not sufficient to explain the spread among the models. The ocean-sea ice mechanisms that we proposed constitute an alternative explanation to the pattern of changes in winter temperatures over Europe in the mid-Holocene, which is in better agreement with available model simulations of this period. Finally, we evaluate if reconstructions of the AMOC for the mid-Holocene can provide interesting emerging constraints on key changes in European climate, and indirectly on AMOC response to on-going and future radiative changes. Although there is a significant link between the response of the mid-Holocene and projections, it remains limited. The proposed mechanism does not appear to be sufficient to explain the large discrepancies between models and reconstruction data for the summertime period.
... Despite these consistent dynamical arguments, earlier studies do indicate that atmospheric circulation changes can link sea ice loss to more widespread warming on GrIS (e.g., Knudsen et al., 2015;Liu et al., 2016). We speculate that these circulation-related patterns observed in relation to recent sea ice loss may not dominate in the long-term view or that they are not represented in our model ensemble. ...
Observed and model‐projected sea ice loss enhances warming in the Arctic. We investigate to
what extent warming on Greenland can be attributed to changes in the sea ice cover in different parts of
the Arctic. Using Climate Model Intercomparison Project phase 5 model projections of the future, we
perform multilinear regressions to separate the simulated warming on Greenland in two parts; one
following global warming and one following regional sea ice changes. This reveals the magnitude and
spatial pattern of warming on Greenland, which can be attributed to sea ice loss in different Arctic regions.
The results indicate that the impact of sea ice loss is largely confined to the coastal parts of Greenland. We
find the strongest links to sea ice loss in adjacent regions; remote regions only have a limited impact.
Overall, warming attributable to sea ice variability is a minor contribution but can be a dominant signal
locally in coastal regions.
... On one hand, there are lots of studies (e.g., Francis and Vavrus, 2012;Miller et al., 2010 ;Zhang et al., 2012;Knudsen et al., 2015) pointing out clear evidences of the Arctic amplification to affect atmospheric conditions and result in anomalous behavior not only in the polar region, but in the midlatitudes as well. Several analyses (Overland et al., 2012;Peings and Magnusdottir, 2014) conclude that the increasing (at least in some seasons) amplitude in Rossby waves causes their slower eastward propagation, and consequently, more persistent weather conditions in the midlatitudes. ...
In recent years, several unusual (or at least very rare) weather events occurred in the Carpathian Basin, e.g., the severe snow in March 2013. We are assuming that this anomaly may be a part of the climate change-related macro-scale circulation changes, especially the changes in the characteristics of polar jetstream. For evaluating this hypothesis, we performed a detailed statistical analysis of the high level wind fields of the region for 22 vertical layers above the 500 hPa pressure level, including the detailed analysis of average wind speed and wind directions, trend analysis of daily wind speed values, and extreme wind speed values. The results show negative trends in the mean wind speed at the higher tropospheric levels, and positive trends at the very high stratospheric levels in the Carpathian Basin. Furthermore, statistically significant trends mostly occurred in westerly winds, which is the most frequent wind direction.
... This weakening has been detected in the westerly jet (following the thermal-wind balance), the total kinetic energy of synoptic storm systems (by about 15%) and the number of strong cyclones 26,71,74 . Similarly, strong Arctic sea-ice melting years are characterized by a weakened circulation 75 . While, the satellite era is most reliable when analyzing wind field characteristics, its limited timespan compromises long-term trend analyses. ...
Accelerated warming in the Arctic, as compared to the rest of the globe, might have profound impacts on mid-latitude weather. Most studies analyzing Arctic links to mid-latitude weather focused on winter, yet recent summers have seen strong reductions in sea-ice extent and snow cover, a weakened equator-to-pole thermal gradient and associated weakening of the mid-latitude circulation. We review the scientific evidence behind three leading hypotheses on the influence of Arctic changes on mid-latitude summer weather: Weakened storm tracks, shifted jet streams, and amplified quasi-stationary waves. We show that interactions between Arctic teleconnections and other remote and regional feedback processes could lead to more persistent hot-dry extremes in the mid-latitudes. The exact nature of these non-linear interactions is not well quantified but they provide potential high-impact risks for society.
... Only on 28 June (indicated by the negative tendency in surface pressure in Ny-Ålesund on 27 June in Fig. 2a), a cyclone passed the region and pre- vented any flight activities. Hence, analysis of synoptic-scale dynamics related to cyclones similar to, for example, Knudsen et al. (2015), Akperov et al. (2018), or Zahn et al. (2018), is not needed in this paper. ...
The two concerted
field campaigns, Arctic CLoud Observations Using airborne measurements during
polar Day (ACLOUD) and the Physical feedbacks of Arctic planetary boundary
level Sea ice, Cloud and AerosoL (PASCAL), took place near Svalbard from
23 May to 26 June 2017. They were focused on studying Arctic mixed-phase
clouds and involved observations from two airplanes (ACLOUD), an icebreaker
(PASCAL) and a tethered balloon, as well as ground-based stations. Here, we
present the synoptic development during the 35-day period of the campaigns,
using near-surface and upper-air meteorological observations, as well as
operational satellite, analysis, and reanalysis data. Over the campaign
period, short-term synoptic variability was substantial, dominating over the
seasonal cycle. During the first campaign week, cold and dry Arctic air from
the north persisted, with a distinct but seasonally unusual cold air
outbreak. Cloudy conditions with mostly low-level clouds prevailed. The
subsequent 2 weeks were characterized by warm and moist maritime air from
the south and east, which included two events of warm air advection. These
synoptical disturbances caused lower cloud cover fractions and
higher-reaching cloud systems. In the final 2 weeks, adiabatically warmed
air from the west dominated, with cloud properties strongly varying within the range of the two other periods. Results presented here provide
synoptic information needed to analyze and interpret data of upcoming studies
from ACLOUD/PASCAL, while also offering unprecedented measurements in a
sparsely observed region.
... For instance, Wu et al. (2013) suggested that winter sea ice concentration conditions west of Greenland influences the following summer atmospheric circulation over northern Eurasia. Using observations, Knudsen et al. (2015) found a link between anomalous Arctic sea ice melt and changes in midlatitude atmospheric patters during summer, as did Screen (2013) using an atmospheric general circulation model. Petrie et al. (2015), using a fully coupled climate model, found that sea ice loss together with increased SST in the Labrador Sea affects the summer atmospheric circulation over the North Atlantic region. ...
... Subsequent studies have shown that this coupling goes in both directions, and that year-to-year variations in cyclone activity also drive changes in sea ice. Years of anomalously (high) low May-August cyclone activity over the Arctic Ocean tending to be associated with anomalously (high) low September sea ice minima (Screen et al. 2011;Knudsen et al. 2015). A recent example of this was the August 2012 'Great Arctic Cyclone' which is thought to have influenced the record low September sea ice extent seen that year (Zhang et al. 2013). ...
The dramatic warming of the Arctic over the last three decades has reduced both the thickness and extent of sea ice, opening opportunities for business in diverse sectors and increasing human exposure to meteorological hazards in the Arctic. It has been suggested that these changes in environmental conditions have led to an increase in extreme cyclones in the region, therefore increasing this hazard. In this study, we investigate the response of Arctic synoptic scale cyclones to climate change in a large initial value ensemble of future climate projections with the CESM1-CAM5 climate model (CESM-LE). We find that the response of Arctic cyclones in these simulations varies with season, with significant reductions in cyclone dynamic intensity across the Arctic basin in winter, but with contrasting increases in summer intensity within the region known as the Arctic Ocean cyclone maximum. There is also a significant reduction in winter cyclogenesis events within the Greenland–Iceland–Norwegian sea region. We conclude that these differences in the response of cyclone intensity and cyclogenesis, with season, appear to be closely linked to changes in surface temperature gradients in the high latitudes, with Arctic poleward temperature gradients increasing in summer, but decreasing in winter.
... Moreover, stable atmospheric conditions prevent vertical motion and therefore condensation. Knudsen et al. (2015) showed that, in the recent era, Arctic anticyclonic circulation patterns also promote low snowfall in summer over the Russian sector of the Arctic, and a similar association with (too) high pressure could be at play in ERA-20C in the pre-1950s. On the other hand, if compared to station data, the ERA-20C snow depths show a good agreement for anomalies early in the 20th century. ...
Snow cover variability has significant effects on local and global climate evolution. By changing surface energy fluxes and hydrological conditions, changes in snow cover can alter atmospheric circulation and lead to remote climate effects. To document such multi-scale climate effects , atmospheric reanalysis and derived products offer the opportunity to analyze snow variability in great detail far back to the early 20th century. So far only little is know about their quality. Comparing snow depth in four long-term re-analysis datasets with Russian in situ snow depth data, we find a moderately high daily correlation (around 0.6–0.7), which is comparable to correlations for the recent era (1981– 2010), and a good representation of sub-decadal variability. However, the representation of pre-1950 inter-decadal snow variability is questionable, since reanalysis products divert towards different base states. Limited availability of independent long-term snow data makes it difficult to assess the exact cause for this bifurcation in snow states, but initial investigations point towards representation of the atmosphere rather than differences in assimilated data or snow schemes. This study demonstrates the ability of long-term reanalysis to reproduce snow variability accordingly.
... A negative state of the SNAO is consistent with generally strongly positive Atlantic multidecadal oscillation conditions over the last decade (Sutton and Dong 2012). However, evidence is strengthening that reductions in summer Arctic sea ice due to warming of the Arctic may also favor a negative SNAO (e.g., Knudsen et al. 2015, Petrie et al. 2015. The July 2015 MSLP anomaly pattern strongly resembled the negative SNAO. ...
... Rather, both NorESM and CCSM show weak reductions in NEE track density (−11.6 % in CCSM to −0.8 % in NorESM; Table 3) associated with enhancements in the Greenland Sea in September ( Fig. 11a and b). Figure 1 reveals that the latter increase coincides with a sea ice retreat in the Greenland Sea over the century. These results follow those of Deser et al. (2000), Magnusdottir et al. (2004) and Knudsen et al. (2015), who found storm activity to be very sensitive to the sea ice variations east of Greenland. Moreover, Chen et al. (2015) showed a corresponding sensitivity in synoptic activity here associated with variations in the surface mass balance of the Greenland Ice Sheet. ...
Metrics of storm activity in Northern Hemisphere high and
midlatitudes are evaluated from historical output and future projections by
the Norwegian Earth System Model (NorESM1-M) coupled global climate model.
The European Re-Analysis Interim (ERA-Interim) and the Community Climate
System Model (CCSM4), a global climate model of the same vintage as
NorESM1-M, provide benchmarks for comparison. The focus is on the autumn and
early winter (September through December) – the period when the ongoing and
projected Arctic sea ice retreat is the greatest. Storm tracks derived from a
vorticity-based algorithm for storm identification are reproduced well by
NorESM1-M, although the tracks are somewhat better resolved in the
higher-resolution ERA-Interim and CCSM4. The tracks show indications of
shifting polewards in the future as climate changes under the Representative
Concentration Pathway (RCP) forcing scenarios. Cyclones are projected to
become generally more intense in the high latitudes, especially over the
Alaskan region, although in some other areas the intensity is projected to
decrease. While projected changes in track density are less coherent, there
is a general tendency towards less frequent storms in midlatitudes and more
frequent storms in high latitudes, especially the Baffin Bay/Davis Strait
region in September. Autumn precipitation is projected to increase
significantly across the entire high latitudes. Together with the projected
loss of sea ice and increases in storm intensity and sea level, this increase
in precipitation implies a greater vulnerability to coastal flooding and
erosion, especially in the Alaskan region. The projected changes in storm
intensity and precipitation (as well as sea ice and sea level pressure) scale
generally linearly with the RCP value of the forcing and with time through
the 21st century.
... Other than Siberia, these atmospheric modes and trends likely affect the recent Arctic sea ice melt and hydroclimate around Mongolia. The circulation pattern over the Arctic that leads to the sea ice melt is nearly identical to that shown in figure 4(b) (also see Ogi and Wallace 2007, Ogi and Yamazaki 2010, Screen et al 2011, Knudsen et al 2015. Erdenebat and Sato (2016) recently explored the reasons for the recent (since the late 1990s) increase in heat wave frequency around Mongolia. ...
This study investigated the interannual variability and trends in precipitation and atmospheric circulation patterns over northern Eurasia using long-term Precipitation REConstruction over Land and atmospheric Japanese 55-year Reanalysis data (JRA-55) from 1958 to 2012. Special emphasis was placed on the recent increase in summer (June, July and August) precipitation around the Lena river basin in eastern Siberia. We found interdecadal modulation in the relationships between interannual variability in summer precipitation and atmospheric circulation patterns among the three major Siberian river basins (Lena, Yenisei, and Ob). The interannual variations in summer precipitation over the Ob and Lena river basins were negatively correlated from the mid-1970s to the mid-1990s. However, after the mid-1990s, this negative correlation became insignificant. In contrast, a significant positive correlation was apparent between the Yenisei and Lena river basins. We also found that there has been a significant increasing (positive) trend in geopotential height in the low-level troposphere since the mid-1980s over Mongolia and European Russia, resulting in an increasing trend of westerly moisture flux into the Yenisei and Lena river basins. Summer precipitation in both basins was continuously high from 2005 to 2008 under a trough that broadly extended from the Yenisei and Lena river basins, which has been a typical pattern of interannual variation since the mid-1990s. This trough increased the meridional pressure gradient between Mongolia and eastern Siberia in combination with the trend pattern. This further enhanced the eastward moisture flux towards the Lena river basin and its convergence over the basin, resulting in high summer precipitation from 2005 to 2008.
The thermodynamic processes and synoptic circulation features driving lower tropospheric temperature extremes in the high Arctic ( > 80 ◦ N) are investigated. Based on 10-day kinematic backward trajectories from the 5% most intense potential temperature anomalies, the contributions of horizontal and vertical transport, subsidence induced warming, and diabatic processes to the generation of the Arctic temperature anomaly are quantified. Cold extremes are mainly the result of sustained radiative cooling due to a sheltering of the Arctic from meridional air mass exchanges. This is linked to a strengthening of the tropospheric polar vortex, a reduced frequency of high latitude blocking, and in winter also a southward shift of the North Atlantic storm track. The temperature anomaly of 60% of wintertime extremely warm air masses (90% in summer) is due to transport from a potentially warmer region. Subsidence from the Arctic mid-troposphere in blocking anticyclones is the most important warming process with the largest contribution in summer (70% of extremely warm air masses). In both seasons, poleward transport of already warm air masses contributes around 20% and is favoured by a poleward shift of the North Atlantic storm track. Finally, about 40% of the air masses in winter are of an Arctic origin and experience diabatic heating by surface heat fluxes in marine cold air outbreaks. Our study emphasizes the importance of processes in the Arctic and the relevance of anomalous blocking – in winter in the Barents, Kara, and Laptev Seas and in summer in the high Arctic – for the formation of warm extremes.
The contribution of extra-tropical synoptic cyclones to the formation of summer-mean atmospheric circulation patterns in the Arctic is investigated by clustering the dominant Arctic circulation patterns by the self-organizing maps (SOMs) using the daily mean sea level pressure (MSLP) in the Arctic domain (≥ 60°N). Three SOM patterns are identified: one with prevalent low pressure anomalies in the Arctic Circle (SOM1) and two opposite dipoles with primary high pressure anomalies covering the Arctic Ocean (SOM2 and SOM3). The time series of summertime occurrence frequencies demonstrate the largest inter-annual variation in the SOM1, the slight decreasing trend in the SOM2, and the abrupt upswing after 2007 in the SOM3. The relevant analyses with produced cyclone track data confirm that the vital contribution. The Arctic cyclone activity is enhanced in the SOM1 because the meridional temperature gradient increases over the land–Arctic Ocean boundaries co-located with major extra-tropical cyclone pathways. The composite daily synoptic evolutions for each SOM reveal that the persistence of all the three SOMs is less than 5 days on average. These evolutionary short-term weather patterns have substantial variability at inter-annual and longer timescales. Therefore, the synoptic-scale activity is central to forming the seasonal-mean climate of the Arctic.
Previous work has explored the linkages between Arctic sea ice extent (SIE) anomalies at the end of the summer melt season and high-latitude climate. Here we show that Arctic midsummer SIE anomalies provide predictive skill on time scales of ~2–3 months for high-latitude climate. Midsummers characterized by low SIE are associated with significant positive temperature and easterly wind anomalies throughout the high-latitude troposphere through September and significant positive temperature anomalies at the Arctic surface into October. The inferred predictive skill for autumn climate derives from the persistence of the sea ice field. It is robust throughout the Arctic basin and is supported in climate models from the fifth phase of the Coupled Model Intercomparison Project archive and in prediction experiments from the Arctic Predictability and Prediction on Seasonal to Interannual Time scales project. It is theorized that the predictive skill derives from (1) the anomalous storage of heat in the Arctic Ocean during periods of low summertime SIE and (2) the delayed formation of sea ice during the following autumn months.
Snow cover variability has significant effects on local and global climate evolution. By changing surface energy fluxes and hydrological conditions, changes in snow cover can alter atmospheric circulation and lead to remote climate effects. To analyze such multi-scale climate effects, atmospheric reanalysis and derived products offer the opportunity to analyze snow variability in great detail far back in time. So far only little is know about their quality. Comparing four long-term reanalysis datasets with Russian in situ snow depth data, a good representation of daily to sub-decadal snow variability was found. However, the representation of pre-1950 inter-decadal snow variability is questionable, since datasets divert towards different base states. Limited availability of independent long-term snow data hinders investigating this bifurcation of snow states in great detail, but initial investigations reveal a non-stationary performance of snow evolution representation. This study demonstrates the ability of long-term reanalysis to reproduce snow variability accordingly.
