Fig 5 - available via license: Creative Commons Attribution 3.0 Unported
Content may be subject to copyright.
Automatic weather station (left) and ERA-Interim (right) surface wind speed and direction data by season from the WAIS Divide site. Wind rose plots show the frequency with which winds came from a given direction. Each dotted circle represents 5 % of observations. True cardinal directions are indicated.
Source publication
We present the first high-resolution (sub-annual) dust particle dataset from West Antarctica, developed from the West Antarctic Ice Sheet (WAIS) Divide deep ice core (79.468° S, 112.086° W), and use it to reconstruct past atmospheric circulation. We find a background dust flux of ∼4 mg m−2 yr−1 and a mode particle size of 5–8 μm diameter. Through c...
Citations
... Records with a wide geographical scope are necessary to map the meridional movement of the SHW. Studies from northern Patagonia (Villa-Martinez et al, 2003), Tristan da Cunha archipelago (Ljung et al., 2015), Southern Patagonia (Stern et al., 2015, Unkel et al., 2008, South Georgia (Strother et al, 2015), Isla de los Estados (Björck et al., 2012), Tierra del Fuego (Moreno et al., 2010;Vanneste et al., 2015), the northern region of the Antarctic Peninsula (Koffman et al., 2013), Southern Africa (Meadows and Baxter, 1999), New Zealand (Shulmeister et al., 2006;Schaefer et al., 2009), and Australia (Fitzsimmons and Barrows, 2010;Jones et al., 1998) all have addressed past position of the SHW in the middle and high latitudes of the Southern Hemisphere since the last glaciation. ...
... For example, gyttja is an indicator of productive basins, whereas inorganic clays suggest low productivity and typically cooler climate (Dudley and Crow, 1983;Moore, 1983). Inorganic sediments, by far the most common sediment type in the tarns, could be deposited either via surface runoff (Gilli et al., 2005;Lamy et al., 2001) or aeolian activity during times of high wind intensity and/or sediment supply (Koffman et al., 2013). Given 1) the near-lack of any evidence of surface runoff (in the form of channels, rills, etc...), 2) the very small catchment basins scarcely larger than the lakes, and 3) the fine silt and clay grain sizes, I infer that inorganic sediments were brought to the lakes primarily by wind. ...
... Usborne tarns, high MS commonly is associated with organic-poor clays and silts, particularly in glacial and late-glacial time. High MS values have been used elsewhere as an indicator of wind intensity in the Southern Hemisphere (Koffman et al., 2013;Waldmann et al., 2010) but detection is limited by availability of magnetic source material (Vanneste et al., 2016). ...
The Southern Hemisphere Westerlies (SHW) are an important driver of climate in the mid-latitudes of the Southern Hemisphere. Abrupt latitudinal migration of this coupled atmospheric-oceanic system is thought to be linked to the onset of the Termination at the end of the last ice age and to subsequent climatic variation through the late-glacial period and Holocene. However, the timing and spatial extent of these shifts, as well as variations in wind intensity, are poorly constrained, hindering our understanding of abrupt climate change in the Southern Hemisphere. In addition, future changes in the position and intensity of the SHW are a critical part of model projections, because the SHW affect Southern Ocean upwelling and CO2 sequestration. Insight into the future behavior of the SHW can come from examination of past fluctuations. My focus is the South Atlantic region, thought to be a key area for interactions between the SHW and other components of the climate system. However, there are few terrestrial datasets constraining past variations in the SHW in this region and many of these appear contradictory. This study is comprised of two alpine lake sediment cores extracted from tarns occupied by alpine glaciers during the last ice age