Holly Ayres

As a gateway between the Arctic and subtropics, the subpolar North Atlantic plays a key role in large‐scale climate variability. In recent decades, the region has experienced pronounced freshening. While much attention has been paid to the expected long‐term weakening of the Atlantic Meridional Overturning Circulation, the impact of freshwater on surface fluxes and seasonal to decadal air‐sea coupling remains poorly understood. Here, we explore the detailed mechanism by which the sea surface salinity in the subpolar gyre is linked to the local sea surface cooling with the CANARI Large Ensemble of the HadGEM3‐GC3.1 climate model. Our analyses suggest that freshwater fluxes modulate upper ocean dynamics, which enhance local cooling in winter and can explain the North Atlantic warming hole without the need to invoke large‐scale ocean circulation changes as a driver. Moreover, by shaping the local air‐sea heat fluxes, the freshening can have further‐reaching implications.

Antarctic sea ice extent began declining in 2015, reaching its minimum in the post-1970s observational era in 2023. To diagnose the drivers of this decline, we analyze an observationally constrained sea ice–ocean model spanning 2013–2023 and identify three distinct phases of sea ice retreat. First, intensifying westerlies preconditