The effect of noise on the stability of convection in a conceptual model of the North Atlantic subpolar gyre

Following a high-end projection for mass loss from the Greenland and Antarctic ice-sheets, a freshwater forcing was applied to the ocean surface in the coupled climate model EC-Earthv2.2 to study the response to meltwater release assuming an RCP8.5 emission scenario. The meltwater forcing results in an overall freshening of the Atlantic that is dominated by advective changes, strongly enhancing the freshening due to dilution by Greenland meltwater release. The strongest circulation change occurs in the western North Atlantic subpolar gyre and in the gyre in the Nordic Seas, leaving the North Atlantic subpolar gyre the region where most advective salt export occurs. Associated with counteracting changes in both gyre systems, the response of the Atlantic Meridional Overturning Circulation is rather weak over the 190 years of the experiment; it reduces with only 1 Sv (= 10^6 m ^3s ^{-1}), compared to changes in the subpolar gyre of 5 Sv. This relative insensitivity of the AMOC to the forcing is attributed to enhanced convection in the Nordic Seas and stronger overflows that compensate reduced convection in the Labrador and Irminger Seas, and lead to higher sea surface temperatures (SSTs) in the former and lower SSTs in the latter region. The weakened subpolar gyre in the west also shifts the North Atlantic Current and the subpolar-subtropical gyre boundary; with the subtropical gyre expanding, and the western subpolar gyre contracting. The SST changes are associated with obduction of Atlantic waters in the Nordic Seas that would otherwise obduct in the western subpolar gyre. The anomalous SSTs also induce a coupled ocean-atmosphere feedback that further strengthens the Nordic Seas circulation and weakens the western subpolar gyre. This occurs because the anomalous SST-gradient enhances the westerlies, especially between 65^{circ }N and 70^{circ }N; the associated increase in windstress curl consequently enhances the gyre in the Nordic Seas. This feedback is driven by the Greenland mass loss; Antarctic meltwater discharge causes a weaker, opposite response and more particularly affects the South Atlantic salinity budget through northward advection of low-salinity waters from the Southern Ocean. This effect, however, becomes visible only hundred years after the onset of Antarctic mass loss. We conclude that the response to freshwater forcing from both ice caps can lead to a complex response in the Atlantic circulation systems with opposing effects in different subbasins. The relative strength of the response is time-dependent and largely governed by internal feedbacks; the forcing acts mainly as a trigger and is decoupled from the response.

Heat budget in the North Atlantic subpolar gyre: Impacts of atmospheric weather regimes on the 1995 warming event

<p>The Atlantic Meridional Overturning Circulation (AMOC) is a key component of the global climate. Recent observations have highlighted the dominant role of the buoyancy forcing in the transformation of surface waters to the AMOC lower limb at subpolar latitudes. The short (4 years) length of the OSNAP timeseries, however, limits conclusions over longer time scales. To investigate a wide range of temporal scales, we use three 100-years long coupled simulations of HadGEM3-GC3.1, at resolutions ranging from ~130 km atmosphere and 1° ocean to 25 km atmosphere and 1/12° ocean. In line with observations, the models show that the mean overturning and buoyancy-induced transformation are concentrated in the eastern subpolar gyre rather than in the Labrador Sea.</p><p>However, the horizontal resolution of the models impacts the formation of dense water over the subpolar gyre. An unrealistically large sea ice extent induces a weak buoyancy-induced transformation over the western subpolar gyre at low resolution, while a bias in surface density produces too dense water at high resolution. These biases are associated with a shift in the location of dense water formation. The transformation is mainly localized in the interior of the Irminger and Labrador seas at low resolution, and over the boundary current at high resolution. The interannual variability of the transformation is thus driven by different mechanisms between the simulations. In contrast with observations, the interannual variance in air-sea fluxes plays a more prominent role in the variance of transformation along the boundary current at high resolution.</p>