Based on a statistical analysis incorporating 925-hPa wind fields from the NCEP/NCAR Reanalyses, it is shown that the combined effect of winter and summer wind forcing accounts for 50% of the variance of the change in September Arctic sea ice extent from one year to the next (Δ SIE) and it also explains roughly 1/3 of the downward linear trend of SIE over the past 31 years. In both seasons meridional wind anomalies to the north and east of Greenland are correlated with September SIE, presumably because they modulate the export of ice through Fram Strait. Anticyclonic wind anomalies over the Beaufort Sea during summer favor low September SIE and have contributed to the record-low values in recent summers, perhaps by enhancing the flux of ice toward Fram Strait in the trans-polar drift.
The Arctic region has warmed more than twice as fast as the global average - a phenomenon known as Arctic amplification. The rapid Arctic warming has contributed to dramatic melting of Arctic sea ice and spring snow cover, at a pace greater than that simulated by climate models. These profound changes to the Arctic system have coincided with a period of ostensibly more frequent extreme weather events across the Northern Hemisphere mid-latitudes, including severe winters. The possibility of a link between Arctic change and mid-latitude weather has spurred research activities that reveal three potential dynamical pathways linking Arctic amplification to mid-latitude weather: changes in storm tracks, the jet stream, and planetary waves and their associated energy propagation. Through changes in these key atmospheric features, it is possible, in principle, for sea ice and snow cover to jointly influence mid-latitude weather. However, because of incomplete knowledge of how high-latitude climate change influences these phenomena, combined with sparse and short data records, and imperfect models, large uncertainties regarding the magnitude of such an influence remain. We conclude that improved process understanding, sustained and additional Arctic observations, and better coordinated modelling studies will be needed to advance our understanding of the influences on mid-latitude weather and extreme events.
Model performance and future projection of Arctic summertime storm-track activity and associated background states are assessed on the basis of Coupled Model Intercomparison Project Phase 3 (CMIP3)/5 (CMIP5) climate models. Despite some improvement in the CMIP5 models relative to the CMIP3 models, most of the climate models underestimate summertime storm-track activity over the Arctic Ocean compared to six reanalysis data sets as measured locally as the variance of subweekly fluctuations of sea level pressure. Its large inter-model spread (i.e., model-to-model differences) is correlated with that of the intensity of the Beaufort Sea High and the lower-tropospheric westerlies in the Arctic region. Most of the CMIP3/5 models project the enhancement of storm-track activity over the Arctic Ocean off the eastern Siberian and Alaskan coasts, the region called the Arctic Ocean Cyclone Maximum, in association with the strengthening of the westerlies in the warmed climate. A model with stronger enhancement of the storm-track activity tends to accompany stronger land-sea contrast in surface air temperature across the Siberian coast, which reflects greater surface warming over the continent and slower warming over the Arctic Ocean. Other processes, however, may also be likely to contribute to the future changes of the storm-track activity, which gives uncertainty in the projection by multiple climate models. Our analysis suggests that further clarification of those processes that influence storm-track activity over the Arctic is necessary for more reliable future projections of the Arctic climate.
The Arctic cryosphere is an integral part of Earth's climate sys-tem and has undergone unprecedented changes within the past few decades. Rapid warming and sea-ice loss has had significant impacts locally, particularly in late summer and early autumn. September sea ice has declined at a rate of 12.4% per dec-ade since 1979 (ref.1), so that by summer 2012, nearly half of the areal coverage had disappeared. This decrease in ice extent has been accompanied by an approximately 1.8 m (40%) decrease in mean winter ice thickness since 1980 (ref.2) and a 75–80% loss in volume 3 . Though sea-ice loss has received most of the research and media attention, snow cover in spring and summer has decreased at an even greater rate than sea ice. June snow cover alone has decreased at nearly double the rate of September sea ice 4 . The decrease in spring snow cover has contributed to both the rise in warm season surface temperatures over the Northern Hemisphere extratropical landmasses and the decrease in summer Arctic sea ice 5 . The com-bined rapid loss of sea ice and snow cover in the spring and sum-mer has played a role in amplifying Arctic warming. However, snow cover and sea-ice trends diverge in the autumn and winter with sea ice decreasing in all months while snow cover has exhibited a neutral to positive trend in autumn and winter 6
Abnormal sea-ice retreat over the Barents Sea during early winter has been considered a leading driver of recent midlatitude severe winters over Eurasia. However, causal relationships between such retreat and the atmospheric circulation anomalies remains uncertain. Using a reanalysis dataset, we found that poleward shift of a sea surface temperature front over the Gulf Stream likely induces warm southerly advection and consequent sea-ice decline over the Barents Sea sector, and a cold anomaly over Eurasia via planetary waves triggered over the Gulf Stream region. The above mechanism is supported by the steady atmospheric response to the diabatic heating anomalies over the Gulf Stream region obtained with a linear baroclinic model. The remote atmospheric response from the Gulf Stream would be amplified over the Barents Sea region via interacting with sea-ice anomaly, promoting the warm Arctic and cold Eurasian pattern.
Significance
The recent decade has seen an exceptional number of boreal summer weather extremes, some causing massive damage to society. There is a strong scientific debate about the underlying causes of these events. We show that high-amplitude quasi-stationary Rossby waves, associated with resonance circulation regimes, lead to persistent surface weather conditions and therefore to midlatitude synchronization of extreme heat and rainfall events. Since the onset of rapid Arctic amplification around 2000, a cluster of resonance circulation regimes is observed involving wave numbers 7 and 8. This has resulted in a statistically significant increase in the frequency of high-amplitude quasi-stationary waves with these wave numbers. Our findings provide important insights regarding the link between Arctic changes and midlatitude extremes.
The past decade has seen an exceptional number of unprecedented summer extreme weather events in northern mid-latitudes, along with record declines in both summer Arctic sea ice and snow cover on high-latitude land. The underlying mechanisms that link the shrinking cryosphere with summer extreme weather, however, remain unclear. Here, we combine satellite observations of early summer snow cover and summer sea-ice extent with atmospheric reanalysis data to demonstrate associations between summer weather patterns in mid-latitudes and losses of snow and sea ice. Results suggest that the atmospheric circulation responds differently to changes in the ice and snow extents, with a stronger response to sea-ice loss, even though its reduction is half as large as that for the snow cover. Atmospheric changes associated with the combined snow/ice reductions reveal widespread upper-level height increases, weaker upper-level zonal winds at high latitudes, a more amplified upper-level pattern, and a general northward shift in the jet stream. More frequent extreme summer heat events over mid-latitude continents are linked with reduced sea ice and snow through these circulation changes.
The areal extent, concentration and thickness of sea ice in the Arctic Ocean and
adjacent seas have strongly decreased during the recent decades, but cold, snow-rich
winters have been common over mid-latitude land areas since 2005. A review is presented
on studies addressing the local and remote effects of the sea ice decline on weather and
climate. It is evident that the reduction in sea ice cover has increased the heat flux from the
ocean to atmosphere in autumn and early winter. This has locally increased air temperature,
moisture, and cloud cover and reduced the static stability in the lower troposphere.
Several studies based on observations, atmospheric reanalyses, and model experiments
suggest that the sea ice decline, together with increased snow cover in Eurasia, favours
circulation patterns resembling the negative phase of the North Atlantic Oscillation and
Arctic Oscillation. The suggested large-scale pressure patterns include a high over Eurasia,
which favours cold winters in Europe and northeastern Eurasia. A high over the western
and a low over the eastern North America have also been suggested, favouring advection of
Arctic air masses to North America. Mid-latitude winter weather is, however, affected by
several other factors, which generate a large inter-annual variability and often mask the
effects of sea ice decline. In addition, the small sample of years with a large sea ice loss
makes it difficult to distinguish the effects directly attributable to sea ice conditions.
Several studies suggest that, with advancing global warming, cold winters in mid-latitude
continents will no longer be common during the second half of the twenty-first century.
Recent studies have also suggested causal links between the sea ice decline and summer
precipitation in Europe, the Mediterranean, and East Asia.
Rapid Arctic warming and sea-ice reduction in the Arctic Ocean are widely attributed to anthropogenic climate change. The Arctic warming exceeds the global average warming because of feedbacks that include sea-ice reduction and other dynamical and radiative feedbacks. We find that the most prominent annual mean surface and tropospheric warming in the Arctic since 1979 has occurred in northeastern Canada and Greenland. In this region, much of the year-to-year temperature variability is associated with the leading mode of large-scale circulation variability in the North Atlantic, namely, the North Atlantic Oscillation. Here we show that the recent warming in this region is strongly associated with a negative trend in the North Atlantic Oscillation, which is a response to anomalous Rossby wave-train activity originating in the tropical Pacific. Atmospheric model experiments forced by prescribed tropical sea surface temperatures simulate the observed circulation changes and associated tropospheric and surface warming over northeastern Canada and Greenland. Experiments from the Coupled Model Intercomparison Project Phase 5 (ref. 16) models with prescribed anthropogenic forcing show no similar circulation changes related to the North Atlantic Oscillation or associated tropospheric warming. This suggests that a substantial portion of recent warming in the northeastern Canada and Greenland sector of the Arctic arises from unforced natural variability.
