No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
The parameterisation of Mediterranean–Atlantic water exchange in the Hadley Centre model HadCM3, and its effect on modelled North Atlantic climate
Abstract
Multiple palaeo-proxy and modelling studies suggest that Mediterranean Outflow Water (MOW) is an important driver of Atlantic Meridional Overturning Circulation (AMOC), particularly during periods of weak overturning. Here, we employ the HadCM3 ocean–atmosphere General Circulation Model (GCM) to investigate the effect of using different parameterisations of Mediterranean–Atlantic water exchange on global ocean circulation and climate. In HadCM3, simulating flow through the Gibraltar Straits with an ‘open seaway’ rather than a ‘diffusive pipe’ causes a shoaling and strengthening of the MOW plume. This reorganises shallow Atlantic circulation, producing regional surface air temperature anomalies of up to + 11 °C and −7.5 °C. We conclude that when investigating the influence of MOW on modelled ocean circulation and climate, an accurate parameterisation of Mediterranean–Atlantic exchange is important and should match observed fresh water and salinity flux constraints. This probably cannot
... 20 A more likely scenario, consistent with both model results (e.g. Gladstone et al., 2007;Meijer and Krijgsman, 2005;Meijer, 2006Meijer, , 2012 and data (Abouchami et al., 1999;Ivanovic et al., 2013a;Muiños et al., 2008) is that the Mediterranean was often connected to the Atlantic during MSC hyper-and hypo-salinity, particularly during episodes of gypsum formation and near-freshening, with at least periodic Mediter-Introduction to determine the climate variables and geographical regions that are most susceptible to MSC-influenced climate changes. To this end, we here present a series of fullycoupled atmosphere-ocean GCM simulations, which assess Messinian climate sensitivity to extreme end-member changes in MOW that may have occurred during the MSC. ...
... Also, the AMOC is completely changed south of the CAS, with strong Southern Ocean sources, due to the opened exchange with the Pacific (Fig. 5a). 5 Whether or not the Mediterranean ever fully or partially desiccated during the MSC, it seems likely that, at least at times, there was no outflow from the Mediterranean to the Atlantic (e.g. van Assen et al., 2006;Benammi et al., 1996;Betzler et al., 2006;Hüsing et al., 2010;Ivanovic et al., 2013a;Krijgsman et al., 1999b). Although blocking MOW in a modern HadCM3 simulation had little impact on North Atlantic ocean circulation and climate (Ivanovic et al., 2013c), consistent with other similar GCM simulations (Chan and Motoi, 2003;Kahana, 2005;Rahmstorf, 1998), there is considerable model and proxy evidence to suggest that it has the potential to play a more important role during periods of weaker AMOC (e.g. ...
... Other evidence (incl. Abouchami et al., 1999;Gladstone et al., 25 2007;Ivanovic et al., 2013a;Meijer and Krijgsman, 2005;Meijer, 2006Meijer, , 2012Muiños et al., 2008) indicates that at least episodic bursts of MOW may well have occurred during gypsum saturation and brackish water conditions. Nonetheless, we have tested all three scenarios on the basis that none can yet be disproved; the volume of evap- (2012) shows that a gateway has to be extremely shallow before outflow is blocked. ...
Late Miocene tectonic changes in Mediterranean-Atlantic connectivity and
climatic changes caused Mediterranean salinity to fluctuate
dramatically, including a ten-fold increase and near-freshening. Recent
proxy- and model-based evidence suggests that at times during this
Messinian Salinity Crisis (MSC, 5.96-5.33 Ma), highly-saline and
highly-fresh Mediterranean water flowed into the North Atlantic Ocean,
whilst at others, no Mediterranean Outflow Water (MOW) reached the
Atlantic. By running extreme, sensitivity-type experiments with a
fully-coupled ocean-atmosphere general circulation model, we investigate
the potential of these various MSC MOW scenarios to impact global-scale
climate. The simulations suggest that MOW had a greater
influence on North Atlantic Ocean circulation and climate than it does
today. We also find that depending on the presence, strength and
salinity of MOW, the MSC could have been capable of cooling mid-high
northern latitudes by more than 1.2 °C, with the greatest cooling
taking place in the Labrador, Greenland-Iceland-Norwegian and Barents
Seas. With hypersaline-MOW, a component of North Atlantic Deep Water
formation shifts to the Mediterranean, strengthening the Atlantic
Meridional Overturning Circulation (AMOC) south of 35° N by 3-7 Sv.
With hyposaline-MOW, AMOC completely shuts down, inducing a bipolar
climate anomaly with strong cooling in the North (up to -10.5 °C)
and weaker warming in the South (up to +2.5 °C). These
simulations identify key target regions and climate variables for future
proxy-reconstructions to provide the best and most robust test cases for
(a) assessing Messinian model performance, (b) evaluating
Mediterranean-Atlantic connectivity during the MSC and (c) establishing
whether or not the MSC could ever have affected global-scale climate.
... The impact of regional changes on global-scale processes are generally ideal questions for the Earth System Models theme. However, because of the inherent small scale of marine gateways relative to global circulation model grid cells, the gateways are either hugely enlarged in the model or the transport of heat and water through them is parameterized rather than explicitly modeled (e.g., Dietrich et al., 2008;Ivanovic et al., 2013). An excellent example of the problem occurs at Gibraltar (Figure F1), where model grid cells of ~400 km 2 , which are suitable for the long global simulations necessary for paleoclimate studies, are ill-equipped to simulate hydraulic control in a strait ~15 km in width and consequently generate exchange behavior that differs from observations (Ivanovic et al., 2013;Alhammoud et al., 2010). ...
... However, because of the inherent small scale of marine gateways relative to global circulation model grid cells, the gateways are either hugely enlarged in the model or the transport of heat and water through them is parameterized rather than explicitly modeled (e.g., Dietrich et al., 2008;Ivanovic et al., 2013). An excellent example of the problem occurs at Gibraltar (Figure F1), where model grid cells of ~400 km 2 , which are suitable for the long global simulations necessary for paleoclimate studies, are ill-equipped to simulate hydraulic control in a strait ~15 km in width and consequently generate exchange behavior that differs from observations (Ivanovic et al., 2013;Alhammoud et al., 2010). Consequently, the codependence of ocean and marginal sea in simulations is reduced, preconditioning models to be insensitive to exchange ...
... The impact of regional changes on global-scale processes are generally ideal questions for the Earth System Models theme. However, because of the inherent small-scale of marine gateways relative to global circulation model grid cells, the gateways are either hugely enlarged in the model or the transport of heat and water through them is parameterized rather than explicitly modeled (e.g., Dietrich et al., 2008;Ivanovic et al., 2013). An excellent example of the problem occurs at Gibraltar (Figure F1), where model grid cells of ~400 km 2 , which are suitable for the long global simulations necessary for paleoclimate studies, are ill-equipped to simulate hydraulic control in a strait ~12 km in width and consequently generate exchange behavior which differs from observations (Ivanovic et al., 2013;Alhammoud et al., 2010). ...
... However, because of the inherent small-scale of marine gateways relative to global circulation model grid cells, the gateways are either hugely enlarged in the model or the transport of heat and water through them is parameterized rather than explicitly modeled (e.g., Dietrich et al., 2008;Ivanovic et al., 2013). An excellent example of the problem occurs at Gibraltar (Figure F1), where model grid cells of ~400 km 2 , which are suitable for the long global simulations necessary for paleoclimate studies, are ill-equipped to simulate hydraulic control in a strait ~12 km in width and consequently generate exchange behavior which differs from observations (Ivanovic et al., 2013;Alhammoud et al., 2010). Consequently, the codependence of ocean and marginal sea in simulations is reduced, preconditioning models to be insensitive to exchangedriven change. ...
... The hydrodynamics of this basin are controlled by wind forcing, heat and freshwater fluxes at the surface, North Atlantic water inflow through the Straits of Gibraltar, and the outflow at depth of Mediterranean water (e.g., Bryden and Kinder, 1991;Tsimplis and Bryden, 2000;García-Lafuente et al., 2011;Struglia et al., 2004). A link with the Mediterranean overflow water (MOW), which might impact on the Atlantic Meridional Overturning Circulation (AMOC), has been proposed (Cacho et al., 1999(Cacho et al., , 2000(Cacho et al., , 2001Bigg and Wadley, 2001;Sierro et al., 2005;Voelker et al., 2006;Penaud et al., 2011;Ivanovic et al., 2013;Bahr et al., 2015). In particular, it has been suggested that the variations in the thermohaline circulation of the Mediterranean Sea, and in consequence the intensity of the MOW, could play a significant role in the modulation of the AMOC through the injection of saline Mediterranean waters into the intermediate North Atlantic (Rogerson et al., 2006;Voelker et al., 2006;Khélifi et al., 2009;Swingedouw et al., 2019). ...
... More recently Swingedouw et al. (2019) investigated the impact of a freshwater release in the Mediterranean Sea on North Atlantic circulation, with a specific focus on the AMOC. They argued that the parameterization of the MOW in the IPSL-CM5 model (used in this study) is a significant improvement as compared to previous studies that analysed the potential impact of the MOW using simpler and coarser ocean models (e.g., Rahmstorf, 1998;Bigg and Wadley, 2001;Chan and Motoi, 2003;Rogerson et al., 2010;Ivanovic et al., 2013). ...
Palaeo-oceanographical archives in the Mediterranean Sea indicate the occurrence of Sapropel depositions, which are characteristic of anoxic bottom conditions and of a stratified sea. The impact of such drastic changes in the thermohaline circulation of the Mediterranean Sea on the large-scale circulations in the North Atlantic basin is not fully understood. Here we evaluate the impact of a direct perturbation of the Mediterranean Sea circulation through different idealized simulations with freshwater release rates of 20, 50, 100, and 200 mSv (1 mSv = 10³ m³/s), using a the IPSL-CM5A-LR global coupled atmosphere-ocean model, with simulations up to 1000 years long. The simulations are compared to available data for the Mediterranean Sea and the North Atlantic. The Neodymium isotopic composition (εNd) anomalies in the North Atlantic are clearly dependent on the rate of fresh water release in the Mediterranean Sea, and subsequent modification of Atlantic meridional overturning circulation (AMOC) intensity and subpolar gyre (SPG) extension. Based on comparison with seawater εNd records from Rockall Trough, we suggest that a limited release of freshwater around 50 mSv may be more representative of hydrological conditions of the Mediterranean Sea during the Sapropel S1 deposition. Our results indicate that a decrease in Mediterranean overflow was clearly a factor in the intensification of upper AMOC following the Early Holocene period, when a large eastward expansion of the subpolar gyre is simulated. Also, eastward expansion of the subpolar gyre due to freshwater release in the Mediterranean (e.g. Sapropel-like events) and its impact on the εNd signature highlight the fact that the modification of ocean circulation in the North Atlantic basin might be more complex than a straightforward change in the AMOC, since there may also be a large impact on barotropic circulation. The simulations also produce a significant decrease in the εNd values of the Iceland-Scotland Overflow Water (between −3.5 and − 1.3 εNd unit), which is consistent with changes observed during the S1 event in the reconstructions. Thus the 3-dimensional changes in baroclinic and barotropic circulations of the ocean, as reconstructed in some εNd records, are broadly consistent with the sensitivity simulations performed, and might thus be partly related to changes in Mediterranean overflow, which would have contributed to modification of the Atlantic Ocean circulation following a complex 3-dimensional pattern that is detailed in the paper.