Formation and export of deep water in the Labrador and Irminger Seas in a GCM

<p><strong>The decadal to multi-decadal temperature variability of the intermediate (700 – 2000 m) North Atlantic Subpolar Gyre (SPG) significantly imprints the global pattern of ocean heat uptake. Here, the origins and dominant pathways of this variability are investigating with an ocean analysis product (EN4), an ocean state estimate (ECCOv4), and idealized modeling approaches. Sustained increases and decreases of intermediate temperature in the SPG correlate with long-lasting warm and cold states of the upper ocean – the Atlantic Multidecadal Variability – with the largest anomalous vertical heat exchanges found in the vicinity of continental boundaries and strong ocean currents. In particular, vertical diffusion along the boundaries of the Labrador and Irminger Seas and advection in the region surrounding Flemish Cap stand as important drivers of the recent warming trend observed during 1996-2014. The impact of those processes is well captured by a 1-dimensional diffusive model with appropriate boundary-like parametrization and illustrated through the continuous downward propagation of a passive tracer in an eddy-permitting numerical simulation. Our results imply that the slow and quasi-periodic variability of intermediate thermohaline properties in the SPG are not strictly driven by the well-known convection-restratification events in the open seas but also receives a key contribution from boundary sinking and mixing. Increased skill for modelling and predicting intermediate-depth ocean properties in the North Atlantic will hence </strong><strong>require the appropriate representation of surface-deep dynamical connections within the boundary currents encircling Greenland and Newfoundland.</strong></p>

The Atlantic Meridional Overturning Circulation (AMOC) transports vast amounts of heat to high latitudes, and is largely responsible for Western Europe’s relatively mild climate. Climate models project the AMOC will weaken substantially over the 21st century, which impacts weather, climate, sea level and the oceanic carbon cycle. In many studies, the AMOC state is described in a condensed two-dimensional view or even by means of a single metric, which leaves many aspects of its complex 3D-structure underexposed. By revealing the sharp contrast in overturning strength between the western and eastern subpolar gyre (SPG), the recent OSNAP observations emphasized the importance of considering the AMOC in 3D.In this study, we explore this further by analyzing the characteristics of the overturning in density space in the North Atlantic SPG on a regional scale, and over time periods ranging from seasons to decades. For this, we use model data from the high-resolution GLORYS12 reanalysis, spanning the period 1993-2020. Following the approach applied in OSNAP, the overturning is assessed from alongstream changes in boundary current transport in specific density classes. This analysis is performed for the entire SPG, for its major basins (Iceland Basin, Irminger Sea, and Labrador Sea) and for smaller segments along the boundary currents, thus providing detailed insights in variations of the overturning varies along the entire SPG boundary.The mean overturning from GLORYS12 for 1993-2020 is 23.8 Sv, distributed as 41%, 29%, and 30% for the Iceland Basin, Irminger Sea, and Labrador Sea respectively, and peaking at increasingly higher densities in alongstream direction. Within each basin, a pronounced seasonal cycle can be identified, with the maximum overturning occurring in March and the minimum in September. Over the entire reanalysis period, the overturning strength in both the Iceland Basin and Irminger Sea exhibits a weak decreasing trend, whereas the Labrador Sea displays a weak increasing trend. The subdivision in shorter segments reveals large spatial differences in overturning, both with regard to its overall strength and its distribution over density classes. However, these outcomes are less robust than the analyses on the scale of the major basins, as the flow is highly variable and numerical uncertainties associated with offline overturning calculations become more prominent.Further research is needed to properly interpret these regional variations, and thereby improve our understanding of the AMOC dynamics and its sensitivity to changing oceanic and atmospheric forcing conditions. Linking them to local processes known to govern the overturning (i.e., formation of dense waters in the interior of marginal seas and their export, formation of dense waters within the boundary current system itself and the exchange of waters via overflows) seems a viable route.