on Mount Usborne of East Falkland (51oS). This terrestrial record, which spans the last 23 ka, uses stratigraphy, organic content, biomarkers (with a focus on plant wax), isotopic composition of plant waxes, and a preliminary pollen record to identify relative wind intensity, wetness, precipitation source, and temperature of the site. Moisture source is particularly useful as it can be tied to the average position of the SHW over time, with enriched precipitation reflecting a southerly location and depleted precipitation indicating a northerly shifted wind belt. My data suggest climate at Mount Usborne was cold and windy until 16.4 ka, when the SHW moved south and the area may have warmed. This shift represents the local expression of the onset of the Termination. Following a brief period at 13.6-14 ka when the SHW returned to a northern position during the Antarctic Cold Reversal, climate became wetter at 12.5-13.6 ka, associated with a southward migration of the SHW. At 11.2-12.5 ka, the westerlies again moved north and climate in the Falkland Islands became more humid. The start of the Holocene was marked by increasingly warmer and wetter conditions, with the SHW migrating south between 9-11 ka. Southward migration from 6-9 ka resulted in drier, windier conditions over the Falkland Islands. A brief event in the mid-Holocene (5.5-6 ka) is wet and less windy. A distinct reversal in the southward trend occurred at 5.5 ka. During the rest of the Holocene, the SHW have slowly migrated to the north. Climate was windy and dry from 3-5.5 ka and less windy and wet from 0-3 ka. My dataset suggests a highly variable position of the SHW over the past 23 ka, with multiple north-south migrations, including 1) southward migration during the Termination 2) periodic northward shifts in the late-glacial period, 3) a southerly position during the early Holocene, and 4) northward movement in the mid to late Holocene, particularly after 5.5 ka. The relative position of the SHW calculated in this study combined with other climate records at 51-54oS suggest that variations in the position of the SHW correspond closely with temperature, wind intensity, and precipitation variations, as well as to well-known climate events, regionally. The correspondence between changes in the SHW and periods of abrupt climate change support the hypothesis that the SHW are linked to much of the climatic variation in the South Atlantic region.
... Similarly, strong decadal-to-centennial temperature signals have been observed in the SO [140,141]. The concentration of opal in cores of bottom sediments, a proxy for the extent of overlying sea ice, oscillates at a period that closely reproduces the periodicity of temperature change in Antarctic ice cores [142] (Figures 3d and 4a, p. 1437), [143,144]. Centennial-scale shifts in wind velocity over the last millennium [143] are also consistent with this hypothesis. Computations using a coupled climate model that incorporates aspects of the RO hypothesis show that variation in deep convection in the SO releases heat in a cyclic temporal pattern that closely reproduces periodic temperature variations of the ACO at EDML and related oscillatory oceanic and atmospheric variations [144]. ...
... The concentration of opal in cores of bottom sediments, a proxy for the extent of overlying sea ice, oscillates at a period that closely reproduces the periodicity of temperature change in Antarctic ice cores [142] (Figures 3d and 4a, p. 1437), [143,144]. Centennial-scale shifts in wind velocity over the last millennium [143] are also consistent with this hypothesis. Computations using a coupled climate model that incorporates aspects of the RO hypothesis show that variation in deep convection in the SO releases heat in a cyclic temporal pattern that closely reproduces periodic temperature variations of the ACO at EDML and related oscillatory oceanic and atmospheric variations [144]. ...
