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Sensitivity of the Atlantic Ocean circulation to a hydraulic overflow parameterisation in a coarse resolution model: Response of the subpolar gyre
Abstract
We investigate the sensitivity of a coarse resolution coupled climate model to the representation of the overflows over the Greenland–Scotland ridge. This class of models suffers from a poor representation of the water mass exchange between the Nordic Seas and the North Atlantic, a crucial part of the large-scale oceanic circulation. We revisit the explicit representation of the overflows using a parameterisation by hydraulic constraints and compare it with the enhancement of the overflow transport by artificially deepened passages over the Greenland–Scotland ridge, a common practice in coarse resolution models. Both configurations increase deep water formation in the Nordic Seas and represent the large-scale dynamics of the Atlantic realistically in contrast to a third model version with realistic sill depths but without the explicit overflow transport. The comparison of the hydrography suggests that for the unperturbed equilibrium the Nordic Seas are better represented with the parameterised overflows. As in previous studies, we do not find a stabilising effect of the overflow parameterisation on the Atlantic meridional overturning circulation but merely on the overflow transport. As a consequence the surface air temperature in the Nordic Seas is less sensitive to anomalous surface fresh water forcing.Special attention is paid to changes in the subpolar gyre circulation. We find it sensitive to the overflow transport and the density of these water masses through baroclinic adjustments. The analysis of the governing equations confirms the presence of positive feedbacks inherent to the subpolar gyre and allows us to isolate the influence of the overflows on its dynamics.
... Meanwhile, its effect on large-scale ocean circulation is not well established. In some models, even modest changes in the sill depth produce large changes in Atlantic overturning circulation, with major consequences for ocean heat transport (OHT) and North Atlantic surface climate (e.g., Roberts and Wood 1997), while in others changes in the sill does not affect AMOC strength (Robinson et al. 2011;Born et al. 2009). ...
... Hence, the presence of the GSR completely changes the dynamics of the North Atlantic-Arctic circulation; as the deep MOC vanishes poleward of 508N, the heat transport across the GSR is dominated by a shallow circulation (driven by a combination of surface winds and buoyancy forcing). This is consistent with earlier model studies suggesting that the SPG accounts for most of the OHT across the GSR in the modern day climate (Spall 2001;Born et al. 2009;Ferrari and Ferreira 2011;Li and Born 2019). ...
... By implementing an overflow parameterization in a fully coupled climate model, Yeager and Danabasoglu (2012) found that the AMOC increases by 4-6 Sv in the subpolar North Atlantic between 408 and 608N, while the maximum AMOC transport at 378N decreases. Meanwhile, other studies using a parameterization based on hydraulic constraints (e.g., Legutke and Maier-Reimer 2002;Kösters et al. 2005;Born et al. 2009) only show a small (less than 2 Sv) effect of Nordic seas overflows on North Atlantic overturning strength. Despite the weak response in overturning, Kösters et al. (2005) show that the GSR overflow has a large impact on North Atlantic climate, by increasing the northward ocean heat transport and warming the Nordic seas. ...
Changes in the geometry of ocean basins have been influential in driving climate change throughout Earth’s history. Here, we focus on the emergence of the Greenland-Scotland Ridge (GSR) and its influence on the ocean state, including large-scale circulation, heat transport, water mass properties and global climate. Using a coupled atmosphere-ocean-sea ice model, we consider the impact of introducing the GSR in an idealized Earth-like geometry, comprising a narrow Atlantic-like basin and a wide Pacific-like basin. Without the GSR, deep-water formation occurs near the North Pole in the Atlantic basin, associated with a deep meridional overturning circulation (MOC). By introducing the GSR, the volume transport across the sill decreases by 64%, and deep convection shifts south of the GSR, dramatically altering the structure of the high-latitude MOC. Due to compensation by the subpolar gyre, the northward ocean heat transport across the GSR only decreases by ∼30%. As in the modern Atlantic ocean, a bidirectional circulation regime is established with warm Atlantic water inflow and a cold dense overflow across the GSR. In sharp contrast to the large changes north of the GSR, the strength of the Atlantic MOC south of the GSR is unaffected. Outside the high-latitudes of the Atlantic basin, the surface climate response is surprisingly small, suggesting that the GSR has little impact on global climate. Our results suggest that caution is required when interpreting paleoproxy and ocean records, which may record large local changes, as indicators of basin-scale changes in the overturning circulation and global climate.
... Several model studies have investigated the importance of the Nordic seas overflow on the SPG circulation (e.g., Bö ning et al. 1996;Redler and Bö ning 1997;Roberts and Wood 1997;Born et al. 2009). Redler and Bö ning (1997) identified the role of the overflow on the upper and lower circulation in the North Atlantic from a high-resolution ocean model. ...
... They concluded that the AMOC was mainly sensitive to the overflow rather than to the open-ocean convection process driven directly by air-sea fluxes over the subpolar region. From a coupled climate model sensitivity experiment, Born et al. (2009) found that the model's representation of the overflow had a large influence on the properties of the SPG and that a more vigorous overflow led to a strengthened SPG. Redler and Bö ning (1997) suggested that the overflow in the Denmark Strait is the main controlling mechanism for the DWBC, while changes in the flow through the Faroe Bank Channel have only a small effect on the deep transport in the western basin. ...
... Fully coupled climate models are valuable tools for investigating the SPG and the mechanisms controlling its variability on decadal to multidecadal time scales (Delworth and Mann 2000;Cooper and Gordon 2002;Gao and Yu 2008;Born et al. 2009). In general, climate variability on time scales longer than a few decades cannot be investigated directly from observations because of the limited length of continuous time series. ...
In the present study the decadal variability in the strength and shape of the subpolar gyre (SPG) in a 600-yr preindustrial simulation using the Bergen Climate Model is investigated. The atmospheric influence on the SPG strength is reflected in the variability of Labrador Sea Water (LSW), which is largely controlled by the North Atlantic Oscillation, the first mode of the North Atlantic atmospheric variability. A combination of the amount of LSW, the overflows from the Nordic seas, and the second mode of atmospheric variability, the East Atlantic Pattern, explains 44% of the modeled decadal variability in the SPG strength. A prior increase in these components leads to an intensified SPG in the western subpolar region. Typically, an increase of one standard deviation (std dev) of the total overflow (1 std dev 5 0.2 Sv; 1 Sv [ 10 6 m 3 s 21) corresponds to an intensification of about one-half std dev of the SPG strength (1 std dev 5 2 Sv). A similar response is found for an increase of one std dev in the amount of LSW, and simultaneously the strength of the North Atlantic Current increases by one-half std dev (1 std dev 5 0.9 Sv).
... To assess the dominant driver of gyre changes in the Nordic Seas we consider the depth-integrated volume transport equation in the meridional direction 24,63 : ...
... The potential energy term is sensitive to density changes in deep water because of the weighting of z in the calculation of Φ. P b is the bottom pressure. However, this term is not considered here since an amplification of the flow can only result from the ageostrophic contributions from the potential energy and the wind stress terms 63 . Derivatives are calculated using centered differences on data regridded onto a regular 1°longitude-latitude grid. ...
The overturning circulation in the Nordic Seas involves the transformation of warm Atlantic waters into cold, dense overflows. These overflow waters return to the North Atlantic and form the headwaters to the deep limb of the Atlantic meridional overturning circulation (AMOC). The Nordic Seas are thus a key component of the AMOC. However, little is known about the response of the overturning circulation in the Nordic Seas to future climate change. Here we show using global climate models that, in contrast to the North Atlantic, the simulated density-space overturning circulation in the Nordic Seas increases throughout most of the 21st century as a result of enhanced horizontal circulation and a strengthened zonal density gradient. The increased Nordic Seas overturning is furthermore manifested in the overturning circulation in the eastern subpolar North Atlantic. A strengthened Nordic Seas overturning circulation could therefore be a stabilizing factor in the future AMOC.
... Realistic simulation of ocean heat anomalies is a prerequisite for understanding and predicting decadal climate variability (Latif and Keenlyside, 2011;Meehl et al., 2014;Guemas et al., 2014). in part on the representation of the ocean exchanges of heat and salt with the Arctic through warm, saline inflow and cold, dense outflows (e.g. Born et al., 2009;Danabasoglu et al., 2010;Köller et al., 2010;Wang et al., 2015). ...
... 5.1). This simplified framework implies that a low biased ventilation rate of 1 Sv will lead to a further freshening of the overflows by 0.05 psu, sufficient to impact on the structure and intensity of the AMOC in the model (Born et al., 2009;Danabasoglu et al., 2010;Köller et al., 2010;Wang et al., 2015). Also, the stability of the AMOC in the climate models of varying complexity is likely related to the intensity of the vertical overturning (e.g. ...
The northern limb of the Atlantic thermohaline circulation and its
transport of heat and salt towards the Arctic strongly modulate the climate
of the Northern Hemisphere. The presence of warm surface waters prevents ice
formation in parts of the Arctic Mediterranean, and ocean heat is directly
available for sea-ice melt, while salt transport may be critical for the
stability of the exchanges. Through these mechanisms, ocean heat and salt
transports play a disproportionally strong role in the climate system, and
realistic simulation is a requisite for reliable climate projections. Across
the Greenland–Scotland Ridge (GSR) this occurs in three well-defined
branches where anomalies in the warm and saline Atlantic inflow across the
shallow Iceland–Faroe Ridge (IFR) have been shown to be particularly
difficult to simulate in global ocean models. This branch (IF-inflow) carries
about 40 % of the total ocean heat transport into the Arctic
Mediterranean and is well constrained by observation during the last 2
decades but associated with significant inter-annual fluctuations. The
inconsistency between model results and observational data is here explained
by the inability of coarse-resolution models to simulate the overflow across
the IFR (IF-overflow), which feeds back onto the simulated IF-inflow. In
effect, this is reduced in the model to reflect only the net exchange across
the IFR. Observational evidence is presented for a substantial and persistent
IF-overflow and mechanisms that qualitatively control its intensity. Through
this, we explain the main discrepancies between observed and simulated
exchange. Our findings rebuild confidence in modelled net exchange across the
IFR, but reveal that compensation of model deficiencies here through other
exchange branches is not effective. This implies that simulated ocean heat
transport to the Arctic is biased low by more than 10 % and associated
with a reduced level of variability, while the quality of the simulated salt
transport becomes critically dependent on the link between IF-inflow and
IF-overflow. These features likely affect sensitivity and stability of
climate models to climate change and limit the predictive skill.
... Realistic simulation of ocean heat anomalies is a prerequisite for understanding and predicting decadal climate variability (Latif and Keenlyside, 2011;Meehl et al., 2014;Guemas et al., 2014). in part on the representation of the ocean exchanges of heat and salt with the Arctic through warm, saline inflow and cold, dense outflows (e.g. Born et al., 2009;Danabasoglu et al., 2010;Köller et al., 2010;Wang et al., 2015). ...