There has been growing interest in the vertical structure of the recent
Arctic warming. We investigated temperatures at the surface, 925, 700,
500 and 300 hPa levels in the Arctic (north of 70° N) using
observations and four reanalyses: ERA-Interim, CFSR, MERRA and NCEP II.
For the period 1979-2011, the layers at 500 hPa and below show a warming
trend in all seasons in all the chosen reanalyses and observations.
Restricting the analysis to the 1998-2011 period, however, all the
reanalyses show a cooling trend in the Arctic-mean 500 hPa temperature
in autumn, and this also applies to both observations and the reanalyses
when restricting the analysis to the locations with available IGRA
radiosoundings. During this period, the surface observations mainly
representing land areas surrounding the Arctic Ocean reveal no
summertime trend, in contrast with the reanalyses whether restricted to
the locations of the available surface observations or not.
In evaluating the reanalyses with observations, we find that the
reanalyses agree better with each other at the available IGRA sounding
locations than for the Arctic average, perhaps because the sounding
observations were assimilated into reanalyses. Conversely, using the
reanalysis data only from locations matching available surface (air)
temperature observations does not improve the agreement between the
reanalyses. At 925 hPa, CFSR deviates from the other three reanalyses,
especially in summer after 2000, and it also deviates more from the IGRA
radiosoundings than the other reanalyses do. The CFSR error in summer
T925 is due mainly to underestimations in the
Canadian-Atlantic sector between 120° W and 0°. The other
reanalyses also have negative biases in this longitude band.
The six summers from 2007 to 2012 were all wetter than average over
northern Europe. Although none of these individual events are
unprecedented in historical records, the sequence of six consecutive wet
summers is extraordinary. Composite analysis reveals that observed wet
summer months in northern Europe tend to occur when the jet stream is
displaced to the south of its climatological position, whereas dry
summer months tend to occur when the jet stream is located further
north. Highly similar mechanisms are shown to drive simulated
precipitation anomalies in an atmospheric model. The model is used to
explore the influence of Arctic sea ice on European summer climate, by
prescribing different sea ice conditions, but holding other forcings
constant. In the simulations, Arctic sea ice loss induces a southward
shift of the summer jet stream over Europe and increased northern
European precipitation. The simulated precipitation response is
relatively small compared to year-to-year variability, but is
statistically significant and closely resembles the spatial pattern of
precipitation anomalies in recent summers. The results suggest a causal
link between observed sea ice anomalies, large-scale atmospheric
circulation and increased summer rainfall over northern Europe. Thus,
diminished Arctic sea ice may have been a contributing driver of recent
wet summers.
The summertime variability of the extratropical storm track over the
Atlantic sector and its links to European climate have been analysed for
the period 1948-2011 using observations and reanalyses. The main
results are as follows. (1) The dominant mode of the summer storm track
density variability is characterized by a meridional shift of the storm
track between two distinct paths and is related to a bimodal
distribution in the climatology for this region. It is also closely
related to the Summer North Atlantic Oscillation (SNAO). (2) A southward
shift is associated with a downstream extension of the storm track and a
decrease in blocking frequency over the UK and northwestern Europe. (3)
The southward shift is associated with enhanced precipitation over the
UK and northwestern Europe and decreased precipitation over southern
Europe (contrary to the behaviour in winter). (4) There are strong
ocean-atmosphere interactions related to the dominant mode of
storm track variability. The atmosphere forces the ocean through
anomalous surface fluxes and Ekman currents, but there is also some
evidence consistent with an ocean influence on the atmosphere, and that
coupled ocean-atmosphere feedbacks might play a role. The ocean
influence on the atmosphere may be particularly important on decadal
timescales, related to the Atlantic Multidecadal Oscillation (AMO).
The summer sea-ice extent in the Arctic has decreased in recent decades,
a feature that has become one of the most distinct signals of the
continuing climate change. However, the inter-annual variability is
large--the ice extent by the end of the summer varies by several million
square kilometres from year to year. The underlying processes driving
this year-to-year variability are not well understood. Here we
demonstrate that the greenhouse effect associated with clouds and water
vapour in spring is crucial for the development of the sea ice during
the subsequent months. In years where the end-of-summer sea-ice extent
is well below normal, a significantly enhanced transport of humid air is
evident during spring into the region where the ice retreat is
encountered. This enhanced transport of humid air leads to an anomalous
convergence of humidity, and to an increase of the cloudiness. The
increase of the cloudiness and humidity results in an enhancement of the
greenhouse effect. As a result, downward long-wave radiation at the
surface is larger than usual in spring, which enhances the ice melt. In
addition, the increase of clouds causes an increase of the reflection of
incoming solar radiation. This leads to the counter-intuitive effect:
for years with little sea ice in September, the downwelling short-wave
radiation at the surface is smaller than usual. That is, the downwelling
short-wave radiation is not responsible for the initiation of the ice
anomaly but acts as an amplifying feedback once the melt is started.
The last six years (2007-2012) show a persistent change in early
summer Arctic wind patterns relative to previous decades. The persistent
pattern, which has been previously recognized as the Arctic Dipole (AD),
is characterized by relatively low sea-level pressure over the Siberian
Arctic with high pressure over the Beaufort Sea, extending across
northern North America and over Greenland. Pressure differences peak in
June. In a search for a proximate cause for the newly persistent AD
pattern, we note that the composite 700 hPa geopotential height
field during June 2007-2012 exhibits a positive anomaly only on
the North American side of the Arctic, thus creating the enhanced mean
meridional flow across the Arctic. Coupled impacts of the new persistent
pattern are increased sea ice loss in summer, long-lived positive
temperature anomalies and ice sheet loss in west Greenland, and a
possible increase in Arctic-subarctic weather linkages through
higher-amplitude upper-level flow. The North American location of
increased 700 hPa positive anomalies suggests that a regional
atmospheric blocking mechanism is responsible for the presence of the AD
pattern, consistent with observations of unprecedented high pressure
anomalies over Greenland since 2007.
Three years ago we proposed that the summer Arctic would be nearly sea
ice free by the 2030s; “nearly” is interpreted as sea ice
extent less than 1.0 million km2. We consider this estimate
to be still valid based on projections of updated climate models (CMIP5)
and observational data. Similar to previous models (CMIP3), CMIP5 still
shows a wide spread in hindcast and projected sea ice loss among
different models. Further, there is no consensus in the scientific
literature for the cause of such a spread in results for CMIP3 and
CMIP5. While CMIP5 model mean sea ice extents are closer to observations
than CMIP3, the rates of sea ice reduction in most model runs are slow
relative to recent observations. All CMIP5 models do show loss of sea
ice due to increased anthropogenic forcing relative to pre-industrial
control runs. Applying the same technique of model selection and
extrapolation approach to CMIP5 as we used in our previous paper, the
interval range for a nearly sea ice free Arctic is 14 to 36 years,
with a median value of 28 years. Relative to a 2007 baseline, this
suggests a nearly sea ice free Arctic in the 2030s.
The rapid retreat and thinning of the Arctic sea ice cover over the past
several decades is one of the most striking manifestations of global
climate change. Previous research revealed that the observed downward
trend in September ice extent exceeded simulated trends from most models
participating in the World Climate Research Programme Coupled Model
Intercomparison Project Phase 3 (CMIP3). We show here that as a group,
simulated trends from the models contributing to CMIP5 are more
consistent with observations over the satellite era (1979-2011).
Trends from most ensemble members and models nevertheless remain smaller
than the observed value. Pointing to strong impacts of internal climate
variability, 16% of the ensemble member trends over the satellite era
are statistically indistinguishable from zero. Results from the CMIP5
models do not appear to have appreciably reduced uncertainty as to when
a seasonally ice-free Arctic Ocean will be realized.
Atmospheric reanalyses were validated against tethersonde sounding data
on air temperature, air humidity and wind speed, collected during the
drifting ice station Tara in the central Arctic in April-August
2007. The data were not assimilated into the reanalyses, providing a
rare possibility for their independent validation, which was here made
for the lowermost 890 m layer. The following reanalyses were
included in the study: the European ERA-Interim, the Japanese JCDAS, and
the U.S. NCEP-CFSR, NCEP-DOE, and NASA-MERRA. All reanalyses included
large errors. ERA-Interim was ranked first; it outperformed the other
reanalyses in the bias and root-mean-square-error (RMSE) for air
temperature as well as in the bias, RMSE, and correlation coefficient
for the wind speed. ERA-Interim suffered, however, from a warm bias of
up to 2°C in the lowermost 400 m layer and a moist bias of 0.3
to 0.5 g kg-1 throughout the 890 m
layer. The NCEP-CFSR, NCEP-DOE, and NASA-MERRA reanalyses outperformed
the other reanalyses with respect to 2-m air temperature and specific
humidity and 10-m wind speed, which makes them, especially NCEP-CSFR,
better in providing turbulent flux forcing for sea ice models.
Considering the whole vertical profile, however, the older NCEP-DOE got
the second highest overall ranking, being better than the new NCEP-CFSR.