The Antarctic Centennial Oscillation (ACO) is a paleoclimate temperature cycle that originates in the Southern Hemisphere, is the presumptive evolutionary precursor of the contemporary Antarctic Oscillation (AAO), and teleconnects to the Northern Hemisphere to influence global temperature. In this study we investigate the internal climate dynamics of the ACO over the last 21 millennia using stable water isotopes frozen in ice cores from 11 Antarctic drill sites as temperature proxies. Spectral and time series analyses reveal that ACOs occurred at all 11 sites over all time periods evaluated, suggesting that the ACO encompasses all of Antarctica. From the Last Glacial Maximum through the Last Glacial Termination (LGT), ACO cycles propagated on a multicentennial time scale from the East Antarctic coastline clockwise around Antarctica in the streamline of the Antarctic Circumpolar Current (ACC). The velocity of teleconnection (VT) is correlated with the geophysical characteristics of drill sites, including distance from the ocean and temperature. During the LGT, the VT to coastal sites doubled while the VT to inland sites decreased fourfold, correlated with increasing solar insolation at 65°N. These results implicate two interdependent mechanisms of teleconnection, oceanic and atmospheric, and suggest possible physical mechanisms for each. During the warmer Holocene, ACOs arrived synchronously at all drill sites examined, suggesting that the VT increased with temperature. Backward extrapolation of ACO propagation direction and velocity places its estimated geographic origin in the Southern Ocean east of Antarctica, in the region of the strongest sustained surface wind stress over any body of ocean water on Earth. ACO period is correlated with all major cycle parameters except cycle symmetry, consistent with a forced, undamped oscillation in which the driving energy affects all major cycle metrics. Cycle period and symmetry are not discernibly different for the ACO and AAO over the same time periods, suggesting that they are the same climate cycle. We postulate that the ACO/AAO is generated by relaxation oscillation of Westerly Wind velocity forced by the equator-to-pole temperature gradient and propagated regionally by identified air-sea-ice interactions.
... Precipitation-based ENSO records from the east, central, and west equatorial Pacific substantiate these findings and show that In the middle to high latitudes of the Southern Hemisphere, the LIA has been associated with strengthened westerlies over Tasmania [Shulmeister et al., 2004, and references therein]. Multiple records from New Zealand, South Africa, South America, and Antarctica during the LIA suggest a negative Southern Annular Mode (SAM) phase that is characterized by an equatorward shift and expansion of the southern westerlies due to cooler surface temperatures and contraction of the Hadley cell, implying weaker gyre circulation [Tyson and Lindesay, 1992;Lamy et al., 2001;Koffman et al., 2013]. It is proposed that at this time, in addition to transporting more 14 C-depleted waters, the EAC was relatively weak and did not penetrate as far south as is presently observed. ...
Variability in the southwest (SW) Pacific Ocean circulation is influenced by the changes in the South Pacific subtropical gyre and its western boundary current, the East Australian Current (EAC). The EAC plays a significant role in transporting warm, well-ventilated, nutrient-poor waters to more temperate higher latitudes. Recent climate changes associated with EAC intensification have led to anomalous warming in the South Tasman, with implications for marine ecosystems and environment. A clear understanding of the significance of these changes requires knowledge of past natural variability. Here we have reconstructed a 4500 year record of regional sea surface radiocarbon reservoir ages (R) and local reservoir effects (ΔR). Our results reveal the centennial-scale variability over the last 4500 years, with R ranges as large as 390 14C yr. Older R (~410 14C yr) between 1610 to 1860 A.D. in our record, corresponding to the “Little Ice Age,” suggests a weaker influence of the EAC in the South Tasman. Between 4000 and 1900 cal years B.P., R and ΔR were significantly younger than the modern, with values of ~170 and −130 14C yr, respectively, indicating increased EAC transport of tropical waters into the South Tasman. We propose that the large R variability was influenced by strong and abrupt El Niño events which punctuated the muted El Niño–Southern Oscillation (ENSO) period in the mid-late Holocene and enabled increased westward flow of gyre waters into the SW Pacific. The strengthening of the EAC extension appears to have been a response to the precession-modulated ENSO-Southern Annular Mode interactions.
... Although there is currently no straightforward record of late Holocene westerly wind speed variations from the Antarctic Peninsula (e.g., Bentley et al., 2009), Koffman et al. (2013 used changes in the grain-size of dust particles in the WAIS Divide ice core to show that westerly wind speed at the southern boundary of the SWWB decreased in 1400e1850 CE. Other Antarctic ice core records have been used to infer past changes in wind strength but these records are mostly based on the sea-salt sodium proxy, which is influenced by multiple parameters, such as atmospheric circulation and sea-ice cover, and can therefore not be unambiguously interpreted as variations in westerly wind speed (Koffman et al., 2013). ...