... 5.1). This simplified framework implies that a low biased ventilation rate of 1 Sv will lead to a further freshening of the overflows by 0.05 psu, sufficient to impact on the structure and intensity of the AMOC in the model (Born et al., 2009;Danabasoglu et al., 2010;Köller et al., 2010;Wang et al., 2015). Also, the stability of the AMOC in the climate models of varying complexity is likely related to the intensity of the vertical overturning (e.g. ...
The northern limb of the Atlantic thermohaline circulation and its transport of heat and salt towards the Arctic strongly modulates the climate of the Northern Hemisphere. Presence of warm surface waters prevents ice formation in parts of the Arctic Mediterranean and ocean heat is in critical regions directly available for sea-ice melt, while salt transport may be critical for the stability of the exchanges. Hereby, ocean heat and salt transports play a disproportionally strong role in the climate system and realistic simulation is a requisite for reliable climate projections. Across the Greenland-Scotland Ridge (GSR) this occurs in three well defined branches where anomalies in the warm and saline Atlantic inflow across the shallow Iceland-Faroe Ridge (IFR) have shown particularly difficult to simulate in global ocean models. This branch (IF-inflow) carries about 40 % of the total ocean heat transport into the Arctic Mediterranean and is well constrained by observation during the last two decades but is associated with significant inter-annual fluctuations. The inconsistency between model results and observational data is here explained by the inability of coarse resolution models to simulate the overflow across the IFR (IF-overflow), which feeds back on the simulated IF-inflow. In effect, this is reduced in the model to reflect only the net exchange across the IFR. Observational evidence is presented for a substantial and persistent IF-overflow and mechanisms that qualitatively control its intensity. Through this, we explain the main discrepancies between observed and simulated exchange. Our findings rebuild confidence in modeled net exchange across the IFR, but reveal that compensation of model deficiencies here through other exchange branches is not effective. This implies that simulated ocean heat transport to the Arctic is biased low by more than 10 % and associated with a reduced level of variability while the quality of the simulated salt transport becomes critically dependent on the link between IF-inflow and IF-overflow. These features likely affect sensitivity and stability of climate models to climate change and limit the predictive skill.
... Transport changes due to wind are two orders of magnitude smaller and thus deemed negligible (Fig. 11). The subpolar gyre is at least partly controlled by the density contrast between its center and rim (Born et al. 2009; Häkkinen and Rhines 2004). A higher density in the center requires a deeper depression of the sea surface, if a level of no motion is assumed at depth where horizontal density gradients disappear . ...
... d Vertically integrated transport as shown in left panels, zonally averaged over the gyre center (40°W–20°W) and meridionally integrated in order to obtain a measure of the gyre strength difference. For details on the calculation, refer to Born et al. (2009). The black curve corresponds to the transport derived from the density field (a), the gray one to the simulated velocities (b). ...
We argue that Arctic sea ice played an important role during early stages of the last glacial inception. Two simulations of
the Institut Pierre Simon Laplace coupled model 4 are analyzed, one for the time of maximum high latitude summer insolation
during the last interglacial, the Eemian, and a second one for the subsequent summer insolation minimum, at the last glacial
inception. During the inception, increased Arctic freshwater export by sea ice shuts down Labrador Sea convection and weakens
overturning circulation and oceanic heat transport by 27 and 15%, respectively. A positive feedback of the Atlantic subpolar
gyre enhances the initial freshening by sea ice. The reorganization of the subpolar surface circulation, however, makes the
Atlantic inflow more saline and thereby maintains deep convection in the Nordic Seas. These results highlight the importance
of an accurate representation of dynamic sea ice for the study of past and future climate changes.
KeywordsSea ice-Ocean circulation-Eemian-Glacial inception-Subpolar gyre-North Atlantic
... It becomes clear that a cyclonic circulation requires a dense core of water around which lighter water can circulate. This is equivalent to outcropping of isopycnals in the center of the SPG, that has been associated with the strength of the subpolar gyre circulation in previous studies (Häkkinen and Rhines, 2004; Levermann and Born, 2007; Lohmann et al., 2009b). ...
... The importance of the density field for the SPG strength is well documented in observations (Mellor et al., 1982; Greatbatch et al., 1991; Mellor , 1999) and numerical models (Myers et al., 1996; Penduff et al., 2000; Born et al., 2009; Montoya et al., 2011). Circulation changes of recent decades have been attributed to local buoyancy forcing (Häkkinen and Rhines, 2004; Lohmann et al., 2009a). ...
Internal variability of the Atlantic subpolar gyre is investigated in a 600years control simulation of a comprehensive coupled
climate model. The subpolar gyre shows irregular oscillations of decadal time scale with most spectral power between 15 and
20years. Positive and negative feedback mechanisms act successively on the circulation leading to an internal oscillation.
This involves periodically enhanced deep convection in the subpolar gyre center and intermittently enhanced air-sea thermal
coupling. As a result, anomalies of the large-scale atmospheric circulation can be transferred to the ocean on the ocean’s
intrinsic time scale, exciting the oscillator stochastically. A detailed understanding of oscillatory mechanisms of the ocean
and their sensitivity to atmospheric forcing holds considerable potential for decadal predictions as well as for the interpretation
of proxy data records.
KeywordsOcean circulation–North Atlantic–Subpolar gyre–Decadal variability
... Whitehead, 1998; Käse and Oschlies, 2000; Girton et al., 2001). Several different model studies showed that an overflow parametrization based on hydraulic constraints leads to a more realistic representation of the Denmark Strait Overflow (Kösters et al., 2005), a stronger AMOC and temperatures and salinities in the Nordic Seas that remain closer to observed values (Born et al., 2009). Moreover, the sea ice edge is located much further northwards, again in a better agreement with observations (Kösters et al., 2005). ...
... As a consequence , the ice edge (not shown) is pushed back from about 65 @BULLET N in NHYDP to about 75 @BULLET N in HYDP which confirms the findings of Kösters et al. (2005) and is in a better accordance with observations. This was also confirmed by Born et al. (2009) who used the Kösters et al. (2005) parametrization in the coupled climate model CLIMBER-3α (Montoya et al., 2005). ...
Several 1000 yr runs of the University of Victoria Earth System Climate Model (UVic ESCM) with a hydraulically controlled overflow in the Denmark Strait are used to analyse the effects of NAO-like variations of the wind stress localized in the subpolar North Atlantic. The focus is laid on improving the representation of the Atlantic meridional overturning circulation (AMOC), the sea surface temperatures in the Nordic Seas and the sea ice coverage without increasing the resolution of the global model. We show that by implementing hydraulic control in the Denmark Strait Overflow the AMOC can be enhanced at depths between 1000 and 3000 m by up to 7 Sverdrup (Sv) towards more realistic values. The stability of the Deep Western Boundary Current is considerably enhanced. The expansion of sea ice into the Nordic Seas in the standard run is pushed back from about 65 degrees N to 75 degrees N when hydraulic parametrization is switched on. In this case sea ice variations at 75 degrees N and Northern Europe air temperatures exhibit a lag of 9 yr to variations in the wind stress curl.
... Based on models and observations, the sill depths of the ridge have been suggested to influence the strength of the Atlantic Meridional Overturning Circulation (AMOC) and, consequently, the ocean heat transport toward the Nordic Seas (Roberts and Wood 1997). However, not all studies find that sill depths have an effect on the AMOC strength (Born, Levermann, and Mignot 2009;Robinson et al. 2011). Nevertheless, the ridge shapes the distribution of waters masses around Iceland, hence the environment (Jochumsen, Schnurr, and Quadfasel 2016) and therefore life, functional processes, and biodiversity (Brix and Svavarsson 2010;Brix, Stransky, et al. 2018;Brix, Lörz, et al. 2018). ...
The Greenland-Scotland Ridge is a submarine mountain that rises up to 500 m below the sea surface and extends from the east coast of Greenland to the continental shelf of Iceland and across the Faroe Islands to Scotland. The ridge not only separates deeper ocean basins on either side, that is, the North Atlantic and Arctic oceans, but also forms a geomorphological barrier between the cold arctic water masses of the Nordic Seas and the comparably contrastingly warmer water of the North Atlantic Ocean. It is therefore situated at a strategic geographical position in relation to the effect of climate change in the Arctic region. Both the Arctic and the Atlantic subpolar ecosystems are facing each other at the ridge, creating oceanic fronts in the Denmark Strait and in the Iceland-Faroe ridge alike. This ridge in the subarctic area forms the southern boundary of the North Atlantic Gateway to the Arctic Ocean, affecting exchanges of oceanic currents and of marine organisms between the two main ecosystems in the Nordic polar region. For example, the appearance of natural invasive species such as the Atlantic mackerel in this region mainly occurred along the ridge, with arrival through the Scotland-Faroe Islands mount with subsequent waves of col-onization which eventually reached the southern tip of Greenland. With the increasing impacts of climate change, such natural colonization through the ridge is likely to happen more frequently and affect regional ecosystems. Yet, the human resources and the economy of the local nations on the ridge are rather limited compared to neighboring countries. With a total of less than half a million people inhabiting the area and a total ocean surface of circa 3 million km 2 of continental shelf, Greenland, Iceland, the Faroe Islands, and Scotland will face critical challenges in the coming years with respect to biodiversity conservation and sustainable management of marine resources. Here is a summary of what we know, what we might expect, and an opening to potential discussions for the future of research in this region. The main objective of this paper is calling attention to much needed additional research effort on the marine environment around the Greenland-Scotland Ridge, instead of presenting a comprehensive overview of research in this area.
... Based on models and observations, the sill depths of the ridge have been suggested to influence the strength of the Atlantic Meridional Overturning Circulation (AMOC) and, consequently, the ocean heat transport toward the Nordic Seas (Roberts and Wood 1997). However, not all studies find that sill depths have an effect on the AMOC strength (Born, Levermann, and Mignot 2009;Robinson et al. 2011). Nevertheless, the ridge shapes the distribution of waters masses around Iceland, hence the environment (Jochumsen, Schnurr, and Quadfasel 2016) and therefore life, functional processes, and biodiversity (Brix and Svavarsson 2010;Brix, Stransky, et al. 2018;Brix, Lörz, et al. 2018). ...
The Greenland-Scotland Ridge is a submarine mountain that rises up to 500 m below the sea surface and extends from the east coast of Greenland to the continental shelf of Iceland and across the Faroe Islands to Scotland. The ridge not only separates deeper ocean basins on either side, that is, the North Atlantic and Arctic oceans, but also forms a geomorphological barrier between the cold arctic water masses of the Nordic Seas and the comparably contrastingly warmer water of the North Atlantic Ocean. It is therefore situated at a strategic geographical position in relation to the effect of climate change in the Arctic region. Both the Arctic and the Atlantic subpolar ecosystems are facing each other at the ridge, creating oceanic fronts in the Denmark Strait and in the Iceland-Faroe ridge alike. This ridge in the subarctic area forms the southern boundary of the North Atlantic Gateway to the Arctic Ocean, affecting exchanges of oceanic currents and of marine organisms between the two main ecosystems in the Nordic polar region. For example, the appearance of natural invasive species such as the Atlantic mackerel in this region mainly occurred along the ridge, with arrival through the Scotland-Faroe Islands mount with subsequent waves of col-onization which eventually reached the southern tip of Greenland. With the increasing impacts of climate change, such natural colonization through the ridge is likely to happen more frequently and affect regional ecosystems. Yet, the human resources and the economy of the local nations on the ridge are rather limited compared to neighboring countries. With a total of less than half a million people inhabiting the area and a total ocean surface of circa 3 million km 2 of continental shelf, Greenland, Iceland, the Faroe Islands, and Scotland will face critical challenges in the coming years with respect to biodiversity conservation and sustainable management of marine resources. Here is a summary of what we know, what we might expect, and an opening to potential discussions for the future of research in this region. The main objective of this paper is calling attention to much needed additional research effort on the marine environment around the Greenland-Scotland Ridge, instead of presenting a comprehensive overview of research in this area.