“Great Salinity Anomalies” (GSAs) have been observed to propagate around the North Atlantic subpolar gyre. Similar anomalies occur in the Third Hadley Centre Coupled Atmosphere–Ocean GCM (HadCM3) of preindustrial climate. It has been hypothesized that these salinity anomalies result from the advection of anomalously low salinity waters around the subpolar gyre. Here, the consequences of using passive tracers in the HadCM3 climate model to tag the anomalously low salinity water associated with a GSA in the Greenland and Labrador Seas are reported. Rather than predominantly advecting around the modeled subpolar gyre in accordance with the upper-ocean salinity anomaly, the tracers mix to intermediate depths, before becoming incorporated into the model's North Atlantic Deep Water. Horizontal advection of the tracer in the upper ocean is limited to around 1000 km, compared with the gyre-scale propagation of the salinity anomalies. It is concluded that GSAs are unlikely to be caused by the advection of salinity anomalies; rather anomalous oceanic currents or surface fluxes are responsible.

<p>The AMOC (Atlantic Meridional Overturning Circulation) is a key driver of climate change and variability. Since continuous, direct measurements of the overturning strength in the North Atlantic subpolar gyre (SPG) have been unavailable until recently, the understanding, based largely on climate models, is that the Labrador Sea has an important role in shaping the evolution of the AMOC. However, a recent high profile observational campaign (Overturning in the Subpolar North Atlantic, OSNAP) has called into question the importance of the Labrador Sea, and hence of the credibility of the AMOC representation in climate models. Here, we reconcile these viewpoints by comparing the OSNAP data with a new, high-resolution coupled climate model: HadGEM3-GC3.1-MM. Unlike many previous models, we find our model compares well to the OSNAP overturning observations. Furthermore, overturning variability across the eastern OSNAP section (OSNAP-E), and not in the Labrador Sea region, appears linked to AMOC variability further south. Labrador Sea densities are shown to be an important indicator of downstream AMOC variability, but these densities are driven by upstream variability across OSNAP-E rather than local processes in the Labrador Sea.</p>

<p>Decadal variability in indices of North Atlantic (NA) atmospheric circulation plays a major role in changing climate over western Europe. However, reproducing characteristics of this variability in climate models presents a major challenge. Climate models broadly exhibit weaker-than-observed multi-decadal variability in atmospheric circulation indices. A prominent explanation for this is that model-simulated links between anomalous sea-surface temperatures (SSTs) and atmospheric variability are too weak. The dominant mode of basin-wide NA SST variability is Atlantic multi-decadal variability (AMV), which on multi-decadal timescales is expressed more strongly over the NA sub-polar gyre (SPG). SSTs over the SPG region (SST<sub>SPG</sub>) are therefore the main focus here.</p><p>Studies to date have shown that variability in the North Atlantic Oscillation (NAO) exhibits strongest correlations with AMV indices in late winter, but the reasons for this are not clear. Here we show that this stronger late-winter correlation is particularly clear for SST<sub>SPG</sub> and coincides with a climatological equatorward shift of the eddy-driven NA westerly jet from early-to-late winter. To help gain dynamical insight, indices of eddy-driven jet latitude (JLI) and speed (JSI) were correlated with SST<sub>SPG</sub> and it was found that they exhibit more pronounced early-to-late winter shifts in correlations than for the NAO; In particular,  correlations strengthen from early-to-late winter for JLI while weaken for JSI. Our results suggest that the jet-SST<sub>SPG</sub> linkages progress through winter from JSI dominant in early winter to JLI dominant in late winter.</p><p>CMIP5 and CMIP6 models were then evaluated for representation of these observed characteristics in ocean-atmosphere linkages. Consistent with the observed sub-seasonal links between climatological jet latitude and atmosphere-ocean correlation strength, CMIP models with larger equatorward jet biases exhibit weaker JSI-SST<sub>SPG</sub> correlations and stronger JLI-SST<sub>SPG</sub> correlations. A pronounced early-winter equatorward bias in jet latitude in CMIP models could partially explain the weaker-than observed linkage between SSTs and atmospheric variability.  </p>

The Last Interglacial period, Marine Isotope Stage 5e (MIS 5e ∼116–128 ka), is thought to have had a warmer, but less stable climate than the present interglacial. One key factor that has the potential to influence the ocean and climate is sea ice, but its presence and extent throughout MIS 5e is poorly constrained. Here we reconstruct the sea surface hydrography and sea ice variability in the Labrador Sea, a region influenced by the subpolar gyre (SPG) and where deep water formation occurs, in order to evaluate the potential of sea ice to drive or amplify ocean variability. We analysed biomarkers (highly branched isoprenoids, HBIs, and sterols), dinoflagellate cyst assemblages and stable oxygen isotopes from the late stages of MIS 6, throughout MIS 5e, into MIS 5d. Our results show that the late glacial MIS 6 was likely characterised by a thick multiyear sea ice cover. During the first phase of MIS 5e, the hydrography was highly variable. The initial 1500 years (128–126.5 ka) were characterised by the presence of a seasonal Marginal Ice Zone (MIZ) accompanied by subsurface warmth. As the sea ice retreated, cool, likely polar-sourced water dominated the surface and subsurface ocean (126.5–124 ka), until an abrupt surge of sea ice marked the final pulse of the remnants of the deglaciation. The second half of MIS 5e (124–116 ka) was characterised by a persistent inflow of warm water, only interrupted by incursions of cold water as summer insolation declined. Seasonal sea ice returned to the Eirik Drift during MIS 5d. We infer that sea ice variability throughout MIS 5e was coupled with the variability of the SPG. Especially the location of a proximal MIZ to the Labrador Sea convection region could have been important for SPG dynamics. In addition, the presence of sea ice at the transitions into and out of MIS 5e could point to its important role in modulating and enhancing the magnitude and coherence of climate signals at major climatic transitions.

<p>The North Atlantic Subpolar Gyre (SPG) has been widely implicated as the source of large-scale changes in the subpolar marine environment. However, inconsistencies between different indices of SPG strength based on Sea Surface Height (SSH) observations have raised questions about the active role SPG strength and size play in determining water properties in the eastern subpolar North Atlantic (ENA). Here, by analyzing SSH-based and various other SPG-strength indices derived from observations and a global coupled model, we show that the interpretation of SPG strength-salinity relationship is dictated by the choice of the SPG index. Our results emphasize that SPG indices should be interpreted cautiously because they represent variability in different regions of the subpolar North Atlantic. Idealized Lagrangian trajectory experiments illustrate that zonal shifts of main current pathways in the ENA and meridional shifts of the North Atlantic Current (NAC) in the western intergyre region during strong and weak SPG circulation regimes are manifestations of variability in the size and strength of the SPG. Such shifts in advective pathways modulate the proportions of subpolar and subtropical water reaching the ENA, and thus impact salinity. Inconsistency among SPG indices arises due to the inability of some indices to capture the meridional shifts of the NAC in the western intergyre region. Overall, our results imply that salinity variability in the ENA is not exclusively sourced from the subtropics, instead the establishment of a dominant subpolar pathway also points to redistribution within the SPG.</p>

We assess the skill of retrospective multiyear forecasts of North Atlantic ocean characteristics obtained with ocean‐atmosphere‐sea ice models that are initialized with estimates from the observed ocean state. We show that these multimodel forecasts can skilfully predict surface and subsurface ocean variability with lead times of 2 to 9 years. We focus on assessment of forecasts of major well‐observed oceanic phenomena that are thought to be related to the Atlantic meridional overturning circulation (AMOC). Variability in the North Atlantic subpolar gyre, in particular that associated with the Atlantic Multidecadal Oscillation, is skilfully predicted 2–9 years ahead. The fresh water content and heat content in major convection areas such as the Labrador Sea are predictable as well, although individual events are not captured. The skill of these predictions is higher than that of uninitialized coupled model simulations and damped persistence. However, except for heat content in the subpolar gyre, differences between damped persistence and the initialized predictions are not significant. Since atmospheric variability is not predictable on multiyear time scales, initialization of the ocean and oceanic processes likely provide skill. Assessment of relationships of patterns of variability and ocean heat content and fresh water content shows differences among models indicating that model improvement can lead to further improvements of the predictions. The results imply there is scope for skilful predictions of the AMOC.