... The model has also been used in several other pre-Quaternary palaeoclimate studies, both for the late Miocene (Bradshaw et al., 2012a(Bradshaw et al., , 2015 and Eocene (Loptson et al., 2014;Lunt et al., 2010;Tindall et al., 2010). Other late Miocene simulations have also been carried out running the higher-resolution-ocean (1.25 • latitude and longitude) version of the model (Ivanovic et al., 2013(Ivanovic et al., , 2014a. However, here we used the lower-resolution and more computationally efficient HadCM3L because of the availability of an existing 2000-year spin-up, for consistency with the Bradshaw et al. (2012a) study, and because of the number of simulations conducted in the ensemble. ...
... In the main ensemble of 22 orbital simulations, the Strait of Gibraltar has been kept closed in order to simulate the significantly reduced or missing Mediterranean-Atlantic exchange of the latest Messinian. Given the shortcomings in the parameterisation of Mediterranean-Atlantic exchange for the HadCM3 model (Ivanovic et al., 2013), we decided to close the gateway as neither solution (open or close) would have been entirely realistic. The Mediterranean-Atlantic connection was not yet fully restricted during our simulated time period, but nonetheless it was different compared to today's exchange through the Strait of Gibraltar. ...
Orbital forcing is a key climate driver over multi-millennial timescales. In
particular, monsoon systems are thought to be driven by orbital cyclicity,
especially by precession. Here, we analyse the impact of orbital forcing on
global climate with a particular focus on the North African monsoon, by
carrying out an ensemble of 22 equally spaced (one every 1000 years)
atmosphere–ocean–vegetation simulations using the HadCM3L model, covering one
full late Miocene precession-driven insolation cycle with varying obliquity
(between 6.568 and 6.589 Ma). The simulations only differ in their prescribed
orbital parameters, which vary realistically for the selected time period. We
have also carried out two modern-orbit control experiments, one with late
Miocene and one with present-day palaeogeography, and two additional
sensitivity experiments for the orbital extremes with varying
CO2 forcing. Our results highlight the high sensitivity of
the North African summer monsoon to orbital forcing, with strongly
intensified precipitation during the precession minimum, leading to a
northward penetration of vegetation up to ~ 21° N. The
modelled summer monsoon is also moderately sensitive to palaeogeography
changes, but it has a low sensitivity to atmospheric CO2
concentration between 280 and 400 ppm. Our simulations allow us to explore
the climatic response to orbital forcing not only for the precession
extremes but also on sub-precessional timescales. We demonstrate the
importance of including orbital variability in model–data comparison studies,
because doing so partially reduces the mismatch between the late Miocene
terrestrial proxy record and model results. Failure to include orbital
variability could also lead to significant miscorrelations in
temperature-based proxy reconstructions for this time period, because of the
asynchronicity between maximum (minimum) surface air temperatures and minimum
(maximum) precession in several areas around the globe. This is of particular
relevance for the North African regions, which have previously been
identified as optimal areas to target for late Miocene palaeodata
acquisition.
... The dimensions of the Straits of Gibraltar (nearly 60 km long, 12 km at its narrowest point, and with a maximum depth of 300 m; Candela et al., 1990) make realistically simulating the thin, dense current spilling over the sill (the MO) in numerical models very challenging (Dietrich et al., 2008). In models with a relatively coarse resolution where the model grid cannot resolve the features of the shallow, narrow strait, the exchange is simulated either using a less realistic wider and deeper open seaway (Ivanović et al., 2013b;Rogerson et al., 2010;Bigg and Wadley, 2001), or using an empirical parameterisation of the exchange (Ivanović et al., 2013b;Wu et al., 2007;Chan and Motoi, 2003;Rahmstorf, 1998). Clearly, the same resolution problem also applies to both the Late Miocene Mediterranean-Atlantic seaways. ...
... The dimensions of the Straits of Gibraltar (nearly 60 km long, 12 km at its narrowest point, and with a maximum depth of 300 m; Candela et al., 1990) make realistically simulating the thin, dense current spilling over the sill (the MO) in numerical models very challenging (Dietrich et al., 2008). In models with a relatively coarse resolution where the model grid cannot resolve the features of the shallow, narrow strait, the exchange is simulated either using a less realistic wider and deeper open seaway (Ivanović et al., 2013b;Rogerson et al., 2010;Bigg and Wadley, 2001), or using an empirical parameterisation of the exchange (Ivanović et al., 2013b;Wu et al., 2007;Chan and Motoi, 2003;Rahmstorf, 1998). Clearly, the same resolution problem also applies to both the Late Miocene Mediterranean-Atlantic seaways. ...
... The dimensions of the Straits of Gibraltar (nearly 60 km long, 12 km at its narrowest point, and with a maximum depth of 300 m; Candela et al., 1990) make realistically simulating the thin, dense current spilling over the sill (the MO) in numerical models very challenging (Dietrich et al., 2008). In models with a relatively coarse resolution where the model grid cannot resolve the features of the shallow, narrow strait, the exchange is simulated either using a less realistic wider and deeper open seaway (Ivanović et al., 2013b;Rogerson et al., 2010;Bigg and Wadley, 2001), or using an empirical parameterisation of the exchange (Ivanović et al., 2013b;Wu et al., 2007;Chan and Motoi, 2003;Rahmstorf, 1998). Clearly, the same resolution problem also applies to both the Late Miocene Mediterranean-Atlantic seaways. ...
... The dimensions of the Straits of Gibraltar (nearly 60 km long, 12 km at its narrowest point, and with a maximum depth of 300 m; Candela et al., 1990) make realistically simulating the thin, dense current spilling over the sill (the MO) in numerical models very challenging (Dietrich et al., 2008). In models with a relatively coarse resolution where the model grid cannot resolve the features of the shallow, narrow strait, the exchange is simulated either using a less realistic wider and deeper open seaway (Ivanović et al., 2013b;Rogerson et al., 2010;Bigg and Wadley, 2001), or using an empirical parameterisation of the exchange (Ivanović et al., 2013b;Wu et al., 2007;Chan and Motoi, 2003;Rahmstorf, 1998). Clearly, the same resolution problem also applies to both the Late Miocene Mediterranean-Atlantic seaways. ...
Marine gateways play a critical role in the exchange of water, heat, salt and nutrients between oceans and seas. As a result, changes in gateway geometry can significantly alter both the pattern of global ocean circulation and associated heat transport and climate, as well as having a profound impact on local environmental conditions. Mediterranean-Atlantic marine corridors that pre-date the modern Gibraltar Strait, closed during the Late Miocene and are now exposed on land in northern Morocco and southern Spain. The restriction and closure of these Miocene connections resulted in extreme salinity fluctuations in the Mediterranean, leading to the precipitation of thick evaporites. This event is known as the Messinian Salinity Crisis (MSC). The evolution and closure of the Mediterranean-Atlantic gateways are a critical control on the MSC, but at present the location, geometry and age of these gateways are still highly controversial, as is the impact of changing Mediterranean outflow on Northern Hemisphere circulation. Here, we present a comprehensive overview of the evolution of the Late Miocene gateways and the nature of Mediterranean-Atlantic exchange as deduced from published studies focussed both on the sediments preserved within the fossil corridors and inferences that can be derived from data in the adjacent basins. We also consider the possible impact of evolving exchange on both the Mediterranean and global climate and highlight the main enduring challenges for reconstructing past Mediterranean-Atlantic exchange.
... The model has also been used in several other pre-Quaternary palaeoclimate studies, both for the late Miocene (Bradshaw et al., 2012a(Bradshaw et al., , 2015 and Eocene (Loptson et al., 2014;Lunt et al., 2010;Tindall et al., 2010). Other late Miocene simulations have also been carried out running the higher-resolution-ocean (1.25 • latitude and longitude) version of the model (Ivanovic et al., 2013(Ivanovic et al., , 2014a. However, here we used the lower-resolution and more computationally efficient HadCM3L because of the availability of an existing 2000-year spin-up, for consistency with the Bradshaw et al. (2012a) study, and because of the number of simulations conducted in the ensemble. ...
... In the main ensemble of 22 orbital simulations, the Strait of Gibraltar has been kept closed in order to simulate the significantly reduced or missing Mediterranean-Atlantic exchange of the latest Messinian. Given the shortcomings in the parameterisation of Mediterranean-Atlantic exchange for the HadCM3 model (Ivanovic et al., 2013), we decided to close the gateway as neither solution (open or close) would have been entirely realistic. The Mediterranean-Atlantic connection was not yet fully restricted during our simulated time period, but nonetheless it was different compared to today's exchange through the Strait of Gibraltar. ...
Orbital forcing is a key climate driver over multi-millennial timescales. In particular, monsoon systems are thought to be driven by orbital cyclicity, especially by precession. Here we analyse the impact of orbital forcing on global climate with a particular focus on the North African monsoon, by carrying out a ensemble of 22 atmosphere-ocean-vegetation simulations, equally-spaced in time and covering one full late Miocene precession cycle (~ 6.5 Ma). Orbital parameters vary realistically for the selected time slice. Our results highlight the high sensitivity of the North African summer monsoon to orbital forcing, with strongly intensified precipitation during the precession minimum, leading to a northward penetration of vegetation up to ~ 21° N. The summer monsoon is also moderately sensitive to palaeogeography changes, but has a low sensitivity to atmospheric CO2 levels between 280 and 400 ppm. Our ensemble of simulations allows us to explore the climatic response to orbital forcing not only for the precession extremes, but also on sub-precessional timescales. We demonstrate the importance of including orbital variability in model-data comparison studies, because doing so partially reduces the mismatch between the late Miocene terrestrial proxy record and model results. Failure to include orbital variability could also lead to significant miscorrelations in temperature-based proxy reconstructions for this time period, because of the asynchronicity between maximum (minimum) surface air temperatures and minimum (maximum) precession in several areas around the globe. This is of particular relevance for the North African regions, which have previously been identified as optimal areas to target for late Miocene palaeodata acquisition.
... In particular, even though the ETS is shallower in our simulation than in Hamon et al. (2013), the flux through Gibraltar toward the Atlantic Ocean is higher (4 vs. 2.8 Sv in Hamon et al. (2013)) because the Gibraltar gateway is deeper in our paleogeography (1,350 m vs. 400 m in Hamon et al. (2013)). We note incidentally that changes in the Gibraltar gateway configuration have been suggested to impact the AMOC later in the Miocene (Capella et al., 2019;Flecker et al., 2015;Ivanovic et al., 2013;Ng et al., 2021). In contrast, the modeling study of Z. Zhang et al. (2011) suggests that the Tethys seaway closure was key to the evolution of NADW. ...