Considering the whole group of reanalyses, the largest air temperature
errors surprisingly occurred during higher-than-average wind speeds. The
observed biases in temperature, humidity, and wind speed were in many
cases comparable or even larger than the climatological trends during
the latest decades.
Arctic sea ice cover has decreased dramatically over the last three
decades. Global climate models under-predicted this decline, most likely
a result of the misrepresentation of one or more processes that
influence sea ice. The cloud feedback is the primary source of
uncertainty in model simulations, especially in the polar regions. A
better understanding of the interaction between sea ice and clouds, and
specifically the impact of decreased sea ice on cloud cover, will
provide valuable insight into the Arctic climate system and may
ultimately help in improving climate model parameterizations. In this
study, an equilibrium feedback assessment is employed to quantify the
relationship between changes in sea ice and clouds, using
satellite-derived sea ice concentration and cloud cover over the period
2000-2010. Results show that a 1% decrease in sea ice
concentration leads to a 0.36-0.47% increase in cloud cover,
suggesting that a further decline in sea ice cover will result in an
even cloudier Arctic.
Extratropical cyclones are identified and compared using data from four recent reanalyses for the winter periods in both hemispheres. Results show the largest differences occur between the older lower resolution 25-yr Japanese Reanalysis (JRA-25) when compared with the newer high resolution reanalyses, particularly in the Southern Hemisphere (SH). Spatial differences between the newest reanalyses are small in both hemispheres and generally not significant except in some common regions associated with cyclogenesis close to orography. Differences in the cyclone maximum intensitites are generally related to spatial resolution except in the NASA Modern Era Retrospective-Analysis for Research and Applications (NASA MERRA), which has larger intensities for several different measures. Matching storms between reanalyses shows the number matched between the ECMWF Interim Re-Analysis (ERA-Interim) and the other reanalyses is similar in the Northern Hemisphere (NH). In the SH the number matched between JRA-25 and ERA-Interim is lower than in the NH; however, for NASA MERRA and the NCEP Climate Forecast System Reanalysis (NCEP CFSR), the number matched is similar to the NH. The mean separation of the identically same cyclones is typically less than 2 degrees geodesic in both hemispheres for the latest reanalyses, whereas JRA-25 compared with the other reanalyses has a broader distribution in the SH, indicating greater uncertainty. The instantaneous intensity differences for matched storms shows narrow distributions for pressure, while for winds and vorticity the distributions are much broader, indicating larger uncertainty typical of smaller-scale fields. Composite cyclone diagnostics show that cyclones are very similar between the reanalyses, with differences being related to the intensities, consistent with the intensity results. Overall, results show NH cyclones correspond well between reanalyses, with a significant improvement in the SH for the latest reanalyses, indicating a convergence between reanalyses for cyclone properties.
The ongoing loss of Arctic sea-ice cover has implications for the wider climate system. The detection and importance of the atmospheric impacts of sea-ice loss depends, in part, on the relative magnitudes of the sea-ice forced change compared to natural atmospheric internal variability (AIV). This study analyses large ensembles of two independent atmospheric general circulation models in order to separate the forced response to historical Arctic sea-ice loss (1979–2009) from AIV, and to quantify signal-to-noise ratios. We also present results from a simulation with the sea-ice forcing roughly doubled in magnitude. In proximity to regions of sea-ice loss, we identify statistically significant near-surface atmospheric warming and precipitation increases, in autumn and winter in both models. In winter, both models exhibit a significant lowering of sea level pressure and geopotential height over the Arctic. All of these responses are broadly similar, but strengthened and/or more geographically extensive, when the sea-ice forcing is doubled in magnitude. Signal-to-noise ratios differ considerably between variables and locations. The temperature and precipitation responses are significantly easier to detect (higher signal-to-noise ratio) than the sea level pressure or geopotential height responses. Equally, the local response (i.e., in the vicinity of sea-ice loss) is easier to detect than the mid-latitude or upper-level responses. Based on our estimates of signal-to-noise, we conjecture that the local near-surface temperature and precipitation responses to past Arctic sea-ice loss exceed AIV and are detectable in observed records, but that the potential atmospheric circulation, upper-level and remote responses may be partially or wholly masked by AIV.
Arctic sea ice area has been decreasing for the past two decades. Apart from melting, the southward drift through Fram Strait is the main ice loss mechanism. We present high resolution sea ice drift data across 79° N from 2004 to 2010. Ice drift has been derived from radar satellite data and corresponds well with variability in local geostrophic wind. The underlying East Greenland current contributes with a constant southward speed close to 5 cm s-1, and drives around a third of the ice export. We use geostrophic winds derived from reanalysis data to calculate the Fram Strait ice area export back to 1957, finding that the sea ice area export recently is about 25% larger than during the 1960's. The increase in ice export occurred mostly during winter and is directly connected to higher southward ice drift velocities, due to stronger geostrophic winds. The increase in ice drift is large enough to counteract a decrease in ice concentration of the exported sea ice. Using storm tracking we link changes in geostrophic winds to more intense Nordic Sea low pressure systems. Annual sea ice area export likely has a significant influence on the summer sea ice variability and we find low values in the 1960's, the late 1980's and 1990's, and particularly high values during 2005-2008. The study highlights the possible role of variability in ice export as an explanatory factor for understanding the dramatic loss of Arctic sea ice during the last decades.
The pace of change in the arctic system during recent decades has captured the world's attention. Observations and model simulations both indicate that the arctic experiences an amplified response to climate forcing relative to that at lower latitudes. At the core of these changes is the arctic hydrologic system, which includes ice, gaseous vapor in the atmosphere, liquid water in soils and fluvial networks on land, and the freshwater content of the ocean. The changes in stores and fluxes of freshwater have a direct impact on biological systems, not only of the arctic region itself, but also well beyond its bounds. In this investigation, we used a heuristic, graphical approach to distill the system into its fundamental parts, documented the key relationships between those parts as best we know them, and identified the feedback loops within the system. The analysis illustrates relationships that are well understood, but also reveals others that are either unfamiliar, uncertain, or unexplored. The graphical approach was used to provide a visual assessment of the arctic hydrologic system in one possible future state in which the Arctic Ocean is seasonally ice free.
We present the Met Office Hadley Centre's sea ice and sea surface temperature (SST) data set, HadISST1, and the nighttime marine air temperature (NMAT) data set, HadMAT1. HadISST1 replaces the global sea ice and sea surface temperature (GISST) data sets and is a unique combination of monthly globally complete fields of SST and sea ice concentration on a 1° latitude-longitude grid from 1871. The companion HadMAT1 runs monthly from 1856 on a 5° latitude-longitude grid and incorporates new corrections for the effect on NMAT of increasing deck (and hence measurement) heights. HadISST1 and HadMAT1 temperatures are reconstructed using a two-stage reduced-space optimal interpolation procedure, followed by superposition of quality-improved gridded observations onto the reconstructions to restore local detail. The sea ice fields are made more homogeneous by compensating satellite microwave-based sea ice concentrations for the impact of surface melt effects on retrievals in the Arctic and for algorithm deficiencies in the Antarctic and by making the historical in situ concentrations consistent with the satellite data. SSTs near sea ice are estimated using statistical relationships between SST and sea ice concentration. HadISST1 compares well with other published analyses, capturing trends in global, hemispheric, and regional SST well, containing SST fields with more uniform variance through time and better month-to-month persistence than those in GISST. HadMAT1 is more consistent with SST and with collocated land surface air temperatures than previous NMAT data sets.
The perennial (September) Arctic sea ice cover exhibits large interannual variability, with changes of over a million square kilometers from one year to the next. Here we explore the role of changes in Arctic cyclone activity, and related factors, in driving these pronounced year-to-year changes in perennial sea ice cover. Strong relationships are revealed between the September sea ice changes and the number of cyclones in the preceding late spring and early summer. In particular, fewer cyclones over the central Arctic Ocean during the months of May, June, and July appear to favor a low sea ice area at the end of the melt season. Years with large losses of sea ice are characterized by abnormal cyclone distributions and tracks: they lack the normal maximum in cyclone activity over the central Arctic Ocean, and cyclones that track from Eurasia into the central Arctic are largely absent. Fewer storms are associated with above-average mean sea level pressure, strengthened anticyclonic winds, an intensification of the transpolar drift stream, and reduced cloud cover, all of which favor ice melt. It is also shown that a strengthening of the central Arctic cyclone maximum helps preserve the ice cover, although the association is weaker than that between low cyclone activity and reduced sea ice. The results suggest that changes in cyclone occurrence during late spring and early summer have preconditioning effects on the sea ice cover and exert a strong influence on the amount of sea ice that survives the melt season.
A novel method is introduced to generate climatological frequency distributions of meteorological features from gridded datasets. The method is used here to derive a climatology of extratropical cyclones from sea level pressure (SLP) fields. A simple and classical conception of cyclones is adopted where a cyclone is identified as the finite area that surrounds a local SLP minimum and is enclosed by the outermost closed SLP contour. This cyclone identification procedure can be applied to individual time instants, and climatologies of cyclone frequency, fc, are obtained by simple time averaging. Therefore, unlike most other climatologies, the method is not based on the application of a tracking algorithm and considers the size of cyclones. In combination with a conventional cyclone center tracking algorithm that allows the determination of cyclone life times and the location of cyclogenesis and cyclolysis, additional frequency fields can be obtained for special categories of cyclones that are generated in, move through, or decay in a specified geographical area.
