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. @en

The east Greenland shelf contains southward currents that carry relatively cold and fresh water of Arctic origin that are important to the Arctic’s freshwater budget and the Atlantic Meridional Overturning Circulation (AMOC). Near the shore lies a current named the East Greenland Coastal Current (EGCC), which is a surface intensified southward jet estimated to have a 0.5-2.0 Sv volume transport. The EGCC carries fresher water than the East Greenland Current (EGC), which is found at the shelf break. Due to sparse observations, little understanding exists of the EGCC mean flow, variability, or drivers north of Denmark Strait. Moreover, literature definitions of the EGCC are inconsistent. Therefore, a high-resolution numerical simulation of the region is being used to study the EGCC, with a focus north of Denmark Strait. The performance of five proposed EGCC definitions is assessed by computing volume transport time-series and analyzing velocity cross sections at five model sections along the east Greenland shelf. In addition, the proposed EGCC definitions are applied to an observational time-series at 60°N from the OSNAP East extension. Results show that the mean EGCC transport values differ by up to ~3 Sv at the southern sections (south of 66°N) and ~0.6 Sv at the northern sections (north of 66°N). EGCC definitions based on a specific isohaline performed well in the southern sections but include part of the EGC in the northern sections and, on some occasions, exclude the bottom part of the EGCC. EGCC definitions based on velocity thresholds work better to capture the EGCC but can extend too far towards the EGC. Therefore, we recommend using an EGCC definition based on a velocity threshold and including a criterion on distance from the shore.@en

In the modern ocean, the transformation of light surface waters to dense deep waters primarily occurs in the Atlantic basin rather than in the North Pacific or Southern Oceans. The reasons for this remain unclear, as both models and paleoclimatic observations suggest that sinking can sometimes occur in the Pacific. We present a six-box model of overturning that combines insights from a number of previous studies. A key determinant of the overturning configuration in our model is whether the Antarctic Intermediate Waters are denser than the northern subpolar waters, something that de-pends on the magnitude and configuration of atmospheric freshwater transport. For the modern ocean, we find that al-though the interbasin atmospheric freshwater flux suppresses Pacific sinking, the poleward atmospheric freshwater flux out of the subtropics enhances it. When atmospheric temperatures are held fixed, North Pacific overturning can strengthen with either increases or decreases in the hydrological cycle, as well as under reversal of the interbasin freshwater flux. Tipping-point behavior, where small changes in the hydrological cycle may cause the dominant location of densification of light waters to switch between basins and the magnitude of overturning within a basin to exhibit large jumps, is seen in both transient and equilibrium states. This behavior is modulated by parameters such as the poorly constrained lateral dif-fusive mixing coefficient. If hydrological cycle amplitude is varied consistently with global temperature, northern polar amplification is necessary for the Atlantic overturning to collapse. Certain qualitative insights incorporated in the model can be validated using a fully coupled climate model.

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On the East Greenland Shelf, downwelling-favorable northerly winds confine the freshest water masses to the inner shelf, creating a strong cross-shelf salinity gradient. This wedge of fresh water supports a southward East Greenland Coastal Current (EGCC) that flows continuously from Fram Strait (80°N) to Cape Farewell at the southern tip of Greenland (60°N). What happens to this freshwater as it rounds Cape Farewell is not well understood. Here, we present results from a Lagrangian experiment in which we deployed 50 Surface Velocity Program (SVP) drifters and 8 profiling floats in the EGCC on the southeast Greenland shelf in August/September 2021 and 2022. The SVP drifters were drogued at 15 m and the profiling floats drifted at 100 m for 6 hours and profiled to the surface every 6 hours. We find that the vast majority of the EGCC continues on the shelf around Cape Farewell and supplies the northward West Greenland Coastal Current (WGCC). Local winds exert strong control on the coastal current’s cross-shelf position, but there is little evidence of shelf-basin exchange there. Investigation of the winds during the deployments revealed that they were similar to the mean conditions, and thus our instruments likely captured the typical ocean circulation. However, the lack of sampling during a strong Greenland tip jet event leaves open the possibility that the shelf-basin exchange in this region is intermittent in response to large wind events.@en