The Modern Ocean is characterized by the formation of deep‐water in the North Atlantic Ocean (i.e., NADW). This feature has been attributed to the modern geography, in which the Atlantic Ocean is a large basin extending from northern polar latitudes to the Southern Ocean, the latter enabling the connection of the otherwise isolated Atlantic with the Pacific and Indian Oceans. Sedimentary data date the establishment of the NADW between the beginning of the Eocene (∼49 Ma) and the beginning of the Miocene (∼23 Ma). The objective of this study is to quantify the impact of Miocene geography (∼20 Ma) on NADW using new simulations performed with the Earth System Model IPSL‐CM5A2. We specifically focus on the closure of the Eastern Tethys Seaway (ETS), dated between 22 and 14 Ma, which allowed the connection between the Atlantic and Indian Oceans, and on the Greenland Ice Sheet, whose earliest onset remains open to discussion but for which evidence suggest a possible existence as early as the Eocene. Our results show that the closure of the ETS does not appear to impact the establishment of NADW, because waters from the Indian Ocean do not reach the NADW formation zone when the seaway is open. Conversely, the existence of an ice sheet over Greenland strengthens the formation of NADW owing to topography induced changes in wind patterns over the North Atlantic, which in turn, results in a salinification of the North Atlantic and Nordic Seas, and in an increase in deep‐water formation.
... Model simulations suggest that the AMOC is significantly influenced by the contribution of the MOW (Reid, 1979;Rahmstorf, 1998;Ivanovic et al., 2013;Wu et al., 2007) but the effects of Mediterranean water masses on AMOC remain unclear (Swingedouw et al., 2018). Indeed, while a strengthening of MOW does not influence North Atlantic Deep Water production (NADW; Khélifi et al., 2014), it stimulates the shallow branch of AMOC Sarnthein et al., 2018). ...
High-resolution records from International Ocean Discovery Program (IODP) sites U1386 and U1387 drilled during IODP Expedition 339 into the Faro Drift, made it possible to assess the impact of intensifications of the upper core (MOWU) of the Mediterranean Outflow Water (MOW) and of changes in sediment supply on the sedimentation in the northern Gulf of Cadíz since the Middle Pleistocene. This work focuses on the comparison of records covering Marine Isotope Stage (MIS) 2–1 and MIS 12–11, in order to investigate the behaviour and circulation regime of the MOWU over two climatic cycles of similar astronomical configurations and their associated deglaciation. The analysis of facies established on the basis of grain-size, XRF core-scanning, and carbonate content revealed contourite beds formed by the MOWU during MIS 11 and MIS 1 and deglaciations (deglaciation V and I). Contourite sequences show that MOWU velocity at the seabed was higher during MIS 2–1 than during MIS 12–11, and that sediment supply was different between these two climatic cycles. While overall low during MIS 12–11, MOWU intensity increased during deglaciationV and MIS 11 and preceded large ice rafted events and cooling in the North Atlantic Ocean. As a major element of the MOW, MOWU strengthening during deglaciation V likely contributed to higher heat and moisture transport towards the high latitudes inducing a slight increase of calving and size of boreal ice sheets. The MOW-derived injection of heat and salt in the North Atlantic Ocean during deglaciation V might have contributed, through reactivation of the upper AMOC, to the switch of the Atlantic thermohaline circulation from a glacial to an interglacial mode.
... General circulation models are regularly used to reconstruct the nature of changing connectivity between oceans and seas. However, these models are not suited to simulate hydraulic controls of narrow gateways, generating results that differ from observations (Alhammoud et al., 2010;Ivanovic et al., 2013). To reconstruct the detailed timing and nature of changing connectivity, a geological record is required. ...
Paleoceanographic information from submarine overflows in the vicinity of oceanic gate-
ways is of major importance for resolving the role of ocean circulation in modulating Earth’s climate. Earth system models are currently the favored way to study the impact of gateways on global-scale processes, but studies on overflow-related deposits are more suitable to under- stand the detailed changes. Such deposits, however, had not yet been documented in outcrop. Here, we present a unique late Miocene contourite channel system from the Rifian Corridor (Morocco) related to the initiation of Mediterranean Outflow Water (MOW). Two channel branches were identified consisting of three vertically stacked channelized sandstone units encased in muddy deposits. Both branches have different channel-fill characteristics. Our findings provide strong evidence for intermittent behavior of overflow controlled by tectonic processes and regional climatic change. These fluctuations in paleo-MOW intermittently influenced global ocean circulation.
... We argue here that this parameterization of the MOW is a significant improvement compared to previous studies that analysed the potential impact on the MOW using simpler and coarser ocean models (e.g. Rahmstorf 1998;Bigg and Wadley 2001;Chan and Motoi 2003;Wu et al. 2007;Rogerson et al. 2010;Ivanovic et al. 2013). The parametrisation is based on present-day observations of the MOW, but is not entirely tuned towards these conditions, since the flow remains free to evolve in response to changes in deep water masses in the Mediterranean catchment. ...
Sediment cores from the Mediterranean Sea have evidenced several periods of Sapropel deposition, which can be explained by events of anoxic bottom conditions. An explanation for such events calls for a very stratified sea, possibly related with freshwater input through increased precipitations and runoff discharges. It has been suggested that such a stratified Mediterranean Sea may in turn weaken the Atlantic meridional overturning circulation (AMOC) through changes in the Mediterranean overflow water (MOW). Nevertheless, models used to establish this result were relatively simple and the mechanisms leading to such an impact remained elusive. To improve on those previous studies, we analyse the impact of different freshwater releases with rates of 0.2, 0.1, 0.05 and 0.02 Sv (1 Sv = 10⁶ m³/s) in the Mediterranean Sea using the IPSL-CM5A-LR model in a few multi-centennial simulations. We focus the analysis on the impact of a decrease in the MOW on the large-scale Atlantic circulation. We find a consistent change in horizontal currents in the upper Atlantic Ocean in all simulations in the first century, with a large enhancement of the northward current west of Rockall in the northeast Atlantic. Concerning the AMOC response, we identify three different processes that impact its fate. The first is related to changes in geostrophic currents at depth induced by the disappearance of the MOW, which tends to weaken the AMOC. On the contrary, the second enhances the AMOC and is associated with the increase in northward currents in the horizontal upper circulation. The last process is due to the spread of surface freshwater anomalies out of the Mediterranean Sea that freshens the North Atlantic convection sites and weakens the AMOC. Depending on the rate of the freshwater release, the strength and balance of these three processes are different. For rates larger than 0.05 Sv, we observe a strong reduction of the AMOC, while for lower rates, we notice an enhancement in the upper cell. The climatic response follows that of the upper AMOC with a warming of the North Atlantic for rates lower than 0.05 Sv and a cooling for higher rates. Given that past estimates of freshwater release in the Mediterranean Sea indicate rates lower than 0.05 Sv, we argue that Sapropel events may have enhanced of the upper AMOC and warmed of the North Atlantic.
... Atlantic-Mediterranean exchange is also a critical component of Atlantic Meridional Overturning Circulation (AMOC) particularly at times of weak North Atlantic Deep Water formation 26,[28][29][30][31][32] , as a consequence of dense, salty water being transported from Gibraltar into the high latitudes. Given the weaker AMOC during the late Miocene 33,34 , the relative contribution of Mediterranean density to late Miocene deep-water formation is likely to have been greater than today 10 . Furthermore, in entraining ambient Atlantic water, the transport of dense Mediterranean Overflow into the Atlantic interior also transfers 0.06 GtC yr 1 of anthropogenic carbon from the ocean surface to intermediate depths 35 , contributing a 2-5% of today's total net ocean carbon sink [36][37][38] . ...
A global Neogene cooling trend culminated ~7 million years ago with the onset of Greenland glaciation. Increased ocean-atmosphere interaction and low- to high-latitude circulation are thought to be key factors in reorganizing late Miocene global temperature and precipitation patterns, but the drivers of this reorganization have yet to be identified. Here, we present new information about the evolution of the Atlantic-Mediterranean gateway that generated Mediterranean overflow. We use sedimentary and palaeogeographic evidence to constrain the timing and dimensions of this gateway and document the initiation of a saline plume of water within the North Atlantic. Today, this saline jet entrains and transports Eastern North Atlantic water and its dissolved inorganic carbon into the interior of the ocean, contributing to the drawdown of CO2 and the sensitivity of the ocean to atmospheric changes. We show that during the Miocene this transport emerged simultaneously with gateway restriction and propose that the resulting interaction of ocean-surface and ocean-interior carbon inventories would have greatly enhanced ocean-atmosphere exchange, preconditioning the Earth System for late Miocene cooling.
... Papadakis et al. (2003) demonstrated that a shallower Mediterranean undercurrent produces higher meddy activity and Artale et al. (2003) concluded that the vertical position of MW could have an impact on the structure of AMOC. However Ivanovic et al. (2013) showed that for a proper AMOC simulation it is necessary to attain a MOW plume centered at 1000 m. As long as the MOW depth is similar in both experiments and around 1100 m this misrepresentation will have no impact on our evaluation of the role of tides in MOW spreading. ...
The impact of tides on the spreading of the Mediterranean Outflow Waters (MOW) in the Gulf of Cadiz is investigated through a series of targeted numerical experiments using an ocean general circulation model. The full ephimeridic luni-solar tidal potential is included as forcing. The model grid is global with a strong zoom around the Iberian Peninsula. Thus, the interaction of processes of different space and time scales, which are involved in the MOW spreading, is enabled. This is of particular importance in the Strait of Gibraltar and the Gulf of Cádiz, where the width of the MOW plume is a few tens of km. The experiment with enabled tides successfully simulates the main tidal features of the North Atlantic and in the Gulf of Cádiz and the Strait of Gibraltar. The comparison of the fields from simulations with and without tidal forcing shows drastically different MOW pathways in the Gulf of Cádiz: The experiment without tides shows an excessive southwestward spreading of Mediterranean Waters along the North African slope, whereas the run with tides is closer to climatology. A detailed analysis indicates that tidal residual currents in the Gulf of Cádiz are the main cause for these differences.
... Grey shading is used over areas with less than 95% statistically significant changes using a welch t-test with a sample size of 100 corresponding to the duration of the climatological means and standard deviation calculations in ocean heat transport caused by a feedback between sea ice and circulation in the Barents Sea (e.g. Lehner et al. 2013;Semenov et al. 2009), it is worth noting that in HadCM3, this region is particularly sensitive to climate forcing under warm pre-industrial or interglacial conditions (Ivanovic et al. 2013) and the sensitivity of this response may be model dependent. ...