The method is applied to the global SLP dataset for the time period 1958–2001 from the latest 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40). In the Northern Hemisphere and during winter, the cyclone frequency field has three maxima in the Pacific storm track (with fc up to 35%), the Atlantic storm track (with fc up to 32%), and the Mediterranean (with fc up to 15%). During the other seasons the fc values are generally reduced in midlatitudes and the subtropical monsoon areas appear as regions with enhanced fc. In the Southern Hemisphere, the seasonal variations are smaller with year-round maxima of fc in the belt from 50° to 70°S (along the coast of Antarctica, with maximum values of almost 40%) and to the east of the Andes (with fc up to 35% during summer). Application of a lifetime threshold value significantly reduces fc, in particular over and close to the continents. Subsets of cyclone frequency fields are calculated for several subjectively chosen regions of cyclone genesis, passage, and lysis. They show some interesting aspects of the behavior of extratropical cyclones; cyclones that decay along the U.S. West Coast, for instance, have a short lifetime and originate almost exclusively from the eastern North Pacific, whereas long-lived and long-distance Pacific cyclones terminate farther north in the Gulf of Alaska.
The approach to calculate frequency distributions of atmospheric flow structures as introduced in this study can be easily applied to gridded data from global atmospheric models and assimilation systems. It combines the counts of atmospheric features with their area of influence, and hence provides a robust and easily interpretable measure of key meteorological structures when comparing and evaluating different analysis datasets and climate model integrations. Further work is required to comprehensively exploit the presented global ERA-40 cyclone climatology, in particular, aspects of its interannual variability.
The atmospheric forcing on the Barents Sea ice extent during winter (December-February (DJF)) has been investigated for the period 1967-2002. The time series for the sea ice extent is updated and includes the winter of 2005, which marks a new record low in the wintertime Barents Sea ice extent, and a linear trend of 3.5% decade1 in the ice extent was found. Covariability between the Barents Sea ice extent and the atmospheric mean seasonal flow and the synoptic cyclones has been discussed separately. For the mean flow, linear correlations and regression analysis reveal that anomalous northerly (southerly) winds prevail in the Nordic Seas during winters with extensive (sparse) Barents Sea ice extent. Some of the variability in the mean flow is captured by the North Atlantic Oscillation (NAO); however, the wintertime link between the Barents Sea ice extent and the NAO is moderate. By studying the cyclone activity in the high-latitude Northern Hemisphere using a dataset of individual cyclones, two regions that influence the wintertime Barents Sea ice extent were identified. The variability in the northward-moving cyclones traveling into the Arctic over East Siberia was found to covary strongly with the Barents Sea ice extent. The main mechanism is believed to be the change in the Arctic winds and in ice advection connected to the cyclones. In addition, cyclone activity of northward-moving cyclones over the western Nordic Seas was identified to strongly influence the Barents Sea ice extent. This relationship was particularly strong on decadal time scales and when the ice extent lagged the cyclone variability by 1-2 yr. The lag indicates that the mechanism is related to the cyclones' ability to modulate the inflow of Atlantic water into the Nordic Seas and the transport time of oceanic heat anomalies from the Nordic Seas into the Barents Sea. Multiple regression indicates that the two mechanisms may explain (or at least covary with) 46% of the wintertime Barents Sea variance over the 1967-2002 period and that 79% of the decadal part of the ice variability may be predicted 2 yr ahead using information about the decadal cyclone variability in the Nordic Seas.
Arctic amplification (AA) - the observed enhanced warming in high
northern latitudes relative to the northern hemisphere - is
evident in lower-tropospheric temperatures and in 1000-to-500 hPa
thicknesses. Daily fields of 500 hPa heights from the National
Centers for Environmental Prediction Reanalysis are analyzed over N.
America and the N. Atlantic to assess changes in north-south (Rossby)
wave characteristics associated with AA and the relaxation of poleward
thickness gradients. Two effects are identified that each contribute to
a slower eastward progression of Rossby waves in the upper-level flow:
1) weakened zonal winds, and 2) increased wave amplitude. These effects
are particularly evident in autumn and winter consistent with sea-ice
loss, but are also apparent in summer, possibly related to earlier snow
melt on high-latitude land. Slower progression of upper-level waves
would cause associated weather patterns in mid-latitudes to be more
persistent, which may lead to an increased probability of extreme
weather events that result from prolonged conditions, such as drought,
flooding, cold spells, and heat waves.
Using the coupled ocean-atmosphere Bergen Climate Model, and a Lagrangian vorticity-based cyclone tracking method, the authors investigate current climate summer cyclones in the Northern Hemisphere and their change by the end of the 21st century, with a focus on Northern Eurasia and the Arctic. The two scenarios A1B and A2 for increasing greenhouse gas concentrations are considered. In the model projections, the total number of cyclones in the Northern Hemisphere is reduced by about 3%−4%, but the Arctic Ocean and adjacent coastal re-gions harbour slightly more and slightly stronger summer storms, compared to the model current climate. This in-crease occurs in conjunction with an increase in the high-latitude zonal winds and in the meridional tempera-ture gradient between the warming land and the ocean across Northern Eurasia. Deficiencies in climate model representations of the summer storm tracks at high lati-tudes are also outlined, and the need for further model inter-comparison studies is emphasized., 2009: Pro-jected changes in Eurasian and Arctic summer cyclones under global warming in the Bergen climate model, At-mos. Oceanic Sci. Lett., 2, 62−67.
Climate change is expected to affect not only the means of climatic variables, but also their variabilities(1,2) and extremes such as heat waves(2-6). In particular, modelling studies have postulated a possible impact of soil-moisture deficit and drought on hot extremes(7-11). Such effects could be responsible for impending changes in the occurrence of heat waves in Europe(7). Here we analyse observational indices based on measurements at 275 meteorological stations in central and southeastern Europe, and on publicly available gridded observations(12). We find a relationship between soil-moisture deficit, as expressed by the standardized precipitation index(13), and summer hot extremes in southeastern Europe. This relationship is stronger for the high end of the distribution of temperature extremes. We compare our results with simulations of current climate models and find that the models correctly represent the soil-moisture impacts on temperature extremes in southeastern Europe, but overestimate them in central Europe. Given the memory associated with soil moisture storage, our findings may help with climate-change-adaptation measures, such as early-warning and prediction tools for extreme heat waves
Arctic cyclone activity is investigated in the context of climate change and variability by using a modified automated cyclone identification and tracking algorithm, which differs from previously used algorithms by single counting each cyclone. The investigation extends earlier studies by lengthening the time period to 55 yr (1948 2002) with a 6-hourly time resolution, by documenting the seasonality and the dominant temporal modes of variability of cyclone activity, and by diagnosing regional activity as quantified by the cyclone activity index (CAI). The CAI integrates information on cyclone intensity, frequency, and duration into a comprehensive index of cyclone activity. Arctic cyclone activity has increased during the second half of the twentieth century, while midlatitude activity generally decreased from 1960 to the early 1990s, in agreement with previous studies. New findings include the following. 1) The number and intensity of cyclones entering the Arctic from the midlatitudes has increased, suggesting a shift of storm tracks into the Arctic, particularly in summer. 2) Positive tendencies of midlatitude cyclone activity before and after the 1960 93 period of decreasing activity correlate most strongly with variations of cyclone activity in the North Atlantic and Eurasian sectors. 3) Synchronized phase and amplitude variations in cyclone activity over the Arctic Ocean (70° 90°N) and the Arctic marginal zone (60° 70°N) play a critical role in determining the variations of cyclone activity in the Arctic as a whole. 4) Arctic cyclone activity displays significant low-frequency variability, with a negative phase in the 1960s and a positive phase in the 1990s, upon which 7.8- and 4.1-yr oscillations are superimposed. The 7.8-yr signal generally corresponds to the alternation of the cyclonic and anticyclonic regimes of the Arctic sea ice and ocean motions.
Recent decreases in Arctic sea ice and increases in Greenland ice sheet surface-melt may have global impacts, but the interactions between these two processes are unknown. Using microwave satellite data, we explore the spatial and temporal covariance of sea ice extent and ice sheet surface-melt around Greenland from 1979 to 2007. Significant covariance is discovered in several loci in the late summer, with the strongest covariance in western Greenland, particularly in the southwest (Kangerlussuaq). In this region, wind direction patterns and a statistical lag analysis of ice retreat/advance and surface-melt event timings suggest that sea ice extent change is a potential driver of ice sheet melt. Here, late summer wind directions facilitate onshore advection of ocean heat, and enhanced melting on the ice sheet commonly occurs after reductions in offshore sea ice. Hence, this study identifies for the first time the covariability patterns of sea ice and ice sheet melt and suggests that a retreating sea ice margin may enhance melting over the ice sheet.