... Fig. 4 and S4 in their study). In particular, the center of negative anomalies located south of Greenland appears to be linked with a weakening of an AMOC cell centered at 50 • N. A downslope of deep western boundary current (DWBC), which is fed by the Nordic Seas overflow system, could result in the bottom vortex stretching with corresponding ocean surface changes on the Northern Recirculation Gyre, associated with adjustment of the Gulf Stream position (Born et al., 2009;Langehaug et al., 2012;Yeager and Danabasoglu, 2014;Zhang et al., 2011;Zhang and Vallis, 2006). An anomalous DWBC affects also the subpolar AMOC cell, through geostrophic balance. ...
Study region
Europe, with a particular focus on the Czech Republic, Poland, Spain, and Norway.
Study focus
Long-lasting droughts have become a semi-permanent feature of the European climate, especially over the last two decades. These prolonged droughts are usually driven by persistent sea surface temperature anomalies over the Pacific and Atlantic basins. By employing complex statistical methods (i.e., Canonical Correlation Analysis and Convergent Cross-Mapping) in this study we make a comprehensive assessment of the observed drying trend over the central and southern parts of Europe and its underlying drivers.
New hydrological insights
Building upon the potential relationship between drought variability and large-scale oceanic and atmospheric circulation, we show that the observed drying trend in the central and southern parts of Europe has been driven by a long-term slowdown of the Atlantic Meridional Overturning Circulation (AMOC), via changes in the large-scale atmospheric circulation. A weakening of AMOC leads to an increase in the frequency of atmospheric-blocking like circulation over the central part of Europe, which in turn inhibits precipitation and favors long-term drying. Since climate projections indicate a slowdown of the AMOC in the future, we suggest that this will potentially lead to an increase in the frequency of dry years, especially over the central and southern parts of Europe (e.g., the eastern part of Germany, the Czech Republic, Poland, Spain and Portugal).
... The corresponding time component is characterized by a centennial-scale trend (Fig. 1a). The Trend Mode (hereafter TM) pattern is marked by three centers of alternating signs, disposed from SW to SE of Greenland (Fig. 1c), a structure which was linked to overturning changes (Born et al. 2009;Zhang et al. 2011;Drijfhout et al. 2012). ...
The Atlantic Meridional Overturning Circulation (AMOC) is a tipping component of the climate system, with a quasi-global impact. Several numerical and observational studies emphasized two modes of AMOC variability, characterized by two distinct Atlantic sea surface temperature patterns. One is associated with centennial changes, the Trend Mode, and the other with the Atlantic Multidecadal Oscillation (AMO). The origin of the different manifestations of these modes it is not fully understood. Using observational data and an ocean general circulation model we present evidence that, whereas the Trend Mode is mainly linked with deep water formation in the Nordic Seas and with a North Atlantic AMOC cell centered at 50° N, AMO is related with deep water formation in the Labrador and Irminger Seas and with an overturning cell centered at 20° N. In combination with previous studies, these results imply that a main route of increasing atmospheric CO 2 concentration influence on AMOC passes through deep water formation in the Nordic Seas and it is reflected in a subpolar North Atlantic meridional cell.
... In addition, STIF shows larger potential density in the subpolar gyre (shading in Fig. 7c), compared to CTRL. The increased potential density corresponds to the intensified subpolar gyre in the Labrador Sea and the Iceland-Scotland Basin, which suggests that the subpolar gyre in part is controlled by the density (e.g., Häkkinen and Rhines 2004;Born et al. 2009). It is considered that AMOC variability is modulated by the buoyancy forcing, which relates to the air-sea surface heat flux and freshwater flux, and wind forcing (e.g., Kuhlbrodt et al. 2007;Marshall et al. 2014). ...
The Atlantic Meridional Overturning Circulation (AMOC) plays a central role in the decadal variability of global and regional climate through changing poleward transport of heat. However, realistic simulation of the AMOC, i.e., its strength and spatial structure, remains a challenge for ocean general circulation models (OGCMs) and coupled climate models. Here, we investigate how the simulated AMOC could be affected by improved accuracy of the seawater equation of state (EOS) with an OGCM. Two EOSs used in this study: the UNESCO EOS80, and the “stiffened” EOS derived from the compressibility of sea water and the UNESCO EOS80. Compared to the model using the UNESCO EOS80, the model using the “stiffened” EOS yields stronger deep convection in the Labrador Sea, the Irminger-Iceland-Scotland Basin, and the Greenland-Iceland-Norwegian (GIN) seas, which leads to an improvement in the simulation of the AMOC: Along 26.5°N, the maximum transport is increased from 14.9 to 17.4 Sv and the interface between the upper clockwise cell and lower counterclockwise cell is deepened from 2.8 to 3.3 km, both matching the observations better. Taken the Labrador Sea as an example, the processes, including both direct and indirect causes, that in part responsible for the improved AMOC are as follows. The use of “stiffened” EOS increases the density throughout the water column and weakens the stability of sea water. Moreover, the enhanced cabbeling and thermobaric effect strengthen the vertical advection, intensifying the deep convection and increasing formation of deep water, which eventually improves the simulation of the AMOC. The intensified AMOC, in turn, speeds up the surface return flow, transporting more warm and saline water to the high latitudes in the North Atlantic, which contributes to the densification of surface water. Similar analyses can be applied to the Iceland-Scotland Basin and GIN seas. Thus, the enhanced deep convection and formation of deep water in the Labrador Sea, as well as in the Iceland–Scotland Basin and GIN seas, improve the simulated AMOC.
... However, the instability depends on salt transport to the subpolar gyre through Denmark Straight and the Irminger Current in the decades prior to the abrupt warming. An additional preconditioning for the convection event is the overflow of extremely dense waters from the Nordic Seas (formed from brine rejected during sea ice formation during the cold stadial) which influences deep convection, the subpolar gyre, and heat/salt transport in the Irminger Current (Levermann and Born, 2007;Born et al., 2009;Born and Stocker, 2014), as well as the sensitivity of the subpolar gyre to wind forcing changes (Montoya et al., 2011). The simulated coolings are more gradual than the warmings, in agreement with observations ( Fig. 1). ...
The Dansgaard-Oeschger events of the last ice age are among the best studied abrupt climate changes, yet a comprehensive explanation is still lacking. They are most pronounced in the North Atlantic, where they manifest as large temperature swings, on timescales of decades or shorter, between persistent cold (stadial) and warm (interstadial) conditions. This review examines evidence that Dansgaard-Oeschger events are an unforced or “spontaneous” oscillation of the coupled atmosphere-ice-ocean system comprising the North Atlantic, Nordic Seas and Arctic, collectively termed the Northern Seas. Insights from reanalysis data, climate model simulations, and idealized box model experiments point to the subpolar gyre as a key coupling region where vigorous wind systems encounter the southernmost extension of sea ice and the most variable currents of the North Atlantic, with connections to the deep ocean via convection. We argue that, under special conditions, these components can interact to produce Dansgaard-Oeschger events. Finding the sweet spot is a matter of understanding when the subpolar region enters a feedback loop whereby changes in wind forcing, sea ice cover, and ocean circulation amplify and sustain perturbations towards cold (ice-covered) or warm (ice-free) conditions. The resulting Dansgaard-Oeschger-like variability is seen in a handful of model simulations, including some “ugly duckling” pre-industrial simulations: these may be judged as undesirable at the outset, but ultimately show value in suggesting that current models include the necessary physics to produce abrupt climate transitions, but exhibit incorrect sensitivity to the boundary conditions. Still, glacial climates are hypothesized to favour larger, more persistent transitions due to differences in large-scale wind patterns. Simplified models and idealized experimental setups may provide a means to constrain how the critical processes act, both in isolation and in combination, to destabilize the subpolar North Atlantic.
... The advection of heat and salt to the high latitudes via the Atlantic inflow is important not only for ameliorating the climate in Western Europe (Rossby, 1996) but also for promoting deep water formation. Additionally, while models disagree on the role that overflows play in AMOC variability, modelling studies have suggested that the density of the overflows and their magnitude can influence the surface hydrography south of the GSR, namely the subpolar gyre circulation (Born et al., 2009;Zhang et al., 2011) and the structure and strength of the AMOC as well as North Atlantic climate (Wang et al., 2015). ...
... A change in the density structure is found to provide positive feedbacks leading to a permanent strengthening of the SPG and convection. The impact of baroclinic adjustments on the SPG strength is well documented[Eden and Willebrand, 2001;Häkkinen and Rhines, 2004;Hátún et al., 2005;Treguier et al., 2005;Levermann and Born, 2007;Born et al., 2009;Lohmann et al., 2009;Born et al., 2010;Montoya et al., 2010]and this study aims to apply this physical understanding to data from the geological record.[5]To investigate the role of the SPG during the 8.2 ka event we make use of the coupled climate model CLIMBER‐3a, which comprises atmosphere and sea ice components and the oceanic general circulation model MOM‐3 (Montoya et al.[2005]and auxiliary material). 1 The oceanic horizontal resolution is 3.75° × 3.75° with 24 unevenly spaced vertical layers. ...
Climate model simulations of the 8.2 ka event show an abrupt strengthening of the Atlantic subpolar gyre that allows us to connect two major but apparently contradictory climate events of the early Holocene: the freshwater outburst from proglacial lakes and the onset of Labrador Sea water formation. The 8.2 ka event is the largest climatic signal of our present interglacial with a widespread cooling in the North Atlantic region about 8200 years before present. It coincides with a meltwater outburst from North American proglacial lakes that is believed to have weakened the Atlantic meridional overturning circulation and northward heat transport, followed by a recovery of the deep ocean circulation and rising temperatures after a few centuries. Marine proxy data, however, date the onset of deep water formation in Labrador Sea to the same time. The subsequent strengthening of the slope current system created a regional signal recorded as an abrupt and persistent surface temperature decrease. Although similarities in timing are compelling, a mechanism to reconcile these apparently contradictory events was missing. Our simulations show that an abrupt and persistent strengthening of the Atlantic subpolar gyre provides a plausible explanation. The intense freshwater pulse triggered a transition of the gyre circulation into a different mode of operation, stabilized by internal feedbacks and persistent after the cessation of the perturbation. As a direct consequence, deep water formation around its center intensifies. This corresponds to the modern flow regime and stabilizes the meridional overturning circulation, possibly contributing to the Holocene's climatic stability.
... Besides slightly different definitions of the mixed layer complicating a direct comparison, it is possible that the high‐resolution IPSL CM4 model requires a longer integration time to equilibrate the deep ocean with possible impact on the formation of deep water. At the same time, a known deficiency of coarse resolution models is the imperfect representation of the southward outflow from the Nordic Seas over the deep and narrow sills of the Greenland‐Scotland ridge that also influences the mixed layer depth [Roberts and Wood, 1997; Thorpe et al., 2004; Born et al., 2009]. As a result bottom waters leaving the Nordic Seas are less dense than observed and the reservoir of deep dense water in the Nordic Seas is not optimally represented in coarse resolution models which again impacts deep convection. ...
We analyze a transient simulation of the last glacial inception in a climate model of intermediate complexity, focusing on sea ice-ocean circulation dynamics in the North Atlantic and Nordic Seas. As northern high-latitude summer insolation decreases toward the end of the Eemian interglacial, Arctic sea ice export to the North Atlantic increases. This surface fresh water transport weakens deep water formation in the North Atlantic and the near-surface circulation of the subpolar gyre. As a consequence, the relative contribution of subpolar gyre waters to the Atlantic inflow into the Nordic Seas is reduced, giving way to more warm and saline subtropical waters from the North Atlantic Current. We thus find an episode of relatively high heat and salt transport into the Nordic Seas during the last glacial inception between 119,000 and 115,000 years before present. This stabilizes deep ocean convection in the region and warms Scandinavia during a phase of low insolation. These findings are in good agreement with proxy data from the Nordic Seas and North Atlantic. At the end of the warm interval, sea surface temperature drops by about 3C, marking the onset of large-scale glacier growth over Scandinavia.
... The density difference between the denser center and the lighter rim of the gyre induces a geostrophic flow which enhances the circulation. Depth integration of the momentum balance equations reveals that this baroclinic contribution is proportional to the difference in potential energy ∆χ ≡ g dz · z∆ρ between the center and the exterior of the SPG (e.g. [Born et al., 2009; Hattermann and Levermann, 2009] . Consequently the gyre strength correlates with the difference in potential energy rather than density which can be verified in our simulations (fig. ...
... At present, the subpolar gyre water is highly influenced by the warm and high salinity water of the NAC because open ocean convection occurs in the Labrador Sea (Schmitz and McCartney, 1993). However, model simulations and oceanic observations suggest that a higher sea ice export through the EGC may change the characteristics of subpolar gyre water masses reducing convection in the Labrador Sea and causing the expansion of winter sea ice (Hakkinen and Rhines, 2004; Hatun et al., 2005; Born et al., 2009; Born et al., 2010). The characteristics of Polar, Arctic and Atlantic waters are well known for the modern NGS (Hansen and Nansen, 1909; Dietrich, 1969; Swift and Aagaard, 1981; Swift, 1986). ...
Surface water conditions at the Integrated Ocean Drilling Program (IODP) Site U1314 (Southern Gardar Drift, 56° 21.8′ N, 27° 53.3′ W, 2820 m depth) were inferred using planktic foraminifer assemblages between Marine Isotope Stage (MIS) 19 and 11 (ca. 800–400 ka). Factor analysis of the planktic foraminifer assemblages suggests that the assemblage was controlled by three factors. The first factor (which explained 49% of the variance) is dominated by transitional and subpolar species and points to warm and salty surface water conditions (Atlantic water). The second factor (37%) is dominated by Neogloboquadrina pachyderma sin and has been associated with the presence of cold and low saline surface waters (Arctic water). Finally, the third factor (9%), linked to a significant presence of Turborotalita quinqueloba, reflects the closeness of the Arctic front (the boundary between Atlantic and Arctic water). The position of the Arctic and Polar fronts has been estimated across the glacial–interglacial cycles studied according to planktic foraminifer abundances from Site U1314 (and their factor analysis) combined with a synthesis of planktic foraminifer and diatom data from other North Atlantic sites. Regarding at the migrations of the Arctic front and the surface water masses distribution across each climatic cycle we determined five phases of development. Furthermore, deep ocean circulation changes observed in glacial–interglacial cycles have been associated with each phase.
... The weaker downward mixing of freshwater in the supercritical case leads to saltier and denser subsurface waters (100–300 m depth) north of and at the latitude of the GSR, i.e. at the northern rim of the SPG, as seen again inFig. 4. The SPG is driven both by the large scale wind pattern and the local density distribution, circulating cyclonically around the dense water in its core, close to geostrophic balance (for a detailed description of the base state of the SPG in the model compare Born et al. 2009). In the supercritical case, the dense anomaly north of and at the latitude of the GSR reduces the meridonal density gradient across the SPG. ...
We use a reduced complexity climate model with a three-dimensional ocean component and realistic topography to investigate
the effect of stratification-dependent mixing on the sensitivity of the North Atlantic subpolar gyre (SPG), and the Atlantic
meridional overturning circulation (AMOC), to idealized CO2 increase and peaking scenarios. The vertical diffusivity of the ocean interior is parameterized as κ∼N
−α, where N is the local buoyancy frequency. For all parameter values 0≤α≤3, we find the SPG, and subsequently the AMOC, to weaken
in response to increasing CO2 concentrations. The weakening is significantly stronger for α≥αcr≈1.5. Depending on the value of α, two separate model states develop. These states remain different after the CO2 concentration is stabilized, and in some cases even after the CO2 concentration has been decreased again to the pre-industrial level. This behaviour is explained by a positive feedback between
stratification and mixing anomalies in the Nordic Seas, causing a persistent weakening of the SPG.
KeywordsSubpolar gyre-Global warming-Ocean mixing-Atlantic overturning
The Atlantic Meridional Overturning Circulation (AMOC), a tipping component of the climate system, is projected to slowdown during the 21st century in response to increased atmospheric CO 2 concentration. The rate and start of the weakening are associated with relatively large uncertainties. Observed sea surface temperature-based reconstructions indicate that AMOC has been weakening since the mid-20th century, but its forcing factors are not fully understood. Here we provide dynamical observational evidence that the increasing atmospheric CO 2 concentration affects the North Atlantic heat fluxes and precipitation rate, and weakens AMOC, consistent with numerical simulations. The inferred weakening, starting in the late 19th century, earlier than previously suggested, is estimated at 3.7 ± 1.0 Sv over the 1854-2016 period, which is larger than it is shown in numerical simulations (1.4 ± 1.4 Sv).
The cyclonic circulation of the Atlantic subpolar gyre is a key mechanism for North Atlantic climate variability on a wide range of time scales. It is generally accepted that it is driven by both cyclonic winds and buoyancy forcing, yet the individual importance and dynamical interactions of the two contributions remain unclear. The authors propose a simplified four-box model representing the convective basin of the Labrador Sea and its shallow and deep boundary current system, the western subpolar gyre. Convective heat loss drives a baroclinic flow of relatively light water around the dense center. Eddy salt flux from the boundary current to the center increases with a stronger circulation, favors the formation of dense waters, and thereby sustains a strong baroclinic flow, approximately 10%-25% of the total. In contrast, when the baroclinic flow is not active, surface waters may be too fresh to convect, and a buoyancy-driven circulation cannot develop. This situation corresponds to a second stable circulation mode. A hysteresis is found for variations in surface freshwater flux and the salinity of the near-surface boundary current. An analytical solution is presented and analyzed.
We propose an idealized four-box model representing the convective basin
of the Labrador Sea and its boundary current system. The simulated
circulation of the subpolar gyre (SPG) shows instabilities with regard
to local and remote freshwater fluxes. A hysteresis of the circulation
exist for a large part of the parameter space and potentially present
day climate. This nonlinearity is due to a positive feedback of
increased eddy salt flux for a stronger SPG circulation, not unlike salt
advection in Stommel-type models of the thermohaline circulation. The
idealized model is compared with simulations of general circulation
models of past and present climates. Potential consequences for decadal
predictability and future climate changes are discussed.
Dense gravity current overflows occur in several regions throughout the
world and are an important process in the meridional overturning
circulation. Overflows are poorly represented in coarse resolution level
coordinate ocean climate models. Here, the embedded Lagrangian model
formulated in the companion paper of Bates et al. (2012) is used in two
idealised test cases to examine the effect on the representation of
dense gravity driven plumes, as well as the effect on the circulation of
the bulk ocean in the Eulerian model. The results are compared with
simulations with no parameterisation for overflows, as well as
simulations that use traditional hydrostatic overflow schemes.The use of
Lagrangian "blobs" is shown to improve three key characteristics that
are poorly represented in coarse resolution level coordinate models: (1)
the depth of the plume, (2) the along slope velocity of the plume, and
(3) the response of the bulk ocean to the bottom boundary layer. These
improvements are associated with the more appropriate set of dynamics
satisfied by the blobs, leading to a more physically sound
representation. Experiments are also conducted to examine sensitivity to
blob parameters. The blob parameters are examined over a large parameter
space.
A new overflow parameterization (OFP) of density-driven flows through ocean ridges via narrow, unresolved channels has been developed and implemented in the ocean component of the Community Climate System Model version 4. It represents exchanges from the Nordic Seas and the Antarctic shelves, associated entrainment, and subsequent injection of overflow product waters into the abyssal basins. We investigate the effects of the parameterized Denmark Strait (DS) and Faroe Bank Channel (FBC) overflows on the ocean circulation, showing their impacts on the Atlantic Meridional Overturning Circulation and the North Atlantic climate. The OFP is based on the Marginal Sea Boundary Condition scheme of Price and Yang (1998), but there are significant differences that are described in detail. Two uncoupled (ocean-only) and two fully coupled simulations are analyzed. Each pair consists of one case with the OFP and a control case without this parameterization. In both uncoupled and coupled experiments, the parameterized DS and FBC source volume transports are within the range of observed estimates. The entrainment volume transports remain lower than observational estimates, leading to lower than observed product volume transports. Due to low entrainment, the product and source water properties are too similar. The DS and FBC overflow temperature and salinity properties are in better agreement with observations in the uncoupled case than in the coupled simulation, likely reflecting surface flux differences. The most significant impact of the OFP is the improved North Atlantic Deep Water penetration depth, leading to a much better comparison with the observational data and significantly reducing the chronic, shallow penetration depth bias in level coordinate models. This improvement is due to the deeper penetration of the southward flowing Deep Western Boundary Current. In comparison with control experiments without the OFP, the abyssal ventilation rates increase in the North Atlantic. In the uncoupled simulation with the OFP, the warm bias of the control simulation in the deep North Atlantic is substantially reduced along with salinity bias reductions in the northern North Atlantic. There are similar but more modest bias reductions in the deep temperature and salinity distributions especially in the northern North Atlantic in the coupled OFP case. In coupled simulations, there are noticeable impacts of the OFP on climate. The sea surface temperatures (SSTs) are warmer by more than 5°C off the North American coast and by more than 1°C in the Nordic Sea with the OFP. The surface heat fluxes mostly act to diminish these SST changes. There are related changes in the sea level pressure, leading to about 15% weaker westerly wind stress in the northern North Atlantic. In response to the warmer Nordic Sea SSTs, there are reductions in the sea ice extent, improving comparisons with observations. Although the OFP cases improve many aspects of the simulations compared to observations, some significant biases remain, more in coupled than in uncoupled simulations.
The Atlantic Meridional Overturning Circulation (MOC) is defined, and our present understanding of MOC driving mechanisms is summarized. Evidence for the changing MOC is reviewed, covering recent developments in observing and modeling the MOC, and the climatic consequences of MOC variability. On a timescale of the next 5-10 years, further developments in MOC monitoring, modeling and prediction are both anticipated and recommended. In the context of what is presently known about the MOC, the evidence for a recent slowing trend is considered. Copyright © 2010 John Wiley & Sons, Inc.
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The dynamics of the North Atlantic subpolar gyre (SPG) are assessed under present and glacial boundary conditions by investigating
the SPG sensitivity to surface wind-stress changes in a coupled climate model. To this end, the gyre transport is decomposed
in Ekman, thermohaline, and bottom transports. Surface wind-stress variations are found to play an important indirect role
in SPG dynamics through their effect on water-mass densities. Our results suggest the existence of two dynamically distinct
regimes of the SPG, depending on the absence or presence of deep water formation (DWF) in the Nordic Seas and a vigorous Greenland–Scotland
ridge (GSR) overflow. In the first regime, the GSR overflow is weak and the SPG strength increases with wind-stress as a result
of enhanced outcropping of isopycnals in the centre of the SPG. As soon as a vigorous GSR overflow is established, its associated
positive density anomalies on the southern GSR slope reduce the SPG strength. This has implications for past glacial abrupt
climate changes, insofar as these can be explained through latitudinal shifts in North Atlantic DWF sites and strengthening
of the North Atlantic current. Regardless of the ultimate trigger, an abrupt shift of DWF into the Nordic Seas could result
both in a drastic reduction of the SPG strength and a sudden reversal in its sensitivity to wind-stress variations. Our results
could provide insight into changes in the horizontal ocean circulation during abrupt glacial climate changes, which have been
largely neglected up to now in model studies.
Using the coupled climate model of intermediate complexity, CLIMBER-3, changes in the vertical thermal structure associated with a shutdown of the Atlantic meridional overturning circulation (AMOC) are investigated. When North Atlantic Deep Water formation is inhibited by anomalous freshwater forcing, intermediate depth ventilation can remain active and cool the subsurface water masses (i.e., the "cold case"). However, if intermediate ventilation is completely suppressed, relatively warm water coming from the south penetrates to a high northern latitude beneath the halocline and induces a strong vertical tem- perature inversion between the surface and intermediate depth (i.e., the "warm case"). Both types of temperature anomalies emerge within the first decade after the beginning of the freshwater perturbation. The sign of subsurface temperature anomaly has a strong implication for the recovery of the AMOC once the anomalous freshwater forcing is removed. While the AMOC recovers from the cold case on centennial time scales, the recovery is much more rapid (decadal time scales) when ventilation is completely sup- pressed and intermediate depths are anomalously warm. This is explained by a more rapid destabilization of the water column after cessation of the anomalous flux due to a strong vertical temperature inversion. A suite of sensitivity experiments with varying strength and duration of the freshwater perturbation and a larger value of background vertical diffusivity demonstrate robustness of the phenomenon. Implications of the simulated subsurface temperature response to the shutdown of the AMOC for future climate and abrupt climate changes of the past are discussed.
Because of its relevance for the global climate the Atlantic meridional overturning circulation (AMOC) has been a major research focus for many years. Yet the question of which physical mechanisms ultimately drive the AMOC, in the sense of providing its energy supply, remains a matter of controversy. Here we review both observational data and model results concerning the two main candidates: vertical mixing processes in the ocean's interior and wind-induced Ekman upwelling in the Southern Ocean. In distinction to the energy source we also discuss the role of surface heat and freshwater fluxes, which influence the volume transport of the meridional overturning circulation and shape its spatial circulation pattern without actually supplying energy to the overturning itself in steady state. We conclude that both wind-driven upwelling and vertical mixing are likely contributing to driving the observed circulation. To quantify their respective contributions, future research needs to address some open questions, which we outline.
On the basis of hydrographic observations taken in the vicinity of
Denmark Strait, a primitive equation model is used to investigate
physical mechanisms that control the exchange through the strait. The
dense water transport is topographically controlled and predictions by
Whitehead [1998] and Killworth and McDonald [1993] are consistent with
numerical model results. The distribution of temperature and thickness
of the modeled plume is in good agreement with the high-resolution
hydrographic data.
The heat transported northwards by the North Atlantic thermohaline
circulation warms the climate of western Europe. Previous model studies
have suggested that the circulation is sensitive to increases in
atmospheric greenhouse-gas concentrations, but such models have been
criticised for the use of unphysical `flux adjustments' (artificial
corrections that keep the model from drifting to unrealistic states),
and for their inability to simulate deep-water formation both north and
south of the Greenland-Iceland-Scotland ridge, as seen in observations,.
Here we present simulations of today's thermohaline circulation using a
coupled ocean-atmosphere general circulation model without flux
adjustments. These simulations compare well with the observed
thermohaline circulation, including the formation of deep water on each
side of the Greenland-Iceland-Scotland ridge. The model responds to
forcing with increasing atmospheric greenhouse-gas concentrations by a
collapse of the circulation and convection in the Labrador Sea, while
the deep-water formation north of the ridge remains stable. These
changes are similar intwo simulations with different rates of increase
of CO2 concentrations. The effects of increasing atmospheric
greenhouse-gas concentrations that we simulate are potentially
observable, suggesting that it is possible to set up an oceanic
monitoring system for the detection of anthropogenic influence on ocean
circulation.
The sensitivity of a global thermodynamic-dynamic sea ice model coupled to a one-dimensional upper ocean model to degradations of the model physics is investigated. A single set of parameter values is employed to simultaneously simulate the Arctic and Antarctic ice regimes. A control run carried out with the model demonstrates that it does reasonably well in simulating the seasonal waxing and waning of both ice packs. The sensitivity study focuses on physical processes pertaining to (1) the vertical growth and decay of sea ice, (2) the lateral growth and decay of sea ice, and (3) the sea ice dynamics. A total of nine sensitivity experiments have been performed. It appears that the thermal inertia of the snow-ice system is negligible in the Antarctic but not in the Arctic. It is also found that the inclusion of a prognostic snow layer and of a scheme of snow ice formation is important for sea ice modeling in the Southern Hemisphere. Furthermore, results suggest that the thermodynamic effect of the subgrid-scale snow and ice thickness distributions, the existence of open water areas within the ice cover, and the ice motion play a crucial role in determining the seasonal behavior of both ice packs.
The sensitivity of the Atlantic Ocean meridional overturning circulation to the vertical diffusion coeffi- cient in the global coupled atmosphere-ocean-sea ice model CLIMBER-3 is investigated. An important feature of the three-dimensional ocean model is its low-diffusive tracer advection scheme. The strength Mmax of the Atlantic overturning is decomposed into three components: 1) the flow MS exported southward at 30°S, 2) the large-scale upward flow that balances vertical diffusion in the Atlantic, and 3) a wind- dependent upwelling flux Wbound along the Atlantic boundaries that is not due to vertical diffusion. The export of water at 30°S varies only weakly with , but is strongly correlated with the strength of the overflow over the Greenland-Scotland ridge. The location of deep convection is found to be mixing dependent such that a shift from the Nordic seas to the Irminger Sea is detected for high values of . The ratio R MS/Mmax gives a measure of the interhemispheric overturning efficiency and is found to decrease linearly with . The diffusion-induced upwelling in the Atlantic is mostly due to the uniform background value of while parameterization of enhanced mixing over rough topography and in stratified areas gives only a weak contribution to the overturning strength. It increases linearly with . This is consistent with the classic 2/3 scaling law only when taking the linear variation of the density differenc et o into account. The value of Wbound is roughly constant with but depends linearly on the wind stress strength in the North Atlantic. The pycnocline depth is not sensitive to changes in in the model used herein, and the results suggest that it is primarily set by the forcing of the Southern Ocean winds. The scaling of the total overturning strength with depends on the combined sensitivity of each of the terms to . In the range of background diffusivity values in which no switch in deep convection sites is detected, Mmax scales linearly with the vertical diffusivity. It is argued that scalings have, in general, to be interpreted with care because of the generally very small range of but also because of possible shifts in important physical processes such as deep convection location.
High resolution models indicate that the dynamics of the Nordic Seas and the subpolar gyre is crucial for deep water formation and overturning circulation in the Atlantic. During the last decades significant changes in ocean properties have been observed in this region. We show that large-scale dynamics, as captured by coarse resolution climate models, allows for a bistability of the Atlantic subpolar gyre. We suggest three positive feedbacks which yield the necessary nonlinearity for this behavior. Transitions between the two states can be triggered by small fluctuations in surface freshwater flux to the Nordic Seas, only slightly larger than natural variability presently observed in the region.
The properties of watermass transformation and the thermohaline circulation in marginal seas with topography and subject to a spatially uniform net surface cooling are discussed. The net heat loss within the marginal sea is ultimately balanced by lateral advection from the open ocean in a narrow boundary current that flows cy-clonically around the basin. Heat loss in the interior is offset by lateral eddy fluxes originating in the boundary current. The objectives of this study are to understand better what controls the density of waters formed within the marginal sea, the temperature of the outflowing waters, the amount of downwelling, and the mechanisms of heat transport within the marginal sea. The approach combines heat budgets with linear stability theory for a baroclinic flow over a sloping bottom to provide simple theoretical estimates of each of these quantities in terms of the basic parameters of the system. The theory compares well to a series of eddy-resolving primitive equation model calculations. The downwelling is concentrated within the boundary current in both a diffusive boundary layer near topography and an eddy-driven region on the offshore edge of the boundary current. For most high-latitude regions, the horizontal gyre is expected to transport more heat than does the overturning gyre.
1] Analyses of sea surface height (SSH) records based on satellite altimeter data and hydrographic properties have suggested a considerable weakening of the North Atlantic subpolar gyre during the 1990s. Here we report hindcast simulations with high-resolution ocean circulation models that demonstrate a close correspondence of the SSH changes with the volume transport of the boundary current system in the Labrador Sea. The 1990s-decline, of about 15% of the long-term mean, appears as part of a decadal variability of the gyre transport driven by changes in both heat flux and wind stress associated with the North Atlantic Oscillation (NAO). The changes in the subpolar gyre, as manifested in the deep western boundary current off Labrador, reverberate in the strength of the meridional overturning circulation (MOC) in the subtropical North Atlantic, suggesting the potential of a subpolar transport index as an element of a MOC monitoring system.