The Laurentide ice sheet, which covered Canada during glacial periods, had a major influence on atmospheric circulation and surface climate, but its role in climate during the early Holocene (9–7 ka), when it was thinner and confined around Hudson Bay, is unclear. It has been suggested that the demise of the ice sheet played a role in the 8.2 ka event (an abrupt 1–3 °C Northern Hemisphere cooling lasting ~ 160 years) through the influence of changing topography on atmospheric circulation. To test this hypothesis, and to investigate the broader implications of changing ice sheet topography for climate, we analyse a set of equilibrium climate simulations with ice sheet topographies taken at 500 year intervals from 9.5 to 8.0 ka. Between 9.5 and 8.0 ka, our simulations show a 2 °C cooling south of Iceland and a 1 °C warming between 40° and 50°N in the North Atlantic. These surface temperature changes are associated with a weakening of the subtropical and subpolar gyres caused by a decreasing wind stress curl over the mid-North Atlantic as the ice sheet lowers. The climate response is strongest during the period of peak ice volume change (9.5–8.5 ka), but becomes negligible after 8.5 ka. The climatic effects of the Laurentide ice sheet lowering during the Holocene are restricted to the North Atlantic sector. Thus, topographic forcing is unlikely to have played a major role in the 8.2 ka event and had only a small effect on Holocene climate change compared to the effects of changes in greenhouse gases, insolation and ice sheet meltwater.
... 10). If Blake Plateau hiatuses and MOW intensifications were roughly synchronous, then MOW flowing into the Caribbean Sea may have contributed to a strengthened Gulf Stream flow by way of increased salt-driven Atlantic Meridional Overturning Circulation in the North Atlantic (e.g., Ivanovic et al. 2013). At Site U1396 there are intervals of elevated carbonate deposition, very roughly coincident, with the Blake Plateau hiatuses and MOW intensifications. ...
Site U1396 was piston cored as a part of Integrated Ocean Drilling Project Expedition 340 to establish a long record for Lesser Antilles volcanism. A ∼150 m sediment succession was recovered from three holes on a bathymetric high ∼33 km southwest of Montserrat. A series of shipboard and newly-generated chronostratigraphic tools (biostratigraphy, magnetostratigraphy, astrochronology, and stable isotope chemostratigraphy) were employed to generate an integrated age model. Two possible chronostratigraphic interpretations for the Brunhes chron are presented, with hypotheses to explain the discrepancies seen between this study and Wall-Palmer et al. (2014). The recent Wade et al. (2011) planktic foraminiferal biostratigraphic calibration is tested, revealing good agreement between primary datums observed at Site U1396 and calibrated ages, but significant mismatches for some secondary datums. Sedimentation rates are calculated, both including and excluding the contribution of discrete volcanic sediment layers within the succession. Rates are found to be 'pulsed'or highly variable within the Pliocene interval, declining through the 1.5-2.4 Ma interval, and then lower through the Pleistocene. Different explanations for the trends in the sedimentation rates are discussed, including orbitally-forced biogenic production spikes, elevated contributions of cryptotephra (dispersed ash), and changes in bottom water sources and flow rates with increased winnowing in the area of Site U1396 into the Pleistocene.
... The Northeast Atlantic Ocean circulation is of paramount importance for the European climate and is intimately tied to the transport of heat and salt through the gyre and intergyre circulations and the Atlantic Meridional Overturning Circulation (AMOC), as revealed by modern hydrographical observations (Bower et al., 2002;Lavender et al., 2005) and model simulations (Ganopolski and Rahmstorf, 2001). In addition, the Mediterranean Sea Water (MSW) that spreads at mid-depth along the western European margin progressively mixes with central and eastern Atlantic water masses and thus contributes to the heat and salt budget of the Northeast Atlantic and possibly to the strength of the AMOC (Rogerson et al., 2006;Voelker et al., 2006;Khelifi et al., 2009;Ivanovic et al., 2013). It has been proposed that the northward penetration of MSW is linked to the strength of the middepth subpolar gyre (SPG) and that its eastward expansion is influenced by the North Atlantic Oscillation (NAO) (Lozier and Stewart, 2008). ...
Dissolved neodymium (Nd) isotopic composition (expressed as εNd) has been analysed for 82 seawater samples collected from 13 stations stretching from the Alboran Sea to the Iceland Basin. The distribution of the εNd values of water masses was thus investigated for the first time along the western European margin in order to explore whether the water masses flowing in the eastern subpolar and subtropical Atlantic reveal distinct isotopic patterns. The Modified Atlantic Water (MAW) in the Alboran Sea displays εNd values (between -9.2±0.2 and -8.9±0.2) that are significantly more radiogenic than those reported in previous studies (-10.8±0.2 to -9.7±0.2), suggesting temporal variations in the Nd isotopic composition of the water that enters the Mediterranean Sea from the Strait of Gibraltar. The εNd value of the underlying modified Winter Intermediate Water (WIW) has been established for the first time (-9.8±0.3) and is compatible with a Nd signature acquired from the sinking of MAW in the northwestern Mediterranean Sea. Within the Gulf of Cádiz, southern Mediterranean Sea Water (MSW) (-10.6±0.2) differs slightly from the northern MSW (-9.9±0.4) owing to a significant contribution of modified East Antarctic Intermediate Water (EAAIW) (-10.9±0.2). In the northeast Atlantic, the North Atlantic Current surface water located in the inter-gyre region (north of 46°N) displays εNd values of between -14.0±0.3 and -15.1±0.3, reflecting the subpolar gyre signature. Along the western European margin, εNd values of surface water decrease toward the north (from -10.4±1.6 to -13.7±1.0) in agreement with the gradual mixing between subtropical and subpolar water. At intermediate depth, εNd values decrease from -9.9±0.4 within the Gulf of Cádiz to -12.1±0.3 within the Porcupine Seabight, indicating a strong dilution of the MSW with subpolar water. Within the Rockall Trough and the Iceland Basin, the more negative εNd values at mid-depth (< -13.5±0.3) indicate that the MSW has no influence, even during periods of low NAO index. Water masses deeper than 1200 m in the northeast Atlantic are clearly influenced by the less radiogenic Labrador Sea Water (LSW) (-13.2±0.3 and -14.5±0.3) that mixes locally (εNd between -10.3±0.2 and -11.3±0.3) in the Iceland basin with the Iceland-Scotland Overflow Water (ISOW).
... Portugal and Moroccan upwelling centres) at long-term orbital timescales (glacial–interglacial cycles) as well as at millennial sub-orbital ones (i.e. the well-known Greenland Interstadial (GI)–Greenland Stadial (GS) cycles; Dansgaard et al., 1993; Grootes et al., 1993). In fact, it has been demonstrated that productivity changes in this region involve complex hydrographical dynamics, including upwelling (Abrantes, 1991Abrantes, , 1992) and river inputs, and are also probably additionally forced by Mediterranean–Atlantic exchanges (Rogerson et al., 2012; Ivanovic et al., 2013). Our study aims to explore how these changes may have impacted dinoflagellates, here viewed as an index planktonic group, so as to understand complex patterns and couplings of palaeohydrological and palaeo-productivity changes over the last 50 ky in the subtropical NE Atlantic. ...
New dinoflagellate cyst (dinocyst) analyses were carried out at high resolution in core MD99-2339, retrieved from a contouritic field in the central part of the Gulf of Cadiz, for the Marine Isotope Stage (MIS) 3 interval, allowing for discussion of palaeohydrological changes over the last 50 ky in the subtropical NE Atlantic Ocean. Some index dinocyst taxa, according to their (palaeo)ecological significance, shed light on significant sea-surface changes. Superimposed on the general decreasing pattern of dinocyst export to the seafloor over the last 50 ky, paralleling the general context of decreasing aeolian dust fertilization, a complex variability in dinocyst assemblages was detected at the millennial timescale. Enhanced fluvial discharges occurred during Greenland Interstadials (GIs), especially GI 1, 8 and 12, while enhanced upwelling cell dynamics were suggested during the Last Glacial Maximum and Heinrich Stadials. Finally, during the early Holocene, and more specifically during the Sapropel 1 interval (around 7-9 ka BP), we evidenced a strong decrease in dinocyst fluxes, which occurred synchronously to a strong reduction in Mediterranean Outflow Water strength and which we attributed to an advection of warm and nutrient-poor subtropical North Atlantic Central Waters. Over the last 50 ky, our study thus allows for capturing and documenting the fine tuning existing between terrestrial and marine realms in North Atlantic subtropical latitudes, in response to not only the regional climate pattern but also monsoonal forcing interfering during precession-driven Northern Hemisphere insolation maxima. This mechanism, well expressed during the Holocene, is superimposed on the pervasive role of the obliquity as a first major trigger for explaining migration of dinocyst productive centres in the NE Atlantic margin to the subtropical (temperate) latitudes during glacial (interglacial) periods.
... Consequently, Mediterranean eddies (meddies) and processes of North Atlantic entrainment in MOW are not directly simulated. Meddies are partially represented by µ in the Mediterranean-Atlantic exchange parameterisation, although overall, HadCM3 probably underestimates shallow-intermediate mixing of MOW with ambient NACW (Ivanovic et al., 2013b). North Atlantic entrainment, on the other hand, is represented by diffusive mixing of MOW with Atlantic water as it descends the continental shelf and spreads westwards. ...
Late Miocene tectonic changes in Mediterranean–Atlantic connectivity and
climatic changes caused Mediterranean salinity to fluctuate dramatically,
including a ten-fold increase and near-freshening. Recent proxy- and
model-based evidence suggests that at times during this Messinian
Salinity Crisis (MSC, 5.96–5.33 Ma), highly saline and highly fresh
Mediterranean water flowed into the North Atlantic Ocean, whilst at others,
no Mediterranean Outflow Water (MOW) reached the Atlantic. By running
extreme, sensitivity-type experiments with a fully coupled ocean–atmosphere
general circulation model, we investigate the potential of these various MSC
MOW scenarios to impact global-scale climate.
The simulations suggest that although the effect remains relatively small,
MOW had a greater influence on North Atlantic Ocean circulation and climate
than it does today. We also find that depending on the presence, strength and
salinity of MOW, the MSC could have been capable of cooling mid–high
northern latitudes by a few degrees, with the greatest cooling taking place
in the Labrador, Greenland–Iceland–Norwegian and Barents seas. With
hypersaline MOW, a component of North Atlantic Deep
Water formation shifts to the Mediterranean, strengthening the Atlantic
Meridional Overturning Circulation (AMOC) south of 35° N by
1.5–6 Sv. With hyposaline MOW, AMOC completely shuts down,
inducing a bipolar climate anomaly with strong cooling in the north (mainly
−1 to −3 °C, but up to −8 °C) and weaker warming in the
south (up to +0.5 to +2.7 °C).
These simulations identify key target regions and climate variables for
future proxy reconstructions to provide the best and most robust test cases
for (a) assessing Messinian model performance, (b) evaluating
Mediterranean–Atlantic connectivity during the MSC and (c) establishing
whether or not the MSC could ever have affected global-scale climate.
... 1. Climate modeling studies that assess the impact of varied MOW are hampered by the crude parameterization of Mediterranean-Atlantic exchange in most General Circulation Models (GCM; e.g. Ivanovic et al., 2013b), and by the lack of constraints on the timing and volume of Late Miocene Mediterranean outflow (Ivanovic et al., 2014). GCM modeling experiments focused on the MSC are therefore currently restricted to sensitivity studies rather than more realistic attempts to simulate Late Miocene climate (Ivanovic et al., 2013a(Ivanovic et al., , 2014. ...