... Although there is currently no straightforward record of late Holocene westerly wind speed variations from the Antarctic Peninsula (e.g., Bentley et al., 2009), Koffman et al. (2013 used changes in the grain-size of dust particles in the WAIS Divide ice core to show that westerly wind speed at the southern boundary of the SWWB decreased in 1400e1850 CE. Other Antarctic ice core records have been used to infer past changes in wind strength but these records are mostly based on the sea-salt sodium proxy, which is influenced by multiple parameters, such as atmospheric circulation and sea-ice cover, and can therefore not be unambiguously interpreted as variations in westerly wind speed (Koffman et al., 2013). ...
... Low precipitation in south-central Chile (Fig. 7aed) and high precipitation in southern Patagonia (Fig. 7feh) in 600e1000 CE agree with a southward position of the SSWB, as reflected in sediment core PC29A (Fig. 7e). Similarly, the northward shift of the SWWB that was identified from sediment core PC29A in 1200e1950 CE is supported by higher precipitation in south-central Chile in 1200e1800 CE (Lamy et al., 2001;Jenny et al., 2002;Bertrand et al., 2005;Fletcher and Moreno, 2012), lower precipitation at the southwestern tip of Patagonia in 1200e1850 CE (Moreno et al., 2010;Waldmann et al., 2010;Schimpf et al., 2011), and by the decrease in westerly wind speed over West Antarctica identified by Koffman et al. (2013). These records collectively provide evidence for a more southward location of the SWWB in 600e1000 CE, a northward shift in 1200e1500 CE and a sustained northward position in 1500e1900 CE. ...
The climate of Chilean Patagonia is strongly influenced by the southern westerlies, which control the amount and latitudinal distribution of precipitation in the southern Andes. In austral summer, the Southern Westerly Wind Belt (SWWB) is restricted to the high latitudes. It expands northward in winter, which results in a strong precipitation seasonality between ∼35 and 45°S. Here, we present a new precipitation seasonality proxy record from Quitralco fjord (46°S), where relatively small latitudinal shifts of the SWWB result in large changes in precipitation seasonality. Our 1400 yr record is based on sedimentological and geochemical data obtained on a sediment core collected in front of a small river that drains the Patagonian Andes, which makes this site particularly sensitive to changes in river discharge. Our results indicate Fe/Al and Ti/Al values that are low between 600 and 1200 CE, increasing at 1200-1500 CE, and high between 1500 and 1950 CE. Increasing Fe/Al and Ti/Al values reflect a decrease in mean sediment grain-size from 30 to 20 μm, which is interpreted as a decrease in seasonal floods resulting from an equatorward shift of the SWWB. Our results suggest that, compared to present-day conditions, the SWWB was located in a more poleward position before 1200 CE. It gradually shifted towards the equator in 1200-1500 CE, where it remained in a sustained position until 1950 CE. This pattern is consistent with most precipitation records from central and southern Chile. The comparison of our record with published regional sea surface temperature (SST) reconstructions for the late Holocene shows that equatorward shifts of the SWWB are systematically coeval with decreasing SSTs and vice versa, which resembles fluctuations over glacial-interglacial timescales. We argue that the synchronicity between SST and SWWB changes during the last 1400 years represents the response of the SWWB to temperature changes in the Southern Hemisphere.
... Under this scenario, the PUE should present higher productivity during the NH warm periods (RWP and MCA) compared to the NH cool periods (DACP and LIA). On the other hand, the southern range of the SPSH also experienced important meridional displacements during the last two millennia as inferred from the meridional changes in the position of the Southern Westerlies (Lamy et al., 2001;Mohtadi et al., 2007;Moy et al., 2009;Koffman et al., 2013), suggesting changes in the SPSH expansion/contraction. According to this idea, the SPSH contracted during the cold periods (DACP and LIA) and expanded during the RWP and part of the MCA (Lamy et al., 2001). ...