This study uses an extensive dataset of monthly surface air temperature (SAT) records (including previously unutilized) from high-latitude (>60°N) meteorological land stations. Most records have been updated by very recent observations (up to December 2008). Using these data, a high-latitude warming rate of 1.36°C century -1 is documented for 1875-2008-the trend is almost 2 times stronger than the Northern Hemisphere trend (0.79°C century -1), with an accelerated warming rate in the most recent decade (1.35°C decade -1). Stronger warming in high-latitude regions is a manifestation of polar amplification (PA). Changes in SAT suggest two spatial scales of PA-hemispheric and local. A new stable statistical measure of PA linking high-latitude and hemispheric temperature anomalies via a regression relationship is proposed. For 1875-2008, this measure yields PA of ~1.62. Local PA related to the ice-albedo feedback mechanisms is autumnal and coastal, extending several hundred kilometers inland. Heat budget estimates suggest that a recent reduction of arctic ice and anomalously high SATs cannot be explained by ice-albedo feedback mechanisms alone, and the role of large-scale mechanisms of PA of global warming should not be overlooked.
The Arctic can be viewed as an integrated system, characterised by intimate couplings between its atmosphere, ocean and land, linked in turn to the larger global system. This comprehensive, up-to-date assessment begins with an outline of early Arctic exploration and the growth of modern research. Using an integrated systems approach, subsequent chapters examine the atmospheric heat budget and circulation, the surface energy budget, the hydrologic cycle and interactions between the ocean, atmosphere and sea ice cover. Reviews of recent directions in numerical modelling and the characteristics of past Arctic climates set the stage for detailed discussion of recent climate variability and trends, and projected future states. Throughout, satellite remote sensing data and results from recent major field programs are used to illustrate key processes. The Arctic Climate System provides a comprehensive and accessible overview of the subject for researchers and advanced students in a wide range of disciplines.
The ability of the climate models participating in phase 5 of the Coupled Model Intercomparison Project (CMIP5) to simulate North Atlantic extratropical cyclones in winter [December-February (DJF)] and summer [June-August (JJA)] is investigated in detail. Cyclones are identified as maxima in T42 vorticity at 850 hPa and their propagation is tracked using an objective feature-tracking algorithm. By comparing the historical CMIP5 simulations (1976-2005) and the ECMWF Interim Re-Analysis (ERA-Interim; 1979-2008), the authors find that systematic biases affect the number and intensity of North Atlantic cyclones in CMIP5 models. In DJF, the North Atlantic storm track tends to be either too zonal or displaced southward, thus leading to too few and weak cyclones over the Norwegian Sea and too many cyclones in central Europe. In JJA, the position of the North Atlantic storm track is generally well captured but some CMIP5 models underestimate the total number of cyclones. The dynamical intensity of cyclones, as measured by either T42 vorticity at 850 hPa or mean sea level pressure, is too weak in both DJF and JJA. The intensity bias has a hemispheric character, and it cannot be simply attributed to the representation of the North Atlantic large-scale atmospheric state. Despite these biases, the representation of Northern Hemisphere (NH) storm tracks has improved since CMIP3 and some CMIP5 models are able of representing well both the number and the intensity of North Atlantic cyclones. In particular, some of the higher-atmospheric-resolution models tend to have a better representation of the tilt of the North Atlantic storm track and of the intensity of cyclones in DJF.
This study uses cluster analysis to investigate the interdecadal poleward shift of the subtropical and eddy-driven jets and its relationship to intraseasonal teleconnections. For this purpose, self-organizing map (SOM) analysis is applied to the ECMWF Interim Re-Analysis (ERA-Interim) zonal-mean zonal wind. The resulting SOM patterns have time scales of 4.8–5.7 days and undergo notable interdecadal trends in their frequency of occurrence. The sum of these trends closely resembles the observed interdecadal trend of the subtropical and eddy-driven jets, indicating that much of the interdecadal climate forcing is manifested through changes in the frequency of intraseasonal teleconnection patterns.
Two classes of jet cluster patterns are identified. The first class of SOM pattern is preceded by anomalies in convection over the warm pool followed by changes in the poleward wave activity flux. The second class of patterns is preceded by sea ice and stratospheric polar vortex anomalies; when the Arctic sea ice area is reduced, the subsequent planetary wave anomalies destructively interfere with the climatological stationary waves. This is followed by a decrease in the vertical wave activity flux and a strengthening of the stratospheric polar vortex. An increase in sea ice area leads to the opposite chain of events. Analysis suggests that the positive trend in the Arctic Oscillation (AO) up until the early 1990s might be attributed to increased warm pool tropical convection, while the subsequent reversal in its trend may be due to the influence of tropical convection being overshadowed by the accelerated loss of Arctic sea ice.
Over the past half century, the Arctic has warmed at about twice the global rate. The reduction of sea ice and snow cover has contributed to the high-latitude warming, as the maximum of the amplification during autumn is a fingerprint of the ice-albedo feedback. There is evidence that atmospheric water vapor, a greenhouse gas, has increased in the Arctic over the past several decades. Ocean heat fluxes into the Arctic from the North Atlantic and North Pacific have also contributed to the Arctic warming through a reduction of sea ice. Observational and modeling studies suggest that reduced sea ice cover and a warmer Arctic in autumn may affect the middle latitudes by weakening the west-to-east wind speeds in the upper atmosphere, by increasing the frequency of wintertime blocking events that in turn lead to persistence or slower propagation of anomalous temperatures in middle latitudes, and by increasing continental snow cover that can in turn influence the atmospheric circulation. While these effects on middle latitudes have been suggested by some analyses, natural variability has thus far precluded a conclusive demonstration of an impact of the Arctic on mid-latitude weather and climate.
[1] The Arctic-wide melt season has lengthened at a rate of 5 days dec-1 from 1979 to 2013, dominated by later autumn freeze-up within the Kara, Laptev, East Siberian, Chukchi and Beaufort seas between 6 and 11 days dec-1. While melt onset trends are generally smaller, the timing of melt onset has a large influence on the total amount of solar energy absorbed during summer. The additional heat stored in the upper ocean of approximately 752 MJ m-2 during the last decade, increases sea surface temperatures by 0.5 to 1.5 °C and largely explains the observed delays in autumn freeze-up within the Arctic Ocean's adjacent seas. Cumulative anomalies in total absorbed solar radiation from May through September for the most recent pentad locally exceed 300-400 MJ m-2 in the Beaufort, Chukchi and East Siberian seas. This extra solar energy is equivalent to melting 0.97 to 1.3 m of ice during the summer.
[1] Previous studies have suggested that Arctic amplification has caused planetary-scale waves to elongate meridionally and slow down, resulting in more frequent blocking patterns and extreme weather. Here trends in the meridional extent of atmospheric waves over North America and the North Atlantic are investigated in three reanalyses, and it is demonstrated that previously reported positive trends are likely an artifact of the methodology. No significant decrease in planetary-scale wave phase speeds are found except in October-November-December, but this trend is sensitive to the analysis parameters. Moreover, the frequency of blocking occurrence exhibits no significant increase in any season in any of the three reanalyses, further supporting the lack of trends in wave speed and meridional extent. This work highlights that observed trends in midlatitude weather patterns are complex and likely not simply understood in terms of Arctic amplification alone.
This study examines observed changes (1979-2011) in atmospheric
planetary-wave amplitude over northern mid-latitudes, which have been
proposed as a possible mechanism linking Arctic amplification and
mid-latitude weather extremes. We use two distinct but equally-valid
definitions of planetary-wave amplitude, termed meridional amplitude, a
measure of north-south meandering, and zonal amplitude, a measure of the
intensity of atmospheric ridges and troughs at 45°N. Statistically
significant changes in either metric are limited to few seasons,
wavelengths, and longitudinal sectors. However in summer, we identify
significant increases in meridional amplitude over Europe, but
significant decreases in zonal amplitude hemispherically, and also
individually over Europe and Asia. Therefore, we argue that possible
connections between Arctic amplification and planetary waves, and
implications of these, are sensitive to how waves are conceptualized.
The contrasting meridional and zonal amplitude trends have different and
complex possible implications for midlatitude weather, and we encourage
further work to better understand these.
This study documents the evolving trends in Arctic sea ice extent and
concentration during 1979-2007 and places them within the context of
overlying changes in the atmospheric circulation. Results are based on
5-day running mean sea ice concentrations (SIC) from passive microwave
measurements during January 1979 to October 2007. Arctic sea ice extent
has retreated at all times of the year, with the largest declines (0.65
× 106 km2 per decade, equivalent to 10% per
decade in relative terms) from mid July to mid October. The pace of
retreat has accelerated nearly threefold from the first half of the
record to the second half, and the number of days with SIC less than 50%
has increased by 19 since 1979. The spatial patterns of the SIC trends
in the two halves of the record are distinctive, with regionally
opposing trends in the first half and uniformly negative trends in the
second half. In each season, these distinctive patterns correspond to
the first two leading empirical orthogonal functions of SIC anomalies
during 1979-2007. Atmospheric circulation trends and accompanying
changes in wind-driven atmospheric thermal advection have contributed to
thermodynamic forcing of the SIC trends in all seasons during the first
half of the record and to those in fall and winter during the second
half. Atmospheric circulation trends are weak over the record as a
whole, suggesting that the long-term retreat of Arctic sea ice since
1979 in all seasons is due to factors other than wind-driven atmospheric
thermal advection.
The surface albedo of the Arctic sea-ice zone is a crucial component in
the energy budget of the Arctic region. The treatment of sea-ice albedo
has been identified as an important source of variability in the future
sea-ice mass loss forecasts in coupled climate models. There is a clear
need to establish data sets of Arctic sea-ice albedo to study the
changes based on observational data and to aid future modelling efforts.
Here we present an analysis of observed changes in the mean albedo of
the Arctic sea-ice zone using a data set consisting of 28 years of
homogenized satellite data. Along with the albedo reduction resulting
from the well-known loss of late-summer sea-ice cover, we show that the
mean albedo of the remaining Arctic sea-ice zone is decreasing. The
change per decade in the mean August sea-ice zone albedo is
-0.029+/-0.011. All albedo trends, except for the sea-ice zone in May,
are significant with a 99% confidence interval. Variations in mean
sea-ice albedo can be explained using sea-ice concentration, surface air
temperature and elapsed time from onset of melt as drivers.
To provide a background for ARM`s activities at the North Slope of Alaska/Adjacent Arctic Ocean sites, an overview is given of our current state of knowledge of Arctic cloud and radiation properties and processes. The authors describe the Arctic temperature and humidity characteristics, cloud properties and processes, radiative characteristics of the atmosphere and surface, direct and indirect radiative effects of aerosols, and the modeling and satellite remote sensing of cloud and radiative characteristics. An assessment is given of the current performance of satellite remote sensing and climate modeling in the Arctic as related to cloud and radiation issues. Radiation-climate feedback processes are discussed, and estimates are made of the sign and magnitude of the individual feedback components. Future plans to address these issues are described. 276 refs., 12 figs.
We used 268 cloud-free Moderate-resolution Imaging Spectroradiometer (MODIS) images from 2003 and 2005–2007 to study the seasonal evolution of supra-glacial lakes in three different regions of the Greenland Ice Sheet. Lake area estimates were obtained by developing an automated classification method for their identification based on 250 m resolution MODIS surface reflectance observations. Widespread supra-glacial lake formation and drainage is observed across the ice sheet, with a 2–3 week delay in the evolution of total supra-glacial lake area in the northern areas compared to the south-west. The onset of lake growth varies by up to one month inter-annually, and lakes form and drain at progressively higher altitudes during the melt season. A positive correlation was found between the annual peak in total lake area and modelled annual runoff. High runoff and lake extent years are generally characterised by low accumulation and high melt season temperatures, and vice versa. Our results indicate that, in a future warmer climate [Meehl, G. A., Stocker, T. F., Collins W. D., Friedlingstein, P., Gaye, A. T., Gregory, J. M., Kitoh, A., Knutti, R., Murphy, J. M., Noda, A., Raper, S. C. B., Watterson, I. G., Weaver, A. J. & Zhao, Z. C. (2007). Global Climate Projections. In: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K. B. Averyt, M. Tignor & H. L. Miller (eds.), Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.], Greenland supra-glacial lakes can be expected to form at higher altitudes and over a longer time period than is presently the case, expanding the area and time period over which connections between the ice sheet surface and base may be established [Das, S., Joughin, M., Behn, M., Howat, I., King, M., Lizarralde, D., & Bhatia, M. (2008). Fracture propagation to the base of the Greenland Ice Sheet during supra-glacial lake drainage. Science, 5877, 778–781] with potential consequences for ice sheet discharge [Zwally, H.J., Abdalati, W., Herring, T., Larson, K., Saba, J. & Steffen, K. (2002). Surface melt-induced acceleration of Greenland Ice Sheet flow. Science, 297, 218–221.].
The retreat of Arctic sea ice in recent decades is a pre-eminent signal of climate change. What role has the atmospheric circulation played in driving the sea ice decline? To address this question, we document the evolution of Arctic sea ice concentration trends during the period January 1979–April 2007 in light of changing atmospheric circulation conditions, in particular an upward trend in the wintertime Northern Annular Mode during the first half of the record and a downward trend during the second half. The results indicate that concurrent atmospheric circulation trends contribute to forcing winter and summer sea ice concentration trends in many parts of the marginal ice zone during both periods. However, there is also an emerging signal of overall Arctic sea ice decline since 1979 in both winter and summer that is not directly attributable to a trend in the overlying atmospheric circulation.
Using reanalysis data from the National Centers for Environmental Prediction-National Center for Atmosperic Research, Boulder, Colorado, for the period from 1958 to 2005, we statistically analyzed the relationships of the summer Northern Hemisphere annular mode (summer NAM) with hemispheric-scale anomalous summer weather and the occurrence of blocking highs. The anomalous positive NAM (low-pressure anomaly in the Arctic and high-pressure anomaly in midlatitudes) accounts well for the hemispheric-scale weather associated with anomalous blocking between the polar and subtropical jets, whereas blocking rarely occurs during negative NAM periods. The double jet stream structure is more evident during periods of anomalous positive NAM than during periods of negative NAM. The surface temperatures associated with the anomalous positive NAM clearly show Europe to be hot and East Asia to be cool, as was the case during the anomalous summer of 2003. The occurrence of a positive summer NAM is therefore consistent with the hemispheric-scale anomalous summer weather associated with blocking in 2003. We investigated the abrupt evolution of atmospheric patterns and the geographic distribution of blocking highs associated with the development, maintenance, and decay periods of an anomalous positive NAM. During the development period, blocking tends to occur over Europe and the Atlantic Ocean, but no significant blocking signature is evident over eastern Eurasia. During the maintenance stage, blocking tends to occur in the Far East. During the decay stage, blocking over the Pacific region is obvious. This longitudinal migration of blocking phenomena may be used to predict the evolution through time of the NAM.
It is an old rule-of-thumb that sea ice moves with a speed of about 2% of the surface wind and about 45° to the right of the wind. A similar relationship between the ice velocity and the geostrophic wind is examined here. It is found that only about half of the long-term (several month) average ice motion is directly related to the geostrophic wind, the other half being due to the mean ocean circulation. On shorter time scales and in all seasons, more than 70% of the variance of the ice velocity in the central Arctic Ocean is explained by the geostrophic wind. Within about 400 km of the coasts the geostrophic wind is less successful in explaining the ice motion. The spatial variations in ice velocity are also partly explained by the geostrophic wind. About half of the variance in the large-scale ice vorticity and shear are accounted for. On the other hand, none of the large-scale ice divergence can be explained by the wind. The long-term average ocean current is estimated by subtracting the share of the ice motion caused by the wind from the total ice motion.
A prominent feature of the atmospheric circulation over the Arctic Ocean
is a sharp summer maximum in cyclone activity, in the mean centered
slightly off the North Pole. This pattern contributes to a late summer
peak in precipitation and net precipitation over the region, and can
have pronounced impacts on the circulation of the underlying sea ice
cover. The cyclone maximum is associated with the influx of lows
generated over the Eurasian continent and cyclogenesis over the Arctic
Ocean itself. Many of the lows found within the cyclone maximum exhibit
their maximum deepening over the Arctic Ocean. While onset of the
pattern can be related to a general poleward shift of baroclinicity in
summer, the cold Arctic Ocean may help to constrain the center of the
500 hPa polar vortex to near the pole. The strength of the pattern is
highly variable. When well developed, the 500 hPa polar vortex is
especially strong and symmetric, with negative sea level pressure
anomalies over the pole attended by positive anomalies over
mid-latitudes. When poorly developed, the opposite pattern holds.
Variability in the pattern can be viewed as a summer manifestation of
the Northern Annular Mode. Baroclinic instability during strongly
cyclonic months tends to be focused along the periphery of the cold
vortex over and north of the Eurasian coast.
A dominant feature of the cyclone, close to its centre, was the "cloud head': a region of cloud with a sharp convex outer edge, which formed poleward of the main polar-front cloud band. The cloud head was caused by two flows that entered it from the east, ascending and fanning out within it. One flow (the "cold conveyor belt') brought low wet-bulk potential temperature (θw) air back into the cloud head from low levels ahead of the warm front. The other flow was due to high-θw air that peeled off from the base of the main warm-sector airflow (i.e. part of the "warm conveyor belt') and travelled in the boundary layer back towards the cyclone centre, first undercutting dry air that had earlier descended from the upper troposphere (called a "dry intrusion'), and then ascending at the upper boundary of the cloud head, above the cold conveyor belt. The transverse circulation that gave the ascent within the cloud head also led to the cold front fracturing along its length into two separate sharp surface cold fronts, with a more diffuse frontal region in between ("frontal fracture'). The two sharp surface cold fronts were associated with narrow cold-frontal rainbands. -Authors