The Atlantic Meridional Overturning Circulation (AMOC) is a key feature of the North Atlantic with global ocean impacts. The AMOC's response to past changes in forcings during the Holocene provides important context for the coming centuries. Here, we investigate AMOC trends using an emerging set of transient simulations using multiple global climate models for the past 6,000 years. Although some models show changes, no consistent trend in overall AMOC strength during the mid‐to‐late Holocene emerges from the ensemble. We interpret this result to suggest no overall change in AMOC, which fits with our assessment of available proxy reconstructions. The decadal variability of the AMOC does not change in ensemble during the mid‐ and late‐Holocene. There are interesting AMOC changes seen in the early Holocene, but their nature depends a lot on which inputs are used to drive the experiment.
La sédimentation de la pente moyenne du Golfe de Cadix est fortement influencée par l’écoulement d’un courant de fond d’origine méditerranéenne : la Mediterranean Outflow Water (MOW). La compréhension de la relation entre l’écoulement de la MOW, l’Atlantic Meridional Overturning Circulation (AMOC) et les variations climatiques en Atlantique Nord représente un enjeu majeur et complexe. Ces travaux de thèse, en étudiant les caractéristiques sédimentaires des sédiments des drifts contouritiques du Golfe de Cadix, corps sédimentaires propices à l’étude de la variabilité des courants de fond, visent à comprendre la dynamique de la MOW depuis le Pléistocène inférieur. Cette étude s’appuie essentiellement sur des archives sédimentaires prélevées lors de l’Expédition Integrated Ocean Drilling Program 339 « Mediterranean Outflow » (2011-2012) et valorise également des carottes sédimentaires collectées durant la mission océanographique CADISAR (2001). Ce travail a permis d’identifier les sources des minéraux argileux composant les dépôts du système contouritique de Cadix (apports fluviatiles du Guadalquivir et poussières éoliennes nord-africaines) et mis en évidence les rôles de la circulation océanique de surface et l’écoulement de la MOW, dans la distribution des minéraux argileux dans le Golfe de Cadix. Les assemblages ostracodiques, la minéralogie des argiles ainsi que les mesures granulométriques, effectués à haute résolution sur des archives sédimentaires prélevées à différentes bathymétries, ont permis d’affiner le schéma de circulation de la MOW dans le système contouritique de Cadix pour le dernier cycle climatique. Des séries sédimentaires plus anciennes couvrant l’intervalle des Stades Isotopiques Marins (MIS) 12-11 ont montré que le modèle de fonctionnement général établi demeurait valable mais d’intensité moindre, pour l’analogue climatique le plus récent. Enfin les mesures granulométriques et des cortèges argileux réalisées à haute résolution sur une longue série sédimentaire du drift de Faro, mises en âges par une stratigraphie isotopique inédite, ont permis de déterminer trois phases de construction du drift de Faro ainsi que de reconstruire la dynamique de la MOW depuis le Pléistocène inférieur (1,2 Ma).
Mediterranean Outflow Water (MOW) adds salt and density to open ocean intermediate waters and is therefore an important motor of Atlantic meridional overturning circulation (AMOC) and climate variability. However, the variability in strength and depth of MOW on geological timescales is poorly documented. Here we present new detailed records, with excellent age control, of MOW variability from 416 ka to present from rapidly accumulated marine sediments recovered from the West Iberian Margin during Integrated Ocean Drilling Program (IODP) Expedition 339. Our records of X‐ray fluorescence (XRF), physical grain size, and paleocurrent information from the anisotropy of magnetic susceptibility (AMS) indicate (i) a close relationship between the orientation of principle AMS axes and glacial‐interglacial cycles and (ii) two distinct regimes of MOW behavior over the last ~416 kyr in grain‐size and AMS variability at orbital (mainly precessional) and suborbital timescales. Between marine isotope stage (MIS) 10 and MIS 4, MOW was focused at a generally shallow depth on the West Iberian Margin, and changes in MOW strength were strongly paced by precession. A transition interval occurred during MISs 5 and 4, when MOW deepened and millennial‐scale variability in flow strength was superimposed on orbitally paced change. During MIS 11 and from MIS 3 to present, MOW was deeply focused and millennial‐scale variability dominated. We infer that late Pleistocene variability in MOW strength and depth were strongly climate influenced and that changes in circum‐Mediterranean rainfall climate were likely a primary control.
With the advancement of telecommunications, sensor networks, crowd sourcing, and remote sensing technology in present days, there has been a tremendous growth in the volume of data having both spatial and temporal references. This huge volume of available spatio-temporal (ST) data along with the recent development of machine learning and computational intelligence techniques has incited the current research concerns in developing various data-driven models for extracting useful and interesting patterns, relationships, and knowledge embedded in such large ST datasets. In this survey, we provide a structured and systematic overview of the research on data-driven approaches for spatio-temporal data analysis. The focus is on outlining various state-of-the-art spatio-temporal data mining techniques, and their applications in various domains. We start with a brief overview of spatio-temporal data and various challenges in analyzing such data, and conclude by listing the current trends and future scopes of research in this multi-disciplinary area. Compared with other relevant surveys, this paper provides a comprehensive coverage of the techniques from both computational/methodological and application perspectives. We anticipate that the present survey will help in better understanding various directions in which research has been conducted to explore data-driven modeling for analyzing spatio-temporal data.
The first isotope enabled General Circulation Model (GCM) simulations of the Pliocene are used to discuss the interpretation of δ18O measurements for a warm climate.The model suggests that spatial patterns of Pliocene ocean surface δ18O (δ18Osw) were similar to the preindustrial, however Arctic and coastal regions were relatively depleted, while South Atlantic and Mediterranean regions were relatively enriched. Modeled δ18Osw anomalies are closely related to modeled salinity anomalies, which supports using δ18Osw as a paleosalinity proxy.Modeled Pliocene precipitation δ18O (δ18Op) was enriched relative to the preindustrial (but with depletion of < 2 ‰ over some tropical regions). While usually modest ( < 4 ‰), the enrichment can reach 25 ‰ over ice sheet regions. In the tropics δ18Op anomalies are related to precipitation amount anomalies; although there is usually a spatial offset between the two. This offset suggests the location of precipitation change is more uncertain than the amplitude when interpreting δ18Op. At high latitudes δ18Op anomalies relate to temperature anomalies; however the relationship is neither linear nor spatially coincident: a large δ18Op signal does not always translate to a large temperature signal.These results suggest isotope modeling can lead to enhanced synergy between climate models and climate proxy data. The model can relate proxy data to climate in a physically based way even when the relationship is complex and nonlocal. The δ18O-climate relationships, identified here from a GCM, could not be determined from transfer functions or simple models.
Mediterranean Outflow Water (MOW) is thought to be a key contributor to the strength and stability of Atlantic Meridional Overturning Circulation (AMOC), but the future of Mediterranean-Atlantic water exchange is uncertain. It is chiefly dependent on the difference between Mediterranean and Atlantic temperature and salinity characteristics, and as a semi-enclosed basin, the Mediterranean is particularly vulnerable to future changes in climate and water usage. Certainly, there is strong geologic evidence that the Mediterranean underwent dramatic salinity and sea-level fluctuations in the past. Here, we use a fully coupled atmosphere–ocean General Circulation Model to examine the impact of changes in Mediterranean-Atlantic exchange on global ocean circulation and climate. Our results suggest that MOW strengthens and possibly stabilises the AMOC not through any contribution towards NADW formation, but by delivering relatively warm, saline water to southbound Atlantic currents below 800 m. However, we find almost no climate signal associated with changes in Mediterranean-Atlantic flow strength. Mediterranean salinity, on the other hand, controls MOW buoyancy in the Atlantic and therefore affects its interaction with the shallow-intermediate circulation patterns that govern surface climate. Changing Mediterranean salinity by a factor of two reorganises shallow North Atlantic circulation, resulting in regional climate anomalies in the North Atlantic, Labrador and Greenland-Iceland-Norwegian Seas of ±4 °C or more. Although such major variations in salinity are believed to have occurred in the past, they are unlikely to occur in the near future. However, our work does suggest that changes in the Mediterranean’s hydrological balance can impact global-scale climate.
A subgrid-scale form for mesoscale eddy mixing on isopycnal surfaces is proposed for use in non-eddy-resolving ocean circulation models. The mixing is applied in isopycnal coordinates to isopycnal layer thickness, or inverse density gradient, as well as to passive scalars, temperature and salinity. The transformation of these mixing forms to physical coordinates is also presented.
A conceptual framework for understanding the exchange through Gibraltar and its thermohaline forcing is presented. The Mediterranean Sea annually produces a dense water mass that sinks and accumulates above the level of the sill until the internal pressure gradient generated through the Strait is sufficiently strong to force it out at a rate equal to the rate of its mean interannual production. The dense water forced out creates a sea-level drop through the Strait that drives a compensatory inflow of surface Atlantic water. The two-way exchange can be calculated geostrophically by requiring that the baroclinic outflow equal the opposing barotropic inflow plus the net water balance of the Basin. Bottom friction acts as a retarding force for the outflow and reduces the geostrophic flow by roughly a half. The exchange was calculated from the steric heights derived from a series of historical hydrographic transects across the western Alboran Sea and the eastern Gulf of Cadiz. Bottom Ekman frictional parameters were estimated from the current-meter data of the Gibraltar Experiment. The mean outflow determined from these data was ; 0.84 " 0.3 Sv. It is shown that time-dependent fluctuations of the sea level can generate an additional, net mass exchange through a 'barotropic pumping' mechanism that increases the outflow by ; 50% to 1.26 Sv. This fluctuating flow component is susceptible to hydraulic control during the percentage of the time that Ž . the combined outflow or inflow achieves a supercritical state. This combined outflow suggests an interannual mean value of ; 96 cmryr for the internal water balance the annual value of which has little direct effect on the exchange due to the ; 9-year e-folding time for draining the reservoir of dense water accumulated to ; 180 m above the depth of the sill. This relatively stable accumulation of dense water provides the steady force for the exchange from seasonal to interannual time scales. However, significant variability in the exchange on weekly to seasonal time scales exists owing to the variability in the Basin's internal circulations, that supply the dense water to and evacuate the Atlantic water from the western Alboran, together with the variability in the sea-level fluctuations that drive barotropic-pumping exchange. In addition, variations in the amplitude of the exchange are damped by negative feedback loops that exist due to the interdependency between the exchange and the force generating it. This interpretation of an exchange buffered from the variability in its meteorological forcing and responsive to the variability in local potential energy suggests that any objective to detect a response to climatic trends in the Strait of Gibraltar should be coordinated with observations of the sea level, internal potential energy, water-mass characteristics, and air–sea interaction both locally and within the Basin and its sub-basins. q 1999 Elsevier Science B.V. All rights reserved.