The North Atlantic Subpolar Gyre (SPG) plays an important role in climate predictability and influences climate variability due to its complex coupling with the atmospheric circulation in the North Atlantic and the Atlantic Meridional Overturning Circulation (AMOC). In this study, we investigate the impact of sea surface temperature (SST) variability in the SPG on atmospheric circulation patterns and climate extremes. We use the EC-Earth3 model (T255~80 km) and perform four sets of AMIP-type ensemble experiments with four different prescribed SST anomalies, each with 10 members and spanning 35 years from 1980 to 2014. The experimental design allows the climatic impact of SPG SST variability to be isolated from other global SST modes. Our results show that SPG SST anomalies directly influence atmospheric circulation between 30-75°N, causing zonally oriented wave-like anomalies. Notably, a warm SST anomaly in the subpolar gyre causes strong low-pressure anomalies over the North Atlantic and North Pacific, leading to warming of regions mainly between 45-60°N and cooling of regions mainly between 60-75°N. We find that the anomalous temperatures are particularly pronounced over the North American continent. We also investigate the indirect effects of SPG variability through its synergy with the North Atlantic and North Pacific SSTs, as well as the atmospheric teleconnections and extreme events associated with SPG variability. The results underline the importance of the SPG for the atmospheric circulation, the teleconnections, the regional climate and the extreme events.

Slowly varying large-scale ocean circulation can provide climate predictability on decadal time scales. It has been hypothesized that the North Atlantic subpolar gyre (SPG) exerts substantial influence on climate predictability. However, a clear identification of the downstream impact of SPG variations is still lacking. Using the MPI-ESM-LR1.2 decadal prediction system, we show that along the Atlantic water pathway, a dynamical link to the SPG causes salinity to be considerably better predicted than temperature. By modulating the slow northward ocean propagation, the subsurface memory of SPG variations enables salinity to be skillfully predicted up to 8 years ahead. In contrast, the SPG loses influence on temperature before Atlantic water penetrates into the Nordic Seas, and in turn, limits temperature to be predicted only 2 years ahead. This study identifies the key role of SPG signals in downstream prediction and highlights how SPG signals determine prediction time scales for different quantities, opening the door for investigating potentially associated predictions in the subarctic for the earth system, marine ecosystems in particular.

The Barents Sea is a key region in the Earth System and is home to highly productive marine resources. An integrated approach for strategic sustainable management of marine resources in such shelf-sea marine ecosystems requires, among many other aspects, a robust understanding of the impact of climate on local oceanic conditions. Here, using a combined observational and modelling approach, we show that decadal climatic trends associated with the North Atlantic Subpolar Gyre (SPG), within the period 1960–2019, have an impact on oceanic conditions in the Barents Sea. We relate hydrographic conditions in the Barents Sea to the decadal variability of the SPG through its impact on the Atlantic Inflow via the Faroe-Shetland Channel and the Barents Sea Opening. When the SPG warms, an increase in the throughput of subtropical waters across the Greenland-Scotland Ridge is followed by an increase in the volume of Atlantic Water entering the Barents Sea. These changes are reflected in pronounced decadal trends in the sea-ice concentration and primary production in the Barents Sea, which follow the SPG after an advective delay of 4–5 years. This impact of the SPG on sea-ice and primary production provides a dynamical explanation of the recently reported 7-year lagged statistical relationship between SPG and cod (Gadus morhua) biomass in the Barents Sea. Overall, these results highlight a potential for decadal ecosystem predictions in the Barents Sea.