The authors present the first quantitative comparison between new velocity datasets and high-resolution models in the North Atlantic subpolar gyre [ 1 ⁄10° Parallel Ocean Program model (POPNA10), Miami Isopycnic Coordinate Ocean Model (MICOM), 1 ⁄6° Atlantic model (ATL6), and Family of Linked Atlantic Ocean Model Experiments (FLAME)]. At the surface, the model velocities agree generally well with World Ocean Circulation Experiment (WOCE) drifter data. Two noticeable exceptions are the weakness of the East Greenland coastal current in models and the presence in the surface layers of a strong southwestward East Reykjanes Ridge Current. At depths, the most prominent feature of the circulation is the boundary current following the continental slope. In this narrow flow, it is found that gridded float datasets cannot be used for a quantitative comparison with models. The models have very different patterns of deep convec-tion, and it is suggested that this could be related to the differences in their barotropic transport at Cape Farewell. Models show a large drift in watermass properties with a salinization of the Labrador Sea Water. The authors believe that the main cause is related to horizontal transports of salt because models with different forcing and vertical mixing share the same salinization problem. A remarkable feature of the model solutions is the large westward transport over Reykjanes Ridge [10 Sv (Sv 10 6 m 3 s 1) or more].
A 2.5-dimensional climate system model of intermediate complexity CLIMBER-2 and its performance for present climate conditions
are presented. The model consists of modules describing atmosphere, ocean, sea ice, land surface processes, terrestrial vegetation
cover, and global carbon cycle. The modules interact through the fluxes of momentum, energy, water and carbon. The model has
a coarse spatial resolution, nevertheless capturing the major features of the Earth's geography. The model describes temporal
variability of the system on seasonal and longer time scales. Due to the fact that the model does not employ flux adjustments
and has a fast turnaround time, it can be used to study climates significantly different from the present one and to perform
long-term (multimillennia) simulations. The comparison of the model results with present climate data show that the model
successfully describes the seasonal variability of a large set of characteristics of the climate system, including radiative
balance, temperature, precipitation, ocean circulation and cryosphere.
We herein present the CLIMBER-3α Earth System Model of Intermediate Complexity (EMIC), which has evolved from the CLIMBER-2
EMIC. The main difference with respect to CLIMBER-2 is its oceanic component, which has been replaced by a state-of-the-art
ocean model, which includes an ocean general circulation model (GCM), a biogeochemistry module, and a state-of-the-art sea-ice
model. Thus, CLIMBER-3α includes modules describing the atmosphere, land-surface scheme, terrestrial vegetation, ocean, sea
ice, and ocean biogeochemistry. Owing to its relatively simple atmospheric component, it is approximately two orders of magnitude
faster than coupled GCMs, allowing the performance of a much larger number of integrations and sensitivity studies as well
as longer ones. At the same time its oceanic component confers on it a larger degree of realism compared to those EMICs which
include simpler oceanic components. The coupling does not include heat or freshwater flux corrections. The comparison against
the climatologies shows that CLIMBER-3α satisfactorily describes the large-scale characteristics of the atmosphere, ocean
and sea ice on seasonal timescales. As a result of the tracer advection scheme employed, the ocean component satisfactorily
simulates the large-scale oceanic circulation with very little numerical and explicit vertical diffusion. The model is thus
suited for the study of the large-scale climate and large-scale ocean dynamics. We herein describe its performance for present-day
boundary conditions. In a companion paper (Part II), the sensitivity of the model to variations in the external forcing, as
well as the role of certain model parameterisations and internal parameters, will be analysed.
We investigate the model sensitivity of the Atlantic meridional overturning circulation (AMOC) to anomalous freshwater flux in the tropical and northern Atlantic. Forcing in both locations leads to the same qualitative response: a positive freshwater anomaly induces a weakening of the AMOC and a negative freshwater anomaly strengthens the AMOC. Strong differences arise in the temporal characteristics and amplitude of the response. The advection of the tropical anomaly up to the deep water formation area leads to a time delayed response compared to a northern forcing. Thus, in its transient response, the AMOC is less sensitive to a constant anomalous freshwater flux in the tropics than in the north. This difference decreases with time and practically vanishes in equilibrium with constant freshwater forcing. The equilibrium response of the AMOC shows a non-linear dependence on freshwater forcing in both locations, with a stronger sensitivity to positive freshwater forcing. As a consequence, competitive forcing in both regions is balanced when the negative forcing is about 1.5 times larger than the positive forcing. The relaxation time of the AMOC after termination of a freshwater perturbation depends significantly on the AMOC strength itself. A strong overturning exhibits a faster relaxation to its unperturbed state. By means of a set of complementary experiments (pulse-perturbations, constant and stochastic forcing) we quantify these effects and discuss the corresponding time scales and physical processes.
[1] Hydraulic constraints on the Denmark Strait overflow (DSO) are used as a parameterisation to improve the overflow representation in a global climate model. The parameterisation increases deep water formation in the Nordic Seas and strengthens the Norwegian Atlantic Current. Associated higher northward heat transport leads to a northward shift of the sea-ice edge and warming by 2.5 degreesC in the eastern Nordic Seas despite a small effect on the Atlantic Meridional Overturning Circulation (AMO). This emphasises the impact of the DSO on climate even though the response in overturning due to the DSO representation in this model is less than expected from previous studies using ocean only models. In contrast to previous studies almost no stabilising effect of the overflow on the AMO is found to freshwater perturbations.
The NCEP and NCAR are cooperating in a project (denoted "reanalysis") to produce a 40-year record of global analyses of atmospheric fields in support of the needs of the research and climate monitoring communities. This effort involves the recovery of land surface, ship, rawinsonde, pibal, aircraft, satellite, and other data; quality controlling and assimilating these data with a data assimilation system that is kept unchanged over the reanalysis period 1957-96. This eliminates perceived climate jumps associated with changes in the data assimilation system. The NCEP/NCAR 40-yr reanalysis uses a frozen state-of-the-art global data assimilation system and a database as complete as possible. The data assimilation and the model used are identical to the global system implemented opera- tionally at the NCEP on 11 January 1995, except that the horizontal resolution is T62 (about 210 km). The database has been enhanced with many sources of observations not available in real time for operations, provided by different coun- tries and organizations. The system has been designed with advanced quality control and monitoring components, and can produce 1 mon of reanalysis per day on a Cray YMPI8 supercomputer. Different types of output archives are being created to satisfy different user needs, including a "quick look" CD-ROM (one per year) with six tropospheric and stratospheric fields available twice daily, as well as surface, top-of-the-atmosphere, and isentropic fields. Reanalysis information and selected output is also available on-line via the Internet (http//:nic.fb4.noaa.gov:8000). A special CD- ROM, containing 13 years of selected observed, daily, monthly, and climatological data from the NCEP/NCAR Re- analysis, is included with this issue. Output variables are classified into four classes, depending on the degree to which they are influenced by the observations andlor the model. For example, "C" variables (such as precipitation and surface fluxes) are completely determined by the model during the data assimilation and should be used with caution. Never- theless, a comparison of these variables with observations and with several climatologies shows that they generally contain considerable useful information. Eight-day forecasts, produced every 5 days, should be useful for predictabil- ity studies and for monitoring the quality of the observing systems. The 40 years of reanalysis (1957-96) should be completed in early 1997. A continuation into the future through an identical Climate Data Assimilation System will allow researchers to reliably compare recent anomalies with those in earlier decades. Since changes in the observing systems will inevitably produce perceived changes in the climate, par- allel reanalyses (at least 1 year long) will be generated for the periods immediately after the introduction of new ob- serving systems, such as new types of satellite data. NCEP plans currently call for an updated reanalysis using a state-of-the-art system every five years or so. The suc- cessive reanalyses will be greatly facilitated by the generation of the comprehensive database in the present reanalysis.
The overflow of dense water from the Nordic Seas through the Faroe Bank Channel (FBC) has attributes suggesting hydraulic control—primarily an asymmetry across the sill reminiscent of flow over a dam. However, this aspect has never been confirmed by any quantitative measure, nor is the position of the control section known. This paper presents a comparison of several different techniques for assessing the hydraulic criticality of oceanic overflows applied to data from a set of velocity and hydrographic sections across the FBC. These include 1) the cross-stream variation in the local Froude number, including a modified form that accounts for stratification and vertical shear, 2) rotating hydraulic solutions using a constant potential vorticity layer in a channel of parabolic cross section, and 3) direct computation of shallow water wave speeds from the observed overflow structure. Though differences exist, the three methods give similar answers, suggesting that the FBC is indeed controlled, with a critical section located 20–90 km downstream of the sill crest. Evidence of an upstream control with respect to a potential vorticity wave is also presented. The implications of these results for hydraulic predictions of overflow transport and variability are discussed. Author Posting. © American Meteorological Society, 2006. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 36 (2006): 2340-2349, doi:10.1175/JPO2969.1. The Faroe Bank Channel experiment was supported by NSF Grant OCE-9906736. JBG gratefully acknowledges the support of the NOAA/ UCAR Climate and Global Change Postdoctoral Program and NSF Grant OCE-9985840. Author Price was supported in part by the U.S. Office of Naval Research through Grant N00014-04-1-0109.
An ocean circulation model for process studies of the Subpolar North
Atlantic is developed based on the Geophysical Fluid Dynamics Laboratory
(GFDL) Modular Ocean Model (MOM) code. The basic model configuration is
identical with that of the high-resolution model (with a grid size of
1/3° × 2/5°) of the World Ocean Circulation Experiment
(WOCE) Community Modeling Effort (CME), except that the domain of
integration is confined to the area from 43° to 65°N. Open
boundary conditions are used for the inflows and outflows across the
northern and southern boundaries. A comparison with the CME model
covering the whole North Atlantic (from 15°S to 65°N) shows that
the regional model, with inflow conditions at 43°N from a CME
solution, is able to reproduce the CME results for the subpolar area.
Thus the potential of a regional model lies in its use as an efficient
tool for numerical experiments aiming at an identification of the key
physical processes that determine the circulation and water mass
transformations in the subpolar gyre. This study deals primarily with
the representation and role of the overflow waters that enter the domain
at the northern boundary. Sensitivity experiments show the effect of
closed versus open boundaries, of different hydrographic conditions at
inflow points, and of the representation of the narrow Faeroe Bank
Channel. The representation of overflow processes in the Denmark Strait
is the main controlling mechanism for the net transport of the deep
boundary current along the Greenland continental slope and further
downstream. Changes in the Faeroe Bank Channel throughflow conditions
have a comparatively smaller effect on the deep transport in the western
basin but strongly affect the water mass characteristics in the eastern
North Atlantic. The deep water transport at Cape Farewell and further
downstream is enhanced compared to the combined Denmark Strait and
Iceland-Scotland overflows. This enhancement can be attributed to a
barotropic recirculation in the Irminger Basin which is very sensitive
to the outflow conditions in the Denmark Strait. The representation of
both overflow regions determine the upper layer circulation in the
Irminger and Iceland Basins, in particular the path of the North
Atlantic Current.
Theoretical and laboratory models of certain types of strait and sill flows are discussed. Specifically, we consider a two-layer rotating fluid; the upper layer is at rest and the lower layer flows from one large basin to another via a connecting channel. The flow is assumed to be principally in a down-channel direction. The cross-channel balance is therefore geostrophic and the Bernoulli and potential vorticity equations are simplified. We further invoke the usual non-rotating hydraulic principle of maximum transport in flow over a weir - here the end of the channel - and thereby calculate relations between transport, rotation rate, and upstream interface height. One of these relations is tested experimentally with favorable results. A nonsteady decaying flow in the same system is analysed similarly and also compares well with experiment, as does a flow in both layers driven by an initial density imbalance. Some connections with oceanic strait and sill flows are discussed. (A)
Two numerical experiments regarding the North Atlantic circulation are compared. The two experiments are initialized with climatological temperatures and salinities and are integrated for ten years with different surface boundary conditions in the lceland Sea. One case climatological wind stress, SST, and surface salinity to force the model. It reproduces the Atlantic circulation found in similar studies. A change in surface boundary values In the Iceland Sea in order to improve the Denmark Strait Overflow water properties in the second case results in major changes in circulation and hydrography. The meridional overturning more than doubles and horizontal gyres are intensified. Recirculation cells occur north and south of the Gulf Stream. The Gulf Stream separates from the North American coast in a more realistic manner, and its further course is in better agreement with observations than in the reference run. The subsurface temperature distribution differs by several kelvin in the subpolar and ...
[1] Hydraulic constraints on the Denmark Strait overflow (DSO) are used as a parameterisation to improve the overflow representation in a global climate model. The parameterisation increases deep water formation in the Nordic Seas and strengthens the Norwegian Atlantic Current. Associated higher northward heat transport leads to a northward shift of the sea-ice edge and warming by 2.5°C in the eastern Nordic Seas despite a small effect on the Atlantic Meridional Overturning Circulation (AMO). This emphasises the impact of the DSO on climate even though the response in overturning due to the DSO representation in this model is less than expected from previous studies using ocean only models. In contrast to previous studies almost no stabilising effect of the overflow on the AMO is found to freshwater perturbations.
The northeastern part of the North Atlantic is unique in the sense that it is much warmer in the surface than other ocean areas at similar latitudes. The main reason for this is the large northward transport of heat that extends to high latitudes and crosses the Greenland–Scotland Ridge to enter the Nordic Seas and the Arctic. There the warm Atlantic water is converted to colder water masses that return southwards over the ridge partly as surface outflows and partly as overflows through the deep passages across the ridge. In this paper, the state of knowledge on the exchanges especially across the eastern part of the Greenland–Scotland Ridge is reviewed based on results from the ICES NANSEN (North Atlantic–Norwegian Sea Exchanges) project, from the Nordic WOCE project and from other sources. The accumulated evidence allows us to describe the exchanges in fair detail; the origins of the waters, the patterns of their flow towards and over the ridge and their ultimate fate. There is also increasing information on temporal variations of the exchanges although dynamical changes are still not well understood. Quantitative estimates for the volume transport of most of the overflow branches seem reasonably well established, and transport measurements of the Atlantic inflows to the Nordic Seas are approaching acceptable levels of confidence which allows preliminary budgets to be presented. The deep overflows are driven by pressure gradients set up by the formation of deep and intermediate water. The dominance of deep overflows over surface outflows in the water budget argues that this thermohaline forcing also dominates over direct wind stress and estuarine forcing in driving the Atlantic water inflow across the Greenland–Scotland Ridge, while wind stress seems to influence the characteristics and distribution of the Atlantic water north of the ridge.
This paper describes a series of four experiments, each run for 10 years at 1° × 1° resolution on a North Altantic domain, designed to illuminate the sensitivity of a Bryan-Cox-type ocean model to changes in the representation of the ridges that restrict the flow of dense, deep water out of the Greenland-Iceland-Norway (GIN) basin. In reality, much of the outflow takes place through narrow sills, which are subgrid-scale in the model, and small changes in the model topography to reflect these sills have a large impact on the outflow and on the compensating inflow of warm North Atlantic water. The circulation of the GIN basin is dramatically changed depending on the amount of this inflow; with no inflow, the basin cools and freshens, as would be expected, whereas with too much inflow, it becomes warm, salty, and homogeneous to great depths. Moreover, the small changes in topography have wider implications for the simulation. The presence or absence of dense overflows has a great impact on the mixed layer development in the subpolar gyre, with mixed layer depths differing by more than 500 m between two of the cases. This has implications for the formation of subpolar mode water, which is nearly shut off in the two cases with significant overflow. The meridional overturning in the model in year 10 increases by over 50% at its peak between the cases with no dense overflow and those with the greatest overflow, and this partly explains a change in peak heat transport, which increases by around 50% in the cases with significant overflow. The results in this paper imply that careful "tuning" of the model topography is necessary in ocean/climate models in order to get a reasonable simulation of the conveyor belt and of North Atlantic Deep Water formation.
The usefulness of the concept of JEBAR, the joint effect of baroclinicity and relief, in large-scale ocean dynamics is critically analyzed. The authors address two questions. Does the JEBAR term properly characterize the joint impact of stratification and bottom topography on the ocean circulation? Do estimates of the JEBAR term from observational data allow reliable diagnostic calculations? The authors give a negative answer to the first question. The JEBAR term need not give a true measure of the effect of bottom relief in a stratified ocean. A simple two-layer model provides examples. As to the second question, it is demonstrated that the large-scale pattern of the transport streamfunction is captured by the smoothed solution, especially with the Mellor et al. formulation of the JEBAR term. However, the calculated velocity field is very noisy and the relative errors are large.
Quasi-meridional flux estimates for volume, heat, and freshwater, with computed errors, in the region of the subpolar gyre of the North Atlantic between Cape Farewell and Ireland between 53° and 60° N are presented. The dataset consists of three approximately 500-km-sided boxes of CTD stations collected as the U.K. Control Volume Experiment during summer 1991 on the RRS Charles Darwin, supplemented by stations from the International Geophysical Year surveys of the R/V Anton Dohrn to represent the East Greenland Current (EGC), and justification is given for their use. The circulation is determined by an inverse method applied to the boxes of the survey, with in situ current measurements from a vessel-mounted acoustic Doppler current profiler forming an estimate of the strength of the EGC. Climatological wind stress data are used to estimate Ekman fluxes. The principal results are the determination of the poleward heat flux across the survey region as 0.28 ± 0.06 PW, and of the freshwater gain by the Arctic as 0.17 ± 0.06 × 106 m3 s-1. A quantified circulation scheme for the part of the subpolar gyre covered by the survey is presented, for which the upper circulation agrees with that of previous authors (within the errors of the estimates), but the deep circulation, particularly the Denmark Strait overflow, appears significantly weaker than previous estimates.
A diagnostic, finite element, barotropic ocean model has been used to simulate the mean circulation in the North Atlantic. With the inclusion of the joint effect of baroclinicity and relief (JEBAR), the Gulf Stream is found to separate at the correct latitude, ∼35° N, off Cape Hatteras. Results suggest that the JEBAR term in three key regions (offshore of the separation point in the path of the main jet, along the slope region of the North Atlantic Bight, and in the central Irminger Sea) is crucial in determining the separation point. The transport driven by the bottom pressure torque component of JEBAR dominates the solution, except in the subpolar gyre, and is also responsible for the separation of the Gulf Stream. Excluding high latitudes (in the deep-water formation regions) density variations in the upper 1000 m of the water column govern the generation of the necessary bottom pressure torque in our model. Examination of results from the World Ocean Circulation Experiment - Community Modelling Effort indicates that the bottom pressure torque component of JEBAR is underestimated by almost an order of magnitude, when compared to our diagnostic results. The reason for this is unclear but may be associated with the diffuse nature of the modeled thermocline in the CME as suggested by our model's sensitivity to the density field above 1000 m.
In diagnostic calculations of the ocean's circulation, the so-called JEBAR (joint effect of baroclinicity and relief) term may induce a significant depth-average current field, as has been noted in the oceanographic literature. Here we present two consistent interpretations of this term. In the equation governing the vorticity of the depth-averaged current, a topographic vortex-stretching term proportional to the dot product of depth-averaged velocity and the depth gradient arises. We show that JEBAR corrects this term by removing the contribution of the geostrophic flow referenced to the bottom, which cannot generate topographic vortex stretching. In the evolution equation for the vorticity of the depth-integrated flow, a bottom-torque term is present. We show that the JFBAR term discussed above enters here as a contribution to the bottom torque, emphasizing the role of JEBAR as a forcing term. We also show that when a diagnostic calculation is formulated completely in terms of the transport str...
The reliability of Ocean General Circulation Models (OGCMs) strongly depends on the quality of their tracer advection schemes. For the sake of simplicity and computing time, tracer advection schemes most commonly used in large-scale OGCMs tend to be low-order schemes, which suffer from spurious numerical diffusion and dispersion that result in distorted solutions. The application of high-order schemes would reduce numerical errors, but at a considerable cost in terms of computing time. An alternative to the use of high-order methods is the implementation of algorithms that take into account the sub-grid distribution of tracers. One such method is the Second-Order Moments (SOM) scheme of Prather (1986), which is more accurate than a fourth-order scheme, but at the time consumption of a second-order algorithm. This article presents results from coarse-resolution, global-ocean simulations with very low explicit diapycnal mixing, in which active and passive tracers were advected with the SOM method. We compare the performance of the method with that of more traditional schemes, namely, the FCT (flux corrected transport) and QUICKer (quadratic upstream interpolation for convective kinematics) schemes. In general, the use of the SOM method significantly improves tracer distributions and transports compared to FCT and QUICKer, thus leading to a better representation of ocean currents, notably boundary currents and frontal systems. While model simulations employing the FCT and QUICKer schemes recreate a global overturning circulation with strong upwelling occurring in low latitudes, the SOM simulations admit a circulation pattern closer to that known as the ``reconfigured conveyor belt'' (Toggweiler and Samuels, 1993), in which the bulk of the global ocean upwelling occurs in the Southern Ocean.
Observations indicate that vertical diffusivity in the deep ocean is considerably enhanced over rough bathymetry and is very small elsewhere. Here we investigate effects of locally enhanced vertical diffusivity over rough bottom topography on the world ocean circulation by use of a coarse resolution ocean general circulation model. Vertical diffusivity is enhanced in the model, taking into account effects of internal tide breaking. For comparison, two cases of an experiment are carried out, where vertical diffusivity increases with depth without horizontal inhomogeneity. Horizontal distribution of deep upwelling is qualitatively different, depending on whether or not vertical diffusivity is horizontally inhomogeneous. Upwelling of Circumpolar Deep Water in the Pacific and Antarctic Bottom Water in the Atlantic is confined where vertical diffusivity is enhanced in the case of horizontally inhomogeneous vertical diffusivity, whereas it is horizontally uniform in the other case. This difference leads to different three-dimensional structure of the deep circulation.