Climate prediction requires the use of coupled models of the atmosphere-deep ocean-sea ice and land surface. This paper outlines the formulation of processes relevant to the simulation and prediction of climate change in the Arctic of one such model, that of the Hadley Centre for Climate Prediction and Research at the Meteorological Office. Comparison of the simulation of a number of features of the Arctic climate is made against observations and predictions of future climate change resulting from increased concentrations of greenhouse gases from recent runs of the model are discussed.
Parametric representations of oceanic geostrophic eddy transfer of heat and salt are studied ranging from horizontal diffusion to the more physically based approaches of Green and Stone (GS) and Gent and McWilliams (GM). The authors argue for a representation that combines the best aspects of GS and GM: transfer coefficients that vary in space and time in a manner that depends on the large-scale density fields (GS) and adoption of a transformed Eulerian mean formalism (GM). Recommendations are based upon a two-dimensional (zonally or azimuthally averaged) model with parameterized horizontal and vertical fluxes that is compared to three-di-mensional numerical calculations in which the eddy transfer is resolved. Three different scenarios are considered: 1) a convective ''chimney'' where the baroclinic zone is created by differential surface cooling; 2) spindown of a frontal zone due to baroclinic eddies; and 3) a wind-driven, baroclinically unstable channel. Guided by baroclinic instability theory and calibrated against eddy-resolving calculations, the authors recommend a form for the horizontal transfer coefficient given by 2 f M 2 2 k l l , N Ri where Ri f 2 N 2 /M 4 is the large-scale Richardson number and f is the Coriolis parameter; M 2 and N 2 are measures of the horizontal and vertical stratification of the large-scale flow, l measures the width of the baroclinic zone, and is a constant of proportionality. In the very different scenarios studied here the authors find to be a ''universal'' constant equal to 0.015, not dissimilar to that found by Green for geostrophic eddies in the at-mosphere. The magnitude of the implied k, however, varies from 300 m 2 s 1 in the chimney to 2000 m 2 s 1 in the wind-driven channel.
The Mediterranean Sea provides a major route for heat and freshwater
loss from the North Atlantic and thus is an important cause of the high
density of Atlantic waters. In addition to the traditional view that
loss of fresh water via the Mediterranean enhances the general salinity
of the North Atlantic, and the interior of the eastern North Atlantic in
particular, it should be noted that Mediterranean water outflowing at
Gibraltar is in fact cooler than compensating inflowing water. The
consequence is that the Mediterranean is also a region of heat loss from
the Atlantic and contributes to its large-scale cooling. Uniquely, this
system can be understood physically via the constraints placed on it by
a single hydraulic structure: the Gibraltar exchange. Here we review the
existing knowledge about the physical structure of the Gibraltar
exchange today and the evidential basis for arguments that it has been
different in the past. Using a series of quantitative experiments, we
then test prevailing concepts regarding the potential causes of these
past changes. We find that (1) changes in the vertical position of the
plume of Mediterranean water in the Atlantic are controlled by the
vertical density structure of the Atlantic; (2) a prominent Early
Holocene "contourite gap" within the Gulf of Cadiz is a response to
reduced buoyancy loss in the eastern Mediterranean during the time of
"sapropel 1" deposition; (3) changes in buoyancy loss from the
Mediterranean during MIS3 caused changes in the bottom velocity field in
the Gulf of Cadiz, but we note that the likely cause is reduced
freshwater loss and not enhanced heat loss; and (4) strong exchange at
Gibraltar during Atlantic freshening phases implies that the Gibraltar
exchange provides a strong negative feedback to reduced Atlantic
meridional overturning. Given the very counterintuitive way in which the
Strait of Gibraltar system behaves, we recommend that without
quantitative supporting work, qualitative interpretations of how the
system has responded to past external forcing are unlikely to be robust.
Results are presented from a new version of the Hadley Centre coupled model (HadCM3) that does not require flux adjustments
to prevent large climate drifts in the simulation. The model has both an improved atmosphere and ocean component. In particular,
the ocean has a 1.25° × 1.25° degree horizontal resolution and leads to a considerably improved simulation of ocean heat transports
compared to earlier versions with a coarser resolution ocean component. The model does not have any spin up procedure prior
to coupling and the simulation has been run for over 400 years starting from observed initial conditions. The sea surface
temperature (SST) and sea ice simulation are shown to be stable and realistic. The trend in global mean SST is less than 0.009 °C
per century. In part, the improved simulation is a consequence of a greater compatibility of the atmosphere and ocean model
heat budgets. The atmospheric model surface heat and momentum budget are evaluated by comparing with climatological ship-based
estimates. Similarly the ocean model simulation of poleward heat transports is compared with direct ship-based observations
for a number of sections across the globe. Despite the limitations of the observed datasets, it is shown that the coupled
model is able to reproduce many aspects of the observed heat budget.
Recent improvements to the Hadley Centre climate model include the introduction of a new land surface scheme called “MOSES”
(Met Office Surface Exchange Scheme). MOSES is built on the previous scheme, but incorporates in addition an interactive plant
photosynthesis and conductance module, and a new soil thermodynamics scheme which simulates the freezing and melting of soil
water, and takes account of the dependence of soil thermal characteristics on the frozen and unfrozen components. The impact
of these new features is demonstrated by comparing 1×CO2 and 2×CO2 climate simulations carried out using the old (UKMO) and new (MOSES) land surface schemes. MOSES is found to improve the
simulation of current climate. Soil water freezing tends to warm the high-latitude land in the northern Hemisphere during
autumn and winter, whilst the increased soil water availability in MOSES alleviates a spurious summer drying in the mid-latitudes.
The interactive canopy conductance responds directly to CO2, supressing transpiration as the concentration increases and producing a significant enhancement of the warming due to the
radiative effects of CO2 alone.
Results are presented from the latest version of the Hadley Centre climate model, HadAM3 (Hadley Centre Atmospheric Model
version 3). It represents a significant improvement over the previous version, HadAM2b. This is demonstrated using a series
of ten year integrations with AMIP (Atmospheric Model Intercomparison Project) boundary conditions. The work covers three
aspects of model performance: (1) it shows the improvements in the mean climate in changing from HadAM2b to HadAM3; (2) it
demonstrates that the model now compares well with observations and (3) it isolates the impacts of new physical parametrizations.
The Atlantic-Mediterranean exchange of water at Gibraltar represents a significant heat and freshwater sink for the North Atlantic and is a major control on the heat, salt and freshwater budgets of the Mediterranean Sea. Consequently, an understanding of the response of the exchange system to external changes is vital to a full comprehension of the hydrographic responses in both ocean basins. Here, we use a synthesis of empirical (oxygen isotope, planktonic foraminiferal assemblage) and modeling (analytical and general circulation) approaches to investigate the response of the Gibraltar Exchange system to Atlantic freshening during Heinrich Stadials (HSs). HSs display relatively flat W-E surface hydrographic gradients more comparable to the Late Holocene than the Last Glacial Maximum. This is significant, as it implies a similar state of surface circulation during these periods and a different state during the Last Glacial Maximum. During HS1, the gradient may have collapsed altogether, implying very strong water column stratification and a single thermal and d18Owater condition in surface water extending from southern Portugal to the eastern Alboran Sea. Together, these observations imply that inflow of Atlantic water into the Mediterranean was significantly increased during HS periods compared to background glacial conditions. Modeling efforts confirm that this is a predictable consequence of freshening North Atlantic surface water with iceberg meltwater and indicate that the enhanced exchange condition would last until the cessation of anomalous freshwater supply into to the northern North Atlantic. The close coupling of dynamics at Gibraltar Exchange with the Atlantic freshwater system provides an explanation for observations of increased Mediterranean Outflow activity during HS periods and also during the last deglaciation. This coupling is also significant to global ocean dynamics, as it causes density enhancement of the Atlantic water column via the Gibraltar Exchange to be inversely related to North Atlantic surface salinity. Consequently, Mediterranean enhancement of the Atlantic Meridional Overturning Circulation will be greatest when the overturning itself is at its weakest, a potentially critical negative feedback to Atlantic buoyancy change during times of ice sheet collapse.
Within the Central waters of the North Atlantic Ocean there is a significant east–west difference in salinity, similar to that caused by Mediterranean Water at deeper levels. In this paper we hypothesize that the salinity of the Central Water is influenced by the saline Mediterranean Outflow Water, despite physical separation of the two water masses by a salinity minimum over most of the ocean basin. It is suggested that there occurs a cross-isopycnal flux of salinity from the Mediterranean Outflow Water towards the low-density Central Water (detrainment) in the eastern Gulf of Cadiz, not far from the Strait of Gibraltar, where the two water masses are in physical contact. Laboratory experiments, inverse modeling and direct current observations are applied to support the hypothesis.
Warm and saline waters enter the Nordic Seas from the south as part of the warm-to-cold water transformation of the thermohaline circulation of the northern North Atlantic. One explanation for the origin of the Nordic Seas Inflow is a “shallow source hypothesis” under which the Inflow waters are a modification of upper ocean subtropical waters. Warm waters from the subtropical gyre are carried to the eastern North Atlantic by the North Atlantic Current and branch northwards, joined by poleward upper thermocline flow along the upper continental slope, to provide the Nordic Seas Inflow. Along this pathway the upper water column is progressively cooled and freshened by winter convection, the subpolar mode water transformation process, and this sets the Inflow characteristics.
The circulation of water masses in the northeastern North Atlantic Ocean has a strong influence on global climate owing to the northward transport of warm subtropical water to high latitudes. But the ocean circulation at depths below the reach of satellite observations is difficult to measure, and only recently have comprehensive, direct observations of whole ocean basins been possible. Here we present quantitative maps of the absolute velocities at two levels in the northeastern North Atlantic as obtained from acoustically tracked floats. We find that most of the mean flow transported northward by the Gulf Stream system at the thermocline level (about 600 m depth) remains within the subpolar region, and only relatively little enters the Rockall trough or the Nordic seas. Contrary to previous work, our data indicate that warm, saline water from the Mediterranean Sea reaches the high latitudes through a combination of narrow slope currents and mixing processes. At both depths under investigation, currents cross the Mid-Atlantic Ridge preferentially over deep gaps in the ridge, demonstrating that sea-floor topography can constrain even upper-ocean circulation patterns.
A Mediterranean overflow parameterization has been implemented in the ocean component of the NCAR Community Climate System Model (CCSM3) to represent the sub-grid scale exchanges through the Strait of Gibraltar and associated entrainment. It is evaluated by comparing solutions from the coarse resolution (nominally 3 degree) global ocean model to observations and high resolution process model results. The properties (temperature, salinity and volume) and depths of the inflow, outflow, entrainment and product waters are all close to observed estimates and stable after 300 years of integration. The seasonal cycle of the exchange has a reasonable amplitude, but its phase is advanced by three months from summer to spring. The improvement relative to either blocking the strait, or excavating a too wide channel is demonstrated. The effects on the circulation and properties of the North Atlantic are most prominent at about 1100 m. They include the formation of a realistic salt tongue across the basin and modified currents and younger water in the eastern basin offshore of the overflow. Climate impacts were assessed by comparing fully coupled climate models with and without the overflow parameterization after 300 years. The effects on North Atlantic sea surface temperature and heat flux do not match, suggesting an atmospheric response. Only minor differences were evident in the meridional overturning circulation and the ocean heat transport.
