Sheldon Bacon

The Arctic Ocean double estuary is a “three‐legged” overturning system in which inflowing waters are converted into both lighter and denser waters before being exported equatorwards. As the northern terminus of the Atlantic Meridional Overturning Circulation (MOC), it thus both affects, and is affected by, the Atlantic MOC. Here we quantify the magnitudes of the two overturning cells in density space, and then decompose the water mass transformation rates into net pan‐Arctic contributions from surface forcing and diapycnal mixing. We use a high‐resolution, quasi‐synoptic ice and ocean hydrographic data set spanning the four main Arctic Ocean gateways—Fram, Davis and Bering Straits, and the Barents Sea Opening. Two surface flux reanalyses and a hydrographic climatology are used to generate estimates of surface water mass transformation rates by density class. A box model then determines the profiles of turbulent mixing transformation rates, and associated turbulent diffusivities. We show that turbulent mixing and surface forcing drive transformations of similar magnitudes, while mixing dominates in the upper cell and surface fluxes in the lower cell. Consideration of uncertainties and timescales leads to the tentative suggestion that our results might be representative of recent decades. We discuss the possible significance of tides and sea ice brine rejection as energy sources driving turbulent mixing. Finally, we speculate as to whether water mass transformation rates may change in future as ocean heat transport into the Arctic increases. As sea ice declines and the efficiency of atmosphere‐to‐ocean momentum transfer increases, the Arctic Ocean is expected to “spin up,” causing more intense turbulent mixing, with uncertain consequences.

The Arctic Ocean is changing rapidly due to climate change, with significant impacts on subpolar ocean dynamics and mid-latitude regional weather. By utilizing a global, 1/12 degree, ocean sea-ice model (NEMO-SI3), which is forced at its surface by an Earth System Model, UKESM1.1, and simulates from 1981 to 2100 under scenario SSP3-7.0, we will demonstrate significant differences in the spatial structure and energy spectrum of Arctic Ocean currents among the past, the present, and the future. We will then explore the implications of changes in Arctic Ocean currents on mass transport pathways within the Arctic and transport across its boundaries. Subsequently, our study will identify the dominant physical drivers of these changes, such as sea ice melting, freshwater discharge, wind stresses, surface heat fluxes, and tides.

The Arctic Ocean has been covered by sea ice year-round for much of the past, inhibiting the transfer of momentum from atmosphere to ocean, with the consequence that Arctic Ocean currents are generally slow and turbulent mixing weak. However, recent decades have seen accelerated lower tropospheric warming accompanied by declines in sea ice concentration, thickness and extent, and more recently, changes in the ocean, termed ”atlantification”, are beginning to be observed. Against this background, here we explore the nature of the Arctic Ocean ”double estuary”, whereby (mainly) inflowing Atlantic-sourced waters are transformed into both lighter and denser components in a two-cell density-overturning circulation. The double estuary is quantified using measurements, and a box model is employed to determine the relative significance of surface forcing versus turbulent mixing to water mass transformation. We generate a net Arctic Ocean profile of turbulent diffusivity that is used to test the likely contribution of tides to mixing, and we find that the outcome is most sensitive to mixing efficiency. We note that Arctic Ocean dense water formation adds to the recognised sites of dense water formation in the Nordic Seas and northern North Atlantic. Finally, we discuss how mixing rates may change in future as sea ice declines and the efficiency of atmosphere-to-ocean momentum transfer increases, leading to ocean ”spin-up” and more intense turbulent mixing, and the possible consequences thereof.