Sea surface height (SSH) from altimeter observations (1992-2000) and modeling results are investigated to determine the modes of variability and the linkages to the state of oceanic circulation in the North Atlantic. Three different model experiments are used with climatological surface forcing appended by wind stress and/or buoyancy flux anomalies. The simulated SSH and gyre circulation and altimeter data are analyzed using the empirical orthogonal function analysis. It is shown that decadal variability in the leading SSH and gyre circulation mode originates from the basin-scale thermal forcing component, not from wind stress driving. This means that low-frequency variations of SSH along the Gulf Stream reflect predominantly overturning changes. The horizontal gyre circulation changes are also related to the overturning due to the topography which couples baroclinic and barotropic flows. SSH variability outside the western boundary current region, on the eastern side of the North Atlantic basin, is determined by local and remote (Rossby waves) wind stress forcing. The model simulations and altimeter data suggest that there was a large SSH change in 1995-1996 over the Gulf Stream which was associated with an abrupt weakening of the overturning.
The results suggest that the Gulf Stream was some 30 Sv weaker in the 1970-1974 pentad than in the pentad 1955-1959. About 20 Sv of this is due to a dramatic weakening of the circulation of the subtropical gyre. This is traced to a change in bottom pressure torque associated with the bottom topography on the western side of the Mid-Atlantic Ridge. This same general area is also shown to be important for enhancing the transport of the climatological mean subtropical gyre above that predicted by the flat-bottomed Sverdrup relation. The remaining 10 Sv is due to a weakening of the cyclonic gyre in the continental slope region south of Atlantic Canada and north of the Gulf Stream. -from Authors
A model of the Atlantic Ocean was forced with decadal-scale time series of surface fluxes taken from the National Centers for Environmental Prediction-National Center for Atmospheric Research reanalysis. The bulk of the variability of the oceanic circulation is found to be related to the North Atlantic oscillation (NAO). Both realistic experiments and idealized sensitivity studies with the model show a fast (intraseasonal timescale) barotropic response and a delayed (timescale about 6-8 yr) baroclinic oceanic response to the NAO. The fast response to a high NAO constitutes a barotropic anticyclonic circulation anomaly near the subpolar front with a substantial decrease of the northward heat transport and an increase of northward heat transport in the subtropics due to changes in Ekman transport. The delayed response is an increase in subpolar heat transport due to enhanced meridional overturning and due to a spinup of the subpolar gyre. The corresponding subpolar and subtropical heat content changes could in principle act as an immediate positive feedback and a delayed negative feedback to the NAO.
The transport of dense water through Denmark Strait is investigated with a high-resolution numerical model. The results are compared with different transport estimates from rotating hydraulic theory in order to evaluate such theories with respect to the interpretation of hydrographic data. Generally, estimates far upstream result in upper transport limitations that can be appropriately scaled to give time mean transport estimates, whereas estimates from the sill entrance region can provide a good approximation of the variable transport.
The Hadley Centre climate model HadCM3 simulates a stable thermohaline circulation driven by deep water formation in the Norwegian and Labrador Seas without the need for flux adjustments. It has however been suggested that this result is the fortuitous consequence of the local use of the Roussenov convection scheme in this region, and that the model simulation may depend sensitively on this parameterisation. Here we investigate the sensitivity of the thermohaline circulation (THC) to the model’s treatment of the overflows from the Nordic Seas for both pre-industrial and increasing greenhouse gas forcings. We find that although the density structure in the Labrador Sea does depend upon the specifics of how the overflows are modelled, the global thermohaline circulation and climate responses are not sensitive to these details. This result gives credibility to previously published modelling studies on the response of the thermohaline circulation to anthropogenic greenhouse gas forcing, and implies that research may profitably be focussed on the large scale transports, where models are known to disagree.
Updating an earlier account by Dickson et al., (1990), this paper
reviews the initial development phase of North Atlantic Deep Water
(NADW) production from the points where the dense inflows from Nordic
seas cross the Greenland-Scotland Ridge to the point off south Greenland
where the buildup of new production appears almost complete. In
particular, three long-term current meter arrays totaling 91 instruments
and set at ˜160 km intervals south from the Denmark Strait sill
are used to validate earlier short-term arrays by others and, in
combination with these earlier arrays, to describe the downstream
evolution of mean speed, depth and entrainment, the variability of the
overflow current in space and time, and the likely contribution of the
other three main constituents of NADW production at densities greater
than σθ = 27.8. From the points of overflow (5.6
Sv) the transport within this range increases by entrainment and
confluence with other contributory streams to around 13.3 Sv at Cape
Farewell. While recirculating elements prevent us from determining the
net southgoing transport, a NADW transport of this order appears
consistent with recent estimates of net abyssal flow passing south
through the North and South Atlantic.
A project to objectively analyze historical ocean temperature, salinity,
oxygen, and percent oxygen saturation data for the world ocean has
recently been completed at the National Oceanic and Atmospheric
Administration's (NOAA) Geophysical Fluid Dynamics Laboratory,
Princeton, New Jersey. The results of the project are being made
available through distribution of the Climatological Atlas of the World
Ocean (NOAA Professional Paper No. 13), and through distribution of
magnetic tapes containing the objective analyses.The sources of data
used in the project were the Station Data, Mechanical Bathythermograph,
and Expendable Bathythermograph files of the National Oceanographic Data
Center (NODC) in Washington, D.C., updated through 1977-1978. The raw
data were subjected to quality control procedures, averaged by
one-degree squares, and then used as input to an objective analysis
procedure that fills in one-degree squares containing no data and
smooths the results. Due to the lack of synoptic observations for the
world ocean, the historical data are composited by annual, seasonal, and
(for temperature) monthly periods.
Part of the uncertainty in predictions by climate models results fromlimited knowledge of the stability of the thermohaline circulation ofthe ocean. Here we provide estimates of the response of pre-industrial surface climatevariables should the thermohalinecirculation in the Atlantic Ocean collapse. For this we have usedHadCM3, an ocean-atmosphere general circulation model that is run without fluxadjustments. In this model a temporary collapse was forced by applying a strong initial freshening to the top layers of the NorthAtlantic. In the first five decades after the collapse surface air temperatureresponse is dominated by cooling of much of the NorthernHemisphere (locally up to 8 C, 1–2 C on average) and weakwarming of theSouthern Hemisphere (locally up to 1 C, 0.2 C onaverage). Response is strongest around the North Atlantic but significant changesoccur over the entire globe and highlight rapidteleconnections.Precipitation is reduced over large parts of the Northern Hemisphere.A southward shift of the IntertropicalConvergence Zone over the Atlantic and eastern Pacific createschanges in precipitation that are particularly large in South America andAfrica. Colder and drier conditions in much of the Northern Hemisphere reducesoil moisture and net primary productivity of the terrestrial vegetation. Thisis only partlycompensated by more productivity in the Southern Hemisphere.The total global net primary productivity by the vegetation decreases by5%. It should be noted, however, that in this version of the model thevegetation distribution cannotchange, and atmospheric carbon levels are also fixed. After about 100 yearsthe model's thermohaline circulation has largelyrecovered, and most climatic anomalies disappear.
The 'conveyor belt' circulation of the Atlantic Ocean transports large amounts of heat northward, acting as a heating system
for the northern North Atlantic region. It is widely thought that this circulation is driven by atmospheric freshwater export
from the Atlantic catchment region, and that it transports freshwater northward to balance the loss to the atmosphere. Using
results from a simple conceptual model and a global circulation model, it is argued here that the freshwater loss to the atmosphere
arises mainly in the subtropical South Atlantic and is balanced by northward freshwater transport in the wind-driven subtropical
gyre, while the thermohaline circulation transports freshwater southward. It is further argued that the direction of freshwater transport is closely linked to the dynamical regime and stability
of the 'conveyor belt': if its freshwater transport is indeed southward, then its flow is purely thermally driven and inhibited
by the freshwater forcing. In this case the circulation is not far from Stommel's saddle-node bifurcation, and a circulation
state without NADW formation would also be stable.
The work presented in this paper is part of an effort to understand and improve the representation of overflows in large scale, coarse resolution ocean climate models. To this end we developed a regional model of the Faroe Bank Channel overflow using the MITgcm (Massachusetts Institute of Technology General Circulation Model), a typical global ocean model using discrete levels as the vertical co-ordinate. In order to isolate the numerical diffusion resulting from the advection of tracers, the model is run without any turbulence closure schemes, without convective adjustment or any other physically based parameterization of mixing. Comparison between the model results and recent observations of the Faroe Bank Channel plume allows assessment of the model performance, including its ability to correctly represent the mixing and the downslope transport in the plume. It is found that at the highest resolution used in this paper (2.5 km – horizontal and 25 m – vertical) the structure of the modeled plume and the magnitude of the entrainment is comparable to the observed plume.The dependence of the mixing on various model parameters, such as vertical and horizontal resolution, vertical viscosity, drag coefficient and inflow forcing, is tested extensively. The numerical mixing in the model is found to be most sensitive to changes in the horizontal resolution, and to a lesser extent on vertical resolution and vertical viscosity. The inflow forcing and drag coefficient show only a very minor effect on the mixing.The results presented in the paper identify the shortcomings of the model at coarser resolutions which need to be addressed when attempting to represent such overflows realistically in large scale climate and ocean models.
An isopycnal, two-layer, idealized model for a convective basin is proposed, consisting of a convecting, interior region and a surrounding boundary current (buoyancy and wind-driven). Parameterized eddy fluxes govern the exchange between the two. To balance the interior buoyancy loss, the boundary current becomes denser as it flows around the basin. Geostrophy imposes that this densification be accompanied by sinking in the boundary current and hence by an overturning circulation. The poleward heat transport, associated with convection in the basin, can thus be viewed as a result of both an overturning and a horizontal circulation. When adapted to the Labrador Sea, the model is able to reproduce the bulk features of the mean state, the seasonal cycle, and even the shutdown of convection from 1969 to 1972. According to the model, only 40% of the poleward heat (buoyancy) transport of the Labrador Sea is associated with the overturning circulation. An exact solution is presented for the linearized equations when changes in the boundary current are small. Numerical solutions are calculated for variations in the amount of convection and for changes in the remotely forced circulation around the basin. These results highlight how the overturning circulation is not simply related to the amount of dense water formed. A speeding up of the circulation around the basin due to wind forcing, for example, will decrease the intensity of the overturning circulation while the dense water formation remains unvaried. In general, it is shown that the fraction of poleward buoyancy (or heat) transport carried by the overturning circulation is not an intrinsic property of the basin but can vary as a result of a number of factors.
The spatial distributions of certain sea-surface properties, such as
temperature, fluctuate on timescales from months to decades and in
synchrony with the main regional atmospheric patterns comprising the
global climate system. Although it has long been assumed that the ocean
is submissive to the dictates of the atmosphere, recent studies raise
the possibility of an assertive, not merely passive, oceanic role in
which water-mass circulation controls the timescales of climate
fluctuations. Previously held notions of the immutability of the
physical and chemical characteristics of deep water masses are changing
as longer time series of ocean measurements indicate that the signatures
of varying sea-surface conditions are translated to deep waters,. Here
we use such time-series measurements to track signals `imprinted' at the
sea surface in the North Atlantic Ocean's subpolar Labrador Basin into
the deep water of the subtropical basins near Bermuda, and infer an
approximately 6-year transit time. We establish a geographic and
temporal context for a portion of the long-term warming trend reported
for mid-depth subtropical waters over the past 40 or so years,, and we
predict that waters at these depths will continue to cool well into the
next decade.