The Tropical Pacific ocean-atmosphere system influences global climate
on interannual, decadal, as well as at longer timescales. Given the
uncertainties in the response of the Tropical Pacific to the ongoing
greenhouse effect, it is important to assess the natural range of the
Tropical Pacific climate variability in response to global natural
changes, and to understand the underlying mechanisms. The Peruvian
Upwelling Ecosystem (PUE) represents an ideal area to reconstruct past
changes in ocean-atmosphere systems because productivity and subsurface
oxygenation are strongly linked to changes in the strength of the Walker
circulation. Throughout the last 2000 yr, warmer (the Roman Warm Period
[RWP], the Medieval Climate Anomaly [MCA] and the Current Warm Period
[CWP]), and colder (the Dark Ages Cold Period [DACP] and Little Ice Age
[LIA]) intervals occurred with considerable changes around the globe. In
order to reconstruct the PUE response to these climatic periods and
reveal the underlying mechanisms, we use a multi-proxy approach
including organic and inorganic proxies in finely laminated sediments
retrieved off Pisco (~ 14° S), Peru. Our results indicate that the
PUE exhibited a La Niña-like mean state during the warm periods,
characterized by an intense OMZ and high marine productivity. During
cold periods the PUE exhibited an El Niño-like mean state,
characterized by a weak OMZ and low marine productivity. Comparing our
results with other relevant paleoclimatic reconstructions revealed that
changes in the strength of the Walker circulation and the
expansion/contraction of the South Pacific Sub-tropical High controlled
productivity and subsurface oxygenation in the PUE during the last two
millennia. This indicate that large scale circulation changes are the
driving forces in maintaining productivity and subsurface oxygenation
off Peru at centennial time scales during the past two millennia.
... According to this, the SPSH contracted during the cold periods (DACP and LIA) and expanded during the RWP and part of the MCA (Lamy et al., 2001). In addition, a dust particle dataset from the West Antarctica Ice Sheet also indicates that the southern Westerlies were stronger and occupied a more southerly position during part of the MCA (∼ 1050 to 1430 AD), and that 10 the Westerlies were weaker and occupied a more equatorward position during almost the entire LIA (1430-1950 AD; Koffman et al., 2013). ...
The Tropical Pacific ocean-atmosphere system influences global climate on interannual, decadal, as well as at longer timescales. Given the uncertainties in the response of the Tropical Pacific to the ongoing greenhouse effect, it is important to assess the natural range of the Tropical Pacific climate variability in response to global natural changes, and to understand the underlying mechanisms. The Peruvian Upwelling Ecosystem (PUE) represents an ideal area to reconstruct past changes in ocean-atmosphere systems because productivity and subsurface oxygenation are strongly linked to changes in the strength of the Walker circulation. Throughout the last 2000 yr, warmer (the Roman Warm Period [RWP], the Medieval Climate Anomaly [MCA] and the Current Warm Period [CWP]), and colder (the Dark Ages Cold Period [DACP] and Little Ice Age [LIA]) intervals occurred with considerable changes around the globe. In order to reconstruct the PUE response to these climatic periods and reveal the underlying mechanisms, we use a multi-proxy approach including organic and inorganic proxies in finely laminated sediments retrieved off Pisco (~ 14° S), Peru. Our results indicate that the PUE exhibited a La Niña-like mean state during the warm periods, characterized by an intense OMZ and high marine productivity. During cold periods the PUE exhibited an El Niño-like mean state, characterized by a weak OMZ and low marine productivity. Comparing our results with other relevant paleoclimatic reconstructions revealed that changes in the strength of the Walker circulation and the expansion/contraction of the South Pacific Sub-tropical High controlled productivity and subsurface oxygenation in the PUE during the last two millennia. This indicate that large scale circulation changes are the driving forces in maintaining productivity and subsurface oxygenation off Peru at centennial time scales during the past two millennia.