Natural variability at subinertial frequencies (time scale of several days) plays an important role in the interaction between Greenland’s fjords, the continental shelf, and shelf-break exchange with the deep basins. In this study we identified the nature and driving mechanisms of this variability in four fjords in Southeast Greenland, in three high-resolution numerical simulations. We find two dominant frequency ranges in along-fjord velocity, volume transport of Atlantic Water, and along-fjord heat transport: one around 2–4 days and one around 10 days. The higher frequency is most prominent in the two smaller fjords (Sermilik Fjord and Kangerdlugssuaq Fjord), while the lower frequency peak dominates in the larger fjords (Scoresby Sund and King Oscar Fjord). The cross-fjord structure of variability patterns is determined by the fjord's dynamic width, while the vertical structure is determined by the stratification in the fjord. The dominant frequency range is a function of stratification and fjord length, through the travel time of resonant internal Kelvin waves. We find that the subinertial variability is the imprint of Coastal Trapped Waves, which manifest as Rossby-type waves on the continental shelf and as internal Kelvin-type waves inside the fjords. Between 50% and 80% of the variability in the fjord is directly forced by Coastal Trapped Waves propagating in from the shelf, with an additional role played by alongshore wind forcing on the shelf.@en

Topographic features like ridges and islands, such as the Greenland-Iceland-Scotland Ridge (GISR), can obstruct drainage from a deep ocean basin. If the flow in connecting straits is hydraulically controlled, the drainage depends on the conditions in the draining straits as well as the upstream conditions. Importantly, through upstream influence imposed by hydraulic control the draining straits feed information to the upstream basin using long waves to maintain certain constraints on the upstream flow through the controlling sill. However, the impact of hydraulic control on the transport of dense water masses through straits, its relationship with strait geometry, and the interplay between multiple draining straits through long waves remains unclear. This study focuses on investigating the draining time scale of a deep basin through Kelvin waves interacting with one or two hydraulically controlled sill flows. To conduct this research, we employed an idealised 1.5-layer reduced-gravity model featuring two basins connected by one or two straits. When the generated Kelvin wave encounters hydraulically controlled flows in the straits, it is partially reflected back to the upstream basin. The strength of this interaction determines the draining time of the basin. This study explores how the draining time is impacted by the presence of a second strait, their location, their geometry and the water properties. The findings of this research contribute valuable insights into understanding the communication of long waves between multiple draining straits subject to hydraulic control. Additionally, the results are discussed in the context of the GISR.@en

The current configuration of the ocean overturning involves upwelling predominantly in the Southern Ocean and sinking predominantly in the Atlantic basin. The reasons for this remain unclear, as both models and paleoclimatic observations suggest that sinking can sometimes occur in the Pacific. We present a six-box model of the overturning in which temperature, salinity and low-latitude pycnocline depths are allowed to vary prognostically in both the Atlantic and Pacific. The overturning is driven by temperature, winds, and mixing and modulated by the hydrological cycle. In each basin there are three possible flow regimes, depending on whether low-latitude water flowing into northern surface boxes is transformed into dense deep water, somewhat lighter intermediate water, or light water that is returned at the surface. The resulting model combines insights from a number of previous studies and allows for nine possible global flow regimes. For the modern ocean, we find that although the interbasin atmospheric freshwater flux suppresses Pacific sinking, the equator-to-pole flux enhances it. When atmospheric temperatures are held fixed, seven possible flow regimes can be accessed by changing the amplitude and configuration of the modern hydrological cycle . North Pacific overturning can strengthen with either increases or decreases in the hydrological cycle, as well as under reversal of the interbasin freshwater flux. Tipping-point behavior of both transient and equilibrium states is modulated by parameters such as the poorly constrained lateral diffusive mixing. If hydrological cycle amplitude is varied consistently with global temperature, northern polar amplification is necessary for the Atlantic overturning to collapse.@en

We describe a new multidisciplinary effort to understand how Artificial Intelligence (AI) could be used to improve climate tipping point discovery using the collapse of the Atlantic Meridional Overturning Circulation (AMOC) as a case study. Our methodology includes an AI simulated environment where a Generative Adversarial Network (GAN) is used to play an adversarial game between two deep networks. One network, the generator, tries to invoke an AMOC collapse tipping point. The other network, the discriminator, tries to predict which model configurations lead to an AMOC collapse, and which do not. The discriminator uses a climate surrogate model to run model configurations suggested by the generator network. In this study we have explored using a well-researched reduced oceanography four-box model (Gnanadesikan et al. 2018) as the surrogate. In addition, we introduce a new type of surrogate model that is used for expanding to larger models, which is based on modeling stochastic differential equations to estimate bifurcation escape times, enabling a deeper understanding of the unstable areas and how to escape them. We describe how we apply this method to larger Global Circulation Models (GCMs) by means of a CESM2-calibrated dataset. Included in this methodology is a neuro-symbolic representation of the model configurations that translates into natural language questions enabling climate modelers to ask questions of the learned adversarial space and to provide explainability. We describe the results of applying this method to study the AMOC, and evaluate our method by comparing how well it learns to predict AMOC collapse, how well it learns the area of uncertainty in state space, and how effective the neuro-symbolic language is in providing a natural language interface into the adversarial game.@en

We identified the nature and driving mechanisms of subinertial variability (variability at a time scale of several days) in four fjords in Southeast Greenland, in three high-resolution numerical simulations. We find two dominant frequency ranges in along-fjord velocity, volume transport of Atlantic Water, and along-fjord heat transport: one around 2–4 days and one around 10 days. The higher frequency is most prominent in the two smaller fjords (Sermilik Fjord and Kangerdlugssuaq Fjord), while the lower frequency peak dominates in the larger fjords (Scoresby Sund and King Oscar Fjord). The cross-fjord structure of variability patterns is determined by the fjord's dynamic width, while the vertical structure is determined by the stratification in the fjord. The dominant frequency range is a function of stratification and fjord length, through the travel time of resonant internal Kelvin waves. We find that the subinertial variability is the imprint of Coastal Trapped Waves, which manifest as Rossby-type waves on the continental shelf and as internal Kelvin-type waves inside the fjords. Between 50% and 80% of the variability in the fjord is directly forced by Coastal Trapped Waves propagating in from the shelf, with an additional role played by alongshore wind forcing on the shelf.@en

Fjords are the primary conduit for oceanic heat transport to the Greenland Ice Sheet. This heat transport is highly time-variable, with typical frequencies in the subinertial range (with periods of several days). We studied four fjords along the southeast coast of Greenland to determine what drives oceanic heat into these fjords. The four fjords cover a range of widths and lengths, and three different high-resolution model simulations provided a range in stratifications and surface forcings. We find that the heat transport is associated with resonant modes of internal subinertial waves. The variability in transport of deep warm (Atlantic) water is highly correlated to the principal component that is associated with the gravest vertical normal mode (EOF 1 in narrow fjords, EOF 3 in a wide fjord). The dominant frequency in the heat transport time series is thus determined by the shape of the fjord and the stratification, the latter through the wave speed of the gravest normal mode. The oceanic heat supply to the Greenland Ice Sheet is thus governed by variables that are relatively easy to measure observationally.@en

In 2018, the IPCC summarized in a special report the potential risks surrounding climate tipping point. In 2019, Lenton et al. highlighted climate tipping points that could contribute to irreversible changes to our world including ice melt, deforestation, and circulation slowing. In this work we describe a study of the Atlantic Meridional Overturning Circulation (AMOC), and the use of an Artificial Intelligence (AI) assisted climate model methodology capable of performing unsupervised tipping point discovery to discover conditions which could lead to AMOC collapse.

We developed a novel AI generative adversarial network (GAN), where a set of deep learning generators attempt to invoke AMOC collapse by perturbing a constrained set of parameters, while another deep learning network, the discriminator, tries to learn how to avoid AMOC collapse. A surrogate model is used to run model configurations to test this adversarial method. We show that our methodology can be used to discover areas in model space that are consistent with fold bifurcations where the system moves from an on state to an off state. We measured the performance of this method by comparing it to an AMOC four box model and experiments described in (Gnanadesikan 2018) which uses the four box model to understand overturning stability.

We have found that the deep learning method can be used to exploit the area of uncertainty that is consistent with the area that separates the two stable states in a fold bifurcation model. When we compared the results of the adversarial network to the Gnanadesikan experiments we observed that when incorporating information regarding the uncertainty in the loss function, increasing the number of AI generators caused the AI agents to become more focused on this area of uncertainty. This area of uncertainty is consistent with what is described as the separatrix. In this study, we show the benefit of using this novel unsupervised approach as part of an AI assisted climate modeling methodology.@en

This study examines the applicability of the Gnanadesikan four-box model of the Atlantic Meridional Overturning Circulation to a global earth system model. The Community Earth System Model 2 (CESM2) is used to quantify the extent to which pycnocline depth and AMOC strength as predicted by the four-box model from regional densities and Southern Ocean fluxes match CESM2 values. The dynamics of a reduced meridional density difference driving a deepening pycnocline and weakening AMOC are qualitatively matched between the 4-box model and CESM2. We will quantify the fraction of variability in the CESM2 Large Ensemble of historical and SSP3-7.0 runs that is explained by the 4-box model; initial results suggest about two-thirds of the variability in AMOC is explained and over 90% of the variability in the pycnocline depth. This analysis may be repeated for any CMIP6 model, and a comparison of SSP5-8.5 across a handful of models is included as a demonstration.@en

We propose a hybrid Artificial Intelligence (AI) climate modeling approach that enables climate modelers in scientific discovery using a climate-targeted simulation methodology based on a novel combination of deep neural networks and mathematical methods for modeling dynamical systems. The simulations are grounded by a neuro-symbolic language that both enables question answering of what is learned by the AI methods and provides a means of explainability. We describe how this methodology can be applied to the discovery of climate tipping points and, in particular, the collapse of the Atlantic Meridional Overturning Circulation (AMOC). We show how this methodology is able to predict AMOC collapse with a high degree of accuracy using a surrogate climate model for ocean interaction. We also show preliminary results of neuro-symbolic method performance when translating between natural language questions and symbolically learned representations. Our AI methodology shows promising early results, potentially enabling faster climate tipping point related research that would otherwise be computationally infeasible.@en

Increased freshwater input to the Subpolar North Atlantic from Greenland ice melt and the Arctic could strengthen stratification in deep convection regions and impact the overturning circulation. However, freshwater pathways from the east Greenland shelf to deep convection regions are not fully understood. We investigate the role of strong wind events at Cape Farewell in driving surface freshwaters from the East Greenland Current to the Irminger Sea. Using a high-resolution model and an atmospheric reanalysis, we identify strong wind events and investigate their impact on freshwater export. Westerly tip jets are associated with the strongest and deepest freshwater export across the shelfbreak, with a mean of 37.5 mSv of freshwater in the first 100 m (with reference salinity 34.9). These wind events tilt isohalines and extend the front offshore, especially over Eirik Ridge. Moderate westerly events are associated with weaker export across the shelfbreak (mean of 15.9 mSv) but overall contribute to more freshwater export throughout the year, including in summer, when the shelf is particularly fresh. Particle tracking shows that half of the surface waters crossing the shelfbreak during tip jet events are exported away from the shelf, either entering the Irminger Gyre, or being driven over Eirik Ridge. During strong westerly wind events, sea ice detaches from the coast and veers toward the Irminger Sea, but the contribution of sea ice to freshwater export at the shelfbreak is minimal compared to liquid freshwater export due to limited sea ice cover at Cape Farewell.@en

The Atlantic Meridional Overturning Circulation redistributes heat across the Atlantic and is therefore a critical element of the climate system. Increased freshwater fluxes to the subpolar north Atlantic from the Greenland ice sheet and from the Arctic could lead to a strengthening of stratification in deep convection regions, and impact deep water formation and the overturning circulation. However, this additional freshwater first enters the boundary current on the Greenland shelf, and freshwater pathways from the shelf to deep convection regions are still unclear. In this study, we investigate the possible role of winds in driving short-lived freshwater export events from the south-east Greenland shelf to the deep convection region of the Irminger Sea.

Along the south-eastern shelf, strong and consistent north-easterly winds tend to restrain fresh surface waters over the shelf. This wind pattern changes at Cape Farewell, where strong westerly winds could lead to across-shelf export. Using a high-resolution model, we identify strong wind events and investigate their impact on freshwater export. The strongest westerly winds, westerly tip jets, are associated with the strongest and deepest freshwater export across the shelfbreak, with a mean of 40.7 mSv of freshwater in the first 100 m (with reference salinity 34.9). These wind events tilt isohalines and extend the front offshore, especially over Eirik Ridge. Moderate westerly events are associated with weaker export across the shelfbreak (mean of 17 mSv) but overall contribute to more freshwater export throughout the year, including in summer, when the shelf is particularly fresh. Particle tracking shows that half of the surface waters crossing the shelfbreak during tip jet events are exported away from the shelf, either entering the Irminger Gyre, or being driven over Eirik Ridge. During strong westerly wind events, sea-ice detaches from the coast and veers towards the Irminger Sea, but the contribution of sea-ice to freshwater export at the shelfbreak is minimal compared to liquid freshwater export.@en

Computational oceanography is the study of ocean phenomena by numerical simulation, especially dynamical and physical phenomena. Progress in information technology has driven exponential growth in the number of global ocean observations and the fidelity of numerical simulations of the ocean in the past few decades. The growth has been exponentially faster for ocean simulations, however. We argue that this faster growth is shifting the importance of field measurements and numerical simulations for oceanographic research. It is leading to the maturation of computational oceanography as a branch of marine science on par with observational oceanography. One implication is that ultraresolved ocean simulations are only loosely constrained by observations. Another implication is that barriers to analyzing the output of such simulations should be removed. Although some specific limits and challenges exist, many opportunities are identified for the future of computational oceanography. Most important is the prospect of hybrid computational and observational approaches to advance understanding of the ocean.@en

Ocean currents along the Southeast Greenland Coast play an important role in the climate system. They carry dense water over the Denmark Strait sill, fresh water from the Arctic and the Greenland Ice Sheet into the subpolar ocean, and warm Atlantic water into Greenland’s fjords, where it can interact with outlet glaciers. Observational evidence from moorings shows that the circulation in this region displays substantial subinertial variability (typically with periods of several days). For the dense water flowing over the Denmark Strait sill, this variability augments the time-mean transport. It has been suggested that the subinertial variability found in observations is associated with Coastal Trapped Waves, whose properties depend on bathymetry, stratification, and the mean flow. Here, we use the output of a high-resolution realistic simulation to diagnose and characterize subinertial variability in sea surface height and velocity along the coast. The results show that the subinertial signals are coherent over hundreds of kilometers along the shelf. We find Coastal Trapped Waves on the shelf and along the shelf break in two subinertial frequency bands—at periods of 1–3 days and 5–18 days—that are consistent with a combination of Mode I waves and higher modes. Furthermore, we find that northeasterly barrier winds may trigger the 5–18 day shelf waves, whereas the 1–3 day variability is linked to high wind speeds over Sermilik Deep.@en

Theories of the Beaufort Gyre (BG) dynamics commonly represent the halocline as a single layer with a thickness depending on the Eulerian-mean and eddy-induced overturning. However, observations suggest that the isopycnal slope increases with depth, and a theory to explain this profile remains outstanding. Here we develop a multilayer model of the BG, including the Eulerian-mean velocity, mesoscale eddy activity, diapycnal mixing, and lateral boundary fluxes, and use it to investigate the dynamics within the Pacific Winter Water (PWW) layer. Using theoretical considerations, observational data, and idealized simulations, we demonstrate that the eddy overturning is critical in explaining the observed vertical structure. In the absence of the eddy overturning, the Ekman pumping and the relatively weak vertical mixing would displace isopycnals in a nearly parallel fashion, contrary to observations. This study finds that the observed increase of the isopycnal slope with depth in the climatological state of the gyre is consistent with a Gent–McWilliams eddy diffusivity coefficient that decreases by at least 10%–40% over the PWW layer. We further show that the depth-dependent eddy diffusivity profile can explain the relative magnitude of the correlated isopycnal depth and layer thickness fluctuations on interannual time scales. Our inference that the eddy overturning generates the isopycnal layer thickness gradients is consistent with the parameterization of eddies via a Gent–McWilliams scheme but not potential vorticity diffusion. This study implies that using a depth-independent eddy diffusivity, as is commonly done in low-resolution ocean models, may contribute to misrepresentation of the interior BG dynamics.@en

Denmark Strait, the channel located between Greenland and Iceland, is a critical gateway between the Nordic Seas and the North Atlantic. Mesoscale features crossing the strait regularly enhance the volume transport of the Denmark Strait overflow. They interact with the dense water masses descending into the subpolar North Atlantic and therefore are important for the Atlantic Meridional Overturning Circulation. Using a realistic numerical model, we find new evidence of the causal relationship between overflow surges (i.e., mesoscale features associated with high-transport events) and overflow cyclones observed downstream. Most of the cyclones form at the Denmark Strait sill during overflow surges and, because of potential vorticity conservation and stretching of the water column, grow as they move equatorward. A fraction of the cyclones form downstream of the sill, when anticyclonic vortices formed during high-transport events start collapsing. Regardless of their formation mechanism, the cyclones weaken starting roughly 150 km downstream of the sill, and potential vorticity is only materially conserved during the growth phase.@en

Ocean currents along the Southeast Greenland Coast play an important role in North Atlantic circulation and the global climate system. They carry dense water over the Denmark Strait sill, fresh water from the Arctic and the Greenland Ice Sheet into the subpolar ocean, and warm Atlantic water into Greenland’s fjords, where it can interact with outlet glaciers. Observational evidence from the OSNAP array and other mooring records shows that the circulation in this region displays substantial subinertial variability, typically with periods of several days. For the dense water flowing over the Denmark Strait sill, this variability augments the time-mean transport; on the shelf, the variability is large enough to occasionally reverse the mean transport direction of the coastal current, highlighting the importance of characterizing this variability when interpreting synoptic surveys. In this study, we used the output of a high-resolution realistic simulation to diagnose and characterize subinertial variability in sea surface height and velocity along the coast. The results show that the subinertial signals on the shelf and along the shelf break are coherent over hundreds of kilometers, and consistent with Coastal Trapped Waves in two subinertial frequency bands—at periods of 1–3 days and 5–18 days—portraying a combination of Mode I and higher modes waves. Furthermore, we find that northeasterly barrier winds may trigger the 5–18 day shelf waves, whereas the 1–3 day variability is linked to high wind speeds over Sermilik Deep.@en