Time series collected in the Camarinal and Espartel sills, the two main sills of the Strait of Gibraltar, have been analyzed to assess the first transformation of the Mediterranean outflow. In Espartel, west of Camarinal sill, the outflow has been estimated from a 6 year long time series of acoustic Doppler current profiler observations. The near bottom water flowing out at Espartel is around 0.1 units saltier and 0.1C warmer than at Camarinal, which is explained by entrainment and mixing of Atlantic water by the outflow inside the basin bounded by both sills. The constancy of heat and salinity transport implies an Atlantic water entrainment of 0.03 Sv, which is about 4% of the outflow observed at Espartel (0.77 Sv) and a flow of 0.74 Sv at Camarinal (96% of the flow at Espartel). It is also shown that the high energy dissipation rate associated with internal tides is enough to sustain the entrainment and a thorough mixing of the entrained water.
The Princeton Ocean General Circulation model (OGCM) is used to simulate the circulation in the Atlantic Ocean between 80°N and 15°S. Horizontal resolution is Δφ = Δλ = 2°, and the computations are carried out at 12 levels. The model is initialized with Levitus's (1982) data. Robust diagnostic and prognastic experiments with prescribed Mediterranean outflow are carried out to study the gross features of the North Atlantic circulation, and in particular the Mediterranean water tongue. The model simulates westward and west-southwestward flow in the area where Mediterranean water is usually observed. The positive temperature and salinity anomalies extend westward. The simulated water masses and their general features agree well with Levitus's (1982) data. Horizontal density gradients due to temperature and salinity anomalies compensate each other in the Mediterranean water tongue, and the buoyancy signal originating from the Strait of Gibraltar does not substantially affect the general circulation in the interior Atlantic Ocean.
The behaviours of three high-resolution ocean circulation models of the North Atlantic, differing chiefly in their description of the vertical coordinate, are investigated in order to elucidate the routes and mechanisms by which saline water masses of southern origin provide inflows to the Nordic Seas. An existing hypothesis is that Mediterranean Overflow Water (MOW) is carried polewards in an eastern boundary undercurrent, and provides a deep source for these inflows. This study, however, provides an alternative view that the inflows are derived from shallow sources, and are comprised of water masses of western origin, carried by branches of the North Atlantic Current (NAC), and also more saline Eastern North Atlantic Water (ENAW), transported northwards from the Bay of Biscay region via a ‘Shelf Edge Current’ (SEC) flowing around the continental margins. In two of the models, the MOW flows northwards, but reaches only as far as the Porcupine Bank
(53°N). In third model, the MOW also invades the Rockall Trough (extending to 60°N). However, none of the models allows the MOW to flow northwards into the Nordic Seas. Instead, they all support the hyporthesis of there being shallow pathways, and that the saline inflows to the Nordic Seas result from NAC-derived
and ENAW water masses, which meet and partially mix in the Rockall Trough. Volume and salinity transports into the southern Rockall Trough via the SEC are, in the various models, between 25 and 100% of those imported by the NAC, and are also a similarly significant proportion (20–75%) of the transports into the
Nordic Seas. Moreover, the highest salinities are carried northwards by the SEC (these being between 0.13 and 0.19 psu more saline at the southern entrance to the Trough than those in the NAC-derived waters). This reveals for the first time the importance of the SEC in carrying saline water masses through the Rockall
Trough and into the Nordic Seas. Furthermore, the high salinities found on density surfaces appropriate to the MOW in the Nordic Seas are shown to result from the wintertime mixing of the saline near-surface waters advected northwards by the SEC/NAC system. Throughout, we have attempted to demonstrate the
extent to which the models agree or disagree with interpretations derived from observations, so that the study also contributes to an ongoing community effort to assess the realism of our current generation of ocean models.
A numerical model for the simulation of sea ice circulation and thickness over a seasonal cycle is presented. This model is used to investigate the effects of ice dynamics on Arctic ice thickness and air-sea heat flux characteristics by carrying out several numerical simulations over the entire Arctic Ocean region. The essential idea in the model is to couple the dynamics to the ice thickness characteristics by allowing the ice interaction to become stronger as the ice becomes thicker and/or contains a lower areas percentage of thin ice. The dynamics in turn causes high oceanic heat losses in regions of ice divergence and reduced heat losses in regions of convergence. TO model these effects consistently the ice is considered to interact in a plastic manner with the plastic strength chosen to depend on the ice thickness and concentration. The thickness and concentration, in turn, evolve according to continuity equations which include changes in ice mass and percent of open water due to advection, ...
A conceptual framework for understanding the exchange through Gibraltar and its thermohaline forcing is presented. The Mediterranean Sea annually produces a dense water mass that sinks and accumulates above the level of the sill until the internal pressure gradient generated through the Strait is sufficiently strong to force it out at a rate equal to the rate of its mean interannual production. The dense water forced out creates a sea-level drop through the Strait that drives a compensatory inflow of surface Atlantic water. The two-way exchange can be calculated geostrophically by requiring that the baroclinic outflow equal the opposing barotropic inflow plus the net water balance of the Basin. Bottom friction acts as a retarding force for the outflow and reduces the geostrophic flow by roughly a half. The exchange was calculated from the steric heights derived from a series of historical hydrographic transects across the western Alboran Sea and the eastern Gulf of Cadiz. Bottom Ekman frictional parameters were estimated from the current-meter data of the Gibraltar Experiment. The mean outflow determined from these data was ; 0.84 " 0.3 Sv. It is shown that time-dependent fluctuations of the sea level can generate an additional, net mass exchange through a 'barotropic pumping' mechanism that increases the outflow by ; 50% to 1.26 Sv. This fluctuating flow component is susceptible to hydraulic control during the percentage of the time that Ž . the combined outflow or inflow achieves a supercritical state. This combined outflow suggests an interannual mean value of ; 96 cmryr for the internal water balance the annual value of which has little direct effect on the exchange due to the ; 9-year e-folding time for draining the reservoir of dense water accumulated to ; 180 m above the depth of the sill. This relatively stable accumulation of dense water provides the steady force for the exchange from seasonal to interannual time scales. However, significant variability in the exchange on weekly to seasonal time scales exists owing to the variability in the Basin's internal circulations, that supply the dense water to and evacuate the Atlantic water from the western Alboran, together with the variability in the sea-level fluctuations that drive barotropic-pumping exchange. In addition, variations in the amplitude of the exchange are damped by negative feedback loops that exist due to the interdependency between the exchange and the force generating it. This interpretation of an exchange buffered from the variability in its meteorological forcing and responsive to the variability in local potential energy suggests that any objective to detect a response to climatic trends in the Strait of Gibraltar should be coordinated with observations of the sea level, internal potential energy, water-mass characteristics, and air–sea interaction both locally and within the Basin and its sub-basins. q 1999 Elsevier Science B.V. All rights reserved.
A cover article in Eos last year [Johnson, 1997] called for a dam across
the Strait of Gibraltar to prevent a new Ice Age. In this article, R. G.
Johnson argued that reduced Nile River flow after building the Aswan Dam
increases Mediterranean Sea salinity, leading to enhanced outflow of
salty water into the Atlantic Ocean. This, in turn, would alter the
thermohaline (that is, temperature and salinity driven) circulation of
the Atlantic, heat up the Labrador Sea and enhance evaporation there,
and increase snowfall in Canada until a new ice sheet builds up. Ocean
circulation model experiments, however, suggest that this fear is
unfounded. While Mediterranean saltwater outflow (Figure 1a) does appear
to have some effect on North Atlantic circulation and surface climate,
the change in Mediterranean salt budget resulting from the Aswan Dam is
far too small to have any noticeable impact.
An energy and angular-momentum conserving vertical finite-difference scheme is introduced for a general terrain-following vertical coordinate which is a function of pressure and its surface value. A corresponding semi-implicit time scheme is also defined. These schemes are used to compare the usual sigma coordinate with the hybrid coordinate which reduces to pressure above a fixed level and with a modified hybrid coordinate which tends uniformly to pressure at upper levels. -from Authors
High-resolution grain size and magnetic susceptibility records from the eastern Gulf of Cadiz (site MD99-2339; 1170m water depth) reveal contourites formed by the Mediterranean Outflow (MOW) during the last 47kyr BP. Oscillations in the MOW's intensity occurred in phase with Greenland temperature variations with a stronger outflow during northern hemisphere coolings such as Dansgaard-Oeschger stadials, Heinrich events, and the Younger Dryas. Benthic δ13C data implies the Western Mediterranean Deep Water as one of the main sources feeding the outflow current, while differential changes in the properties of
the Mediterranean source and entrained North Atlantic Central Water largely control the MOW's strength. Detailed studies for Heinrich events 1, 4 and 5 show that the flow strength peaked only when subtropical surface waters prevailed in the eastern Gulf of Cadiz, while incursions of icebergs and subpolar surface water were not favorable for the MOW's intensification. As the MOW was strong when the Atlantic's thermohaline circulation (THC) was weakened, the heat and salt injected by the MOW into the intermediate North Atlantic waters might have preconditioned the THC to switch from the stadial to the interstadial mode.
ABSTRACTA simple box-model is used to investigate the effect of intermediate level heat/freshwater fluxes on the variability of the oceanic meridional overturning circulation. The model includes a simple representation of the spreading of the Mediterranean Outflow Water in the North Atlantic. We identify an internal advective feedback affecting the amplitude of the thermohaline oscillations. When a salinity gradient is maintained in the ocean interior the oscillations are amplified. Instead, if the intermediate level fluxes are spread in the ocean deep layers, the model variability is reduced. We suggest that this mechanism may be relevant for climate variability on interdecadal timescales.
Climate and ocean-only models have shown that the ocean will respond abruptly to significant perturbations in surface forcing. Centennial-scale oscillation is a characteristic of circulation in large semi-enclosed ocean basins such as the Arctic, whereas millennial-scale adjustment to changes in surface forcing has been found in the global ocean component of climate models. We show that the millennial time-scale in climate models is likely to be intrinsic to the ocean through its presence in an ocean-only model. The strength of the thermohaline circulation is shown to be very sensitive to the magnitude of ice albedo and, to a lesser extent, perturbation in the surface freshwater flux. Modelled glacial ocean circulation, in contrast to present-day simulations, requires an enhanced freshwater flux over the northern Atlantic, even in its non-Heinrich state, to obtain realistic overturning in the North Atlantic. Copyright © 2001 John Wiley & Sons, Ltd.
Dinoflagellate cyst (dinocyst) analysis was conducted on two cores from the SW Iberian margin and central Alboran Sea from which high quality records of Marine Isotope Stage 3 have been previously derived. Our aim in this study is to compare the dinocyst signature between 50 and 25 ka BP with existing datasets of foraminiferal and geochemical proxies related to hydrological parameters. Quantitative reconstructions of sea-surface temperatures (SSTs) and salinities (SSS) based on dinocysts are performed for the first time in this area. The results are compared to SSTs derived from planktonic foraminifera and alkenone measurements, and to SSS calculated from planktonic delta O-18 and foraminiferal SST. Significant oscillations related to Dansgaard-Oeschger cycles are recorded in both cores. Dinocyst-derived hydrological parameters exhibit synchronous fluctuations and similar values to those derived from the other methods, in particular when considering quantitative reconstructions for February based on foraminifera and dinocysts. Our study shows that the influence of subpolar waters was felt during each Greenland Stadial (GS) off Portugal, and that the amplification of the Heinrich Stadial cooling in the Alboran Sea was related to the penetration of subpolar waters through the Strait of Gibraltar. During Greenland Interstadials (GI), we provide evidence for the occurrence of warm and nutrient-rich sea-surface waters in the Alboran Sea, probably due to gyre-induced upwelling. Finally, the difference between August and February dinocyst SST estimates suggests higher seasonal contrasts during GS compared to Cl at the two core sites. Additionally, precession appears to have an imprint on dinocyst-derived long-term seasonality record. However, this observation needs to be confirmed by longer records. (c) 2011 Elsevier B.V. All rights reserved.
A comprehensive new radiation code based on the two‐stream equations in both the long‐wave and short‐wave spectral regions is described. The spectral resolution of the code is variable, enabling it to be used in a wide range of applications. Because of its flexibility, the code is well‐suited to the investigation of the sensitivity of radiative calculations to changes in the way in which physical processes are parametrized. The gaseous transmission data are derived from a line‐by‐line model. Particular attention is directed towards the treatment of the water vapour continuum, the overlap between gases, and the sensitivity to changing the carbon dioxide concentrations.
The performance of the code is examined both at high spectral resolution and in a lower‐resolution configuration designed for the UK Meteorological Office Unified Forecast/Climate Model (UM). Particularly for use in the UM, the code must be shown to perform satisfactorily across the whole range of atmospheric conditions. Comparisons are therefore made with reference calculations in both the long‐wave and the short‐wave, in clear and cloudy skies, and the accuracy with which various processes may be represented is studied.
For the cloudy calculations in the short‐wave, a new method is presented for deriving the single‐scattering properties in broad bands, based on the analytic expression for the reflectivity of an optically thick cloud. This minimizes the errors in calculating the short‐wave radiative properties of water clouds when the spectral resolution is reduced to that designed for the UM. In contrast, for ice clouds the errors are minimized by deriving the single‐scattering properties using linear averaging, as appropriate for optically thin clouds. In the long‐wave, the vertical distribution of the radiative heating in cirrus clouds is examined at high spectral resolution. The effect of scattering of long‐wave radiation, usually ignored in large‐scale models, is examined in some detail and is explained using a simple model. Taking all these studies into account, it is concluded that the configuration designed for the UM retains the generality of the code, without significantly compromising the overall accuracy.
Abstract A two-dimensional Boussinesq ocean model has been used to investigate the effect of intermediate-level thermal and saline anomalies on the known multiple equilibria structure of the thermohaline circulation. These anomalies are taken as a crude representation of the Mediterranean outflow in the Atlantic Ocean. The associated perturbation drives the system towards an overturning which resembles the present average Atlantic thermohaline circulation. The sensitivity to the depth at which the anomaly is placed is also investigated. We found that near-surface anomalies are more efficient in affecting the structure of the equilibria.
Diagnostics derived from cloud-resolving-model simulations in part I of this study, relating to the vertical transport of horizontal momentum by convection, are used to develop a parametrization of convective momentum-transports for deep convection based upon the mass-flux convection-scheme discussed by Gregory and Rowntree. the importance of cloud pressure-gradients to the flow within the cloud is emphasised, and a simple method of representing their effect is suggested. the scheme is able to reproduce the fluxes derived from the cloud-resolving model studies where cloud organization by the flow is unimportant. Inclusion of the parametrization in a version of the Meteorological Office Unified Model demonstrates that convective momentum-transports play a large role in the momentum balance of the atmosphere. Generally, simulation of the mean atmospheric circulation by the Unified Model is improved by the inclusion of such transports.
This study describes a new coupled ocean-atmosphere general circulation model (OAGCM) developed for studies of climate change
and results from a hindcast experiment. The model includes various physical and technical improvements relative to an earlier
version of the Hadley Centre OAGCM. A coupled spinup process is used to bring the model to equilibrium. Compared to uncoupled
spinup methods this is computationally more expensive, but helps to counter climate drift arising from inadequate sampling
of short time scale coupled variability when the components are equilibrated separately. Including sea ice advection and enhancing
reference surface salinities in high southern latitudes in austral winter to promote bottom water formation during spinup
appears to have stabilized the high-latitude drift exhibited in the earlier model’s control run. In the present study, the
atmospheric control climate is stable on multi-century time scales with a drift in global average surface air temperature
of only +0.016 K/century, despite a small residual drift in the deep ocean. The control climate is an improvement over the
earlier model in several respects, notably in its variability on short time scales. Two anomaly runs are presented incorporating
estimated forcing changes over the period 1860 to 1990 arising from greenhouse gases alone and from greenhouse gases plus
the radiative scattering effect of sulphate aerosols. These allow validation of the model against the instrumental climate
record. Inclusion of aerosol forcing gives a significantly better simulation of historical temperature patterns, although
comparisons against recent sea ice trends are equivocal. These studies emphasize the potential importance of including additional
forcing terms apart from greenhouse gases in climate simulations, and refining estimates of their spatial distribution and
magnitude.
To measure the exchange between the Atlantic and Mediterranean through the Strait of Gibraltar, an array of current meter moorings was deployed for a year in the Strait during 1985–1986. A novel aspect of these measurements is the inclusion of conductivity as well as temperature and pressure sensors on each current meter so that the salinity of the flows could be monitored continuously. These salinity measurements determine the water mass characteristics of the flows crossing the sill; they allow definition of the 37 psu isohaline as the interface between inflowing fresher Atlantic water and outflowing saltier Mediterranean water; and they enable time series to be developed for the depth of this interface, for the upper layer inflow, and for the lower layer outflow.
Mediterranean functioning is linked to continental climate and more particularly to freshwater budget across its surface. In the sediment of the eastern basin, the sapropel layers are signatures of variations in hydrology and climate that have occurred over the last half million years. They may be used to investigate the probable evolutions following global climatic change, and particularly the sensitivity of marine dynamics to freshwater input variations. At the present time, in the water column of the eastern and western basins, there are changes in hydrology originating in evolutions in heat and water budgets across the sea surface. With respect to the water budget of the Mediterranean, some evolutions have occurred as a result of the anthropogenic use of freshwater, and consequently, environmental driving forces must also be considered alongside climatic factors. Marine variations may be used as constraints for the quantification of probable evolutions in external driving forces at the scale of a whole basin. As an example, the previously developed 20-box model provides an explanation for the increasing trends of temperature and salinity observed in the western deep water over the 1960–1996 period by accounting for changes in freshwater and heat budgets, reaching 0.1 m yr−1 and 1.5 W m−2, respectively, in 1995 over the Mediterranean.
The Strait of Gibraltar is the only dynamically relevant communication of the Mediterranean Sea with a large ocean basin, the North Atlantic. Although quite constrictive, important water exchanges occur through it over a broad frequency range.With respect to the tides, considering the Mediterranean as a long zonal channel closed at both ends indicates that the principal tidal signal observed in its interior is astronomically forced. However, the tidal wave incoming from the Atlantic, although strongly reflected (∼ 94%) at the entrance to the strait, has about a 10% integrated contribution to the observed tide within the sea. In the strait most of the tidal flow (∼ 92%) is barotropic, but a clear baroclinic tide is discernible from observations. The correlation between tidal currents and depth variations of the interface (separating Atlantic and Mediterranean waters) at Gibraltar's main sill represents up to of the mean transport in each layer.At subinertial frequencies, periods from days to a few months, the strait restricts the meteorologically forced flows modifying the simple isostatic response of sea level to atmospheric pressure within the sea. A simple analytical model, consisting of two basins (the western and eastern Mediterranean) and two straits (Gibraltar and Sicily), indicates that the atmospheric pressure field over the sea accounts for 65% of the barotropic subinertial flows at Gibraltar, 68% of sea level variability in the western basin but only 41% of that in the eastern basin. These results are obtained by applying a linear friction coefficient to limit the barotropic flows at both straits, Gibraltar and Sicily.The long-term (seasonal to interannual) two-layered baroclinic exchange through the strait has historically been related to the integrated effects of the mass and salt balances in the sea. Recent works, based on the hydraulic behaviour of the two-layer flow in from the Atlantic at the surface and out of the Mediterranean at depth, allow speculation on the possible forcing and control mechanisms for this exchange, as well as their repercussions on the circulation and water characteristics within the sea. However, the true dynamic role played by time-dependent processes on these exchange flows remains a subject of debate and intense research.
The role of the Strait of Gibraltar on the exchanges of substances between Mediterranean Sea and the Atlantic Ocean is reviewed. The previous estimations have been recalculated by using a similar water flux and compared with the river and atmospheric inputs to the Western Mediterranean Sea. The man-induced changes in the dimensions of the Strait of Gibraltar increasing (planning the sill) or reducing of the cross-section by a total or partial dam are discussed. A total dam will control the sea-level rise in the Mediterranean Sea, but an annual increase of major nutrient concentrations of 1-2% could be expected, lower than the rate of increase of the river and atmospheric inputs in the Western Mediterranean Sea. The increase of the cross-section of the Strait by increasing the depth (planning) at the sill could compensate the increase of the external nutrient inputs.
Exchange through the Strait of Gibraltar
- T P Boyer
- J I Antonov
- O K Baranova
- H E Garcia
- D R Johnson
- R A Locarnini
- A V Mishonov
- O Brien
- T D Seidov
- D Smolyar
- I V Zweng
Boyer, T.P., Antonov, J.I., Baranova, O.K., Garcia, H.E., Johnson, D.R., Locarnini, R.A., Mishonov, A.V., O'Brien, T.D., Seidov, D., Smolyar, I.V., Zweng, M.M., 2009. In: Levitus, S. (Ed.),World Ocean Database 2009. NOAA Atlas NESDIS 66, U.S. Gov. Printing Office, Wash. DC, 216. Bryden, H.L., Candela, J., Kinder, T.H., 1994. Exchange through the Strait of Gibraltar. Prog. Oceanogr. 33, 201–248.
- A V Mishonov
- T D O 'brien
- D Seidov
- I V Smolyar
- M M Zweng
Mishonov, A.V., O'Brien, T.D., Seidov, D., Smolyar, I.V., Zweng, M.M., 2009. World Ocean Database 2009. S. Levitus, Ed., NOAA Atlas NESDIS 66, U.S. Gov.
369
Printing Office, Wash. D.C., 216.
The Gibraltar Strait and its Role in the Dynamics of the Mediterranean- 373
- J Candela
Candela, J., 1991. The Gibraltar Strait and its Role in the Dynamics of the Mediterranean-
373