There is growing evidence for rapid Arctic climate change, including the intrusion of warm and saline Atlantic Water into the Arctic, seasonal sea ice loss and shifts in circulation pathways. These rapid Arctic changes can modulate subpolar North Atlantic processes, the connecting link between the Arctic and the North Atlantic, with potentially far-reaching and profound implications for the Northern Hemisphere weather and climate. Across observations and model datasets, untangling this complex set of multiscale phenomena in the Arctic and their potential teleconnections is a major challenge. Yet intercomparison of different data products is necessary for understanding the mechanisms underlying the rapid Arctic changes. In this study, we seek to understand recent changes and their underlying physical processes so that we can better observe, model and predict future climate changes. The spatiotemporal resolution of data needed to cover a sufficient range of multiscale processes tends to be high, which leads to huge volumes of data to analyse. Fortunately, however, there have been new advances in data-driven methods. These methods abstract the problem to a dynamical systems framework of Reduced Order Modelling with the aim of discovering the key physical processes buried within the complexity.Therefore, this study simplifies the complexity of the multiscale climate variability , starting with analysis of the Arctic Subpolar Gyre sTate Estimate (ASTE), spanning 2002-2017. We use sparse sampling methods to robustly approximate the underlying dynamics efficiently, even on huge datasets. An advantage is that we can explore periods of rapid changes in the system as a whole, i.e. not solely via one measure or variable. This provides a natural way to study changes to the dynamical balance across all state variables and to extract teleconnection patterns. By taking advantage of these new, data-driven approaches, the findings of our study will inform the detection and attribution of rapid changes to the Arctic – North Atlantic climate system, especially in regions where the relevant state variables are hard to observe or estimate accurately.

Amplified Arctic ice loss in recent decades has been linked to the increased occurrence of extreme mid-latitude weather. The underlying mechanisms remain elusive, however. One potential link occurs through the ocean as the loss of sea ice and glacial ice leads to increased freshwater fluxes into the ocean. Thus, in this study, we examine the link between North Atlantic freshwater anomalies and European summer weather. Combining a comprehensive set of observational products, we show that stronger freshwater anomalies are associated with a sharper sea surface temperature front between the subpolar and the subtropical North Atlantic in winter, an increased atmospheric instability above the sea surface temperature front, and a large-scale atmospheric circulation that induces a northward shift in the North Atlantic Current, strengthening the sea surface temperature front. In the following summer, the lower-tropospheric winds are deflected northward along the enhanced sea surface temperature front and the European coastline, forming part of a large-scale atmospheric circulation anomaly that is associated with warmer and drier weather over Europe. The identified statistical links are significant on timescales from years to decades and indicate an enhanced predictability of European summer weather at least a winter in advance, with the exact regions and amplitudes of the warm and dry weather anomalies over Europe being sensitive to the location, strength, and extent of North Atlantic freshwater anomalies in the preceding winter.

The overturning streamfunction as measured at the OSNAP (Overturning in the Subpolar North Atlantic Program) mooring array represents the transformation of warm, salty Atlantic Water into cold, fresh North Atlantic Deep Water (NADW). The magnitude of the overturning at the OSNAP array can therefore be linked to the transformation by air–sea buoyancy fluxes and mixing in the region north of the OSNAP array. Here, we estimate these water mass transformations using observational-based, reanalysis-based and model-based datasets. Our results highlight that air–sea fluxes alone cannot account for the time-mean magnitude of the overturning at OSNAP, and therefore a residual mixing-driven transformation is required to explain the difference. A cooling by air–sea heat fluxes and a mixing-driven freshening in the Nordic Seas, Iceland Basin and Irminger Sea precondition the warm, salty Atlantic Water, forming subpolar mode water classes in the subpolar North Atlantic. Mixing in the interior of the Nordic Seas, over the Greenland–Scotland Ridge and along the boundaries of the Irminger Sea and Iceland Basin drive a water mass transformation that leads to the convergence of volume in the water mass classes associated with NADW. Air–sea buoyancy fluxes and mixing therefore play key and complementary roles in setting the magnitude of the overturning within the subpolar North Atlantic and Nordic Seas. This study highlights that, for ocean and climate models to realistically simulate the overturning circulation in the North Atlantic, the small-scale processes that lead to the mixing-driven formation of NADW must be adequately represented within the model's parameterisation scheme.

Freshwater plays a key role in the Arctic - North Atlantic climate system. On the one hand, it has been suggested as a precursor of large-scale weather extremes and as a potential trigger of rapid climate changes in the past. On the other hand, it is, itself, a sensitive climate change indicator that increases in response to the melting cryosphere. Yet, future risks arising from enhanced glacial and sea ice melt remain difficult to assess due to the complexity of the involved ice-ocean-atmosphere feedbacks and the interference of signals on different timescales. Combining observations, models, theory, and a sophisticated statistical approach, we demonstrate the central role of freshwater anomalies in North Atlantic climate variability over the last 70 years, assess the extent to which they have been contributing to weather extremes, and discuss the risk of a more fundamental climate change under increased freshwater fluxes in future.

The overturning streamfunction as measured at the OSNAP (Overturning in the Subpolar North Atlantic Program) mooring array represents the transformation of warm/salty Atlantic Water into cold/fresh North Atlantic Deep Water (NADW). The magnitude of the overturning at the OSNAP mooring array can therefore be linked to the water mass transformation by air--sea buoyancy fluxes and mixing in the region to the north of the OSNAP array. Here, we estimate these water mass transformations using a combination of observational-based, reanalysis-based and model-based datasets. Our results highlight the complementary roles of air--sea buoyancy fluxes and mixing in setting the time-mean magnitude of the overturning at OSNAP. A cooling by air--sea heat fluxes and a mixing-driven freshening in the Nordics Seas, Iceland Basin and Irminger Sea, precondition the warm/salty Atlantic Water, forming subpolar mode water classes. Mixing in the interior of the Nordic Seas, over the Greenland-Scotland ridge and along the boundaries of the Irminger Sea and Iceland Basin drive a water mass transformation that leads to the convergence of volume in the water mass classes associated with NADW. Air--sea buoyancy fluxes and mixing therefore play key and complementary roles in setting the magnitude of the overturning within the subpolar North Atlantic and Nordic Seas. This study highlights that for climate models to realistically simulate the overturning circulation in the North Atlantic, the small scale processes that lead to the mixing-driven formation of NADW must be adequately represented within the model's parameterisation scheme.

The overturning streamfunction as measured at the OSNAP (Overturning in the Subpolar North Atlantic Program) mooring array represents the transformation of warm/salty Atlantic Water into cold/fresh North Atlantic Deep Water (NADW). The magnitude of the overturning at the OSNAP mooring array can therefore be linked to the water mass transformation by air--sea buoyancy fluxes and mixing in the region to the north of the OSNAP array. Here, we estimate these water mass transformations using a combination of observational-based, reanalysis-based and model-based datasets. Our results highlight the complementary roles of air--sea buoyancy fluxes and mixing in setting the time-mean magnitude of the overturning at OSNAP. A cooling by air--sea heat fluxes and a mixing-driven freshening in the Nordics Seas, Iceland Basin and Irminger Sea, precondition the warm/salty Atlantic Water, forming subpolar mode water classes. Mixing in the interior of the Nordic Seas, over the Greenland-Scotland ridge and along the boundaries of the Irminger Sea and Iceland Basin drive a water mass transformation that leads to the convergence of volume in the water mass classes associated with NADW. Air--sea buoyancy fluxes and mixing therefore play key and complementary roles in setting the magnitude of the overturning within the subpolar North Atlantic and Nordic Seas. This study highlights that for climate models to realistically simulate the overturning circulation in the North Atlantic, the small scale processes that lead to the mixing-driven formation of NADW must be adequately represented within the model's parameterisation scheme.

Amplified Arctic ice loss in recent decades has been linked to increased occurrence of extreme mid-latitude weather. The underlying mechanisms remain elusive, however. One potential link occurs through the ocean as the loss of sea ice and glacial ice leads to increased freshwater fluxes into the North Atlantic. Thus, in this study, we examine the extent to which North Atlantic freshwater anomalies constrain the subsequent ocean-atmosphere evolution and assess their implications for European summer weather. Combining remote sensing, atmospheric reanalyses and model simulations, we show that stronger freshwater anomalies are associated with sharper sea surface temperature gradients over the North Atlantic in winter, destabilising the overlying atmosphere and inducing a northward shift in the North Atlantic Current. In turn, the jet stream over the North Atlantic is deflected northward in the following summers, leading to warmer and drier weather over Europe. Our results suggest that growing freshwater fluxes into the North Atlantic will increase the risk of heat waves and droughts over the coming decades, and could yield enhanced predictability of European summer weather, months to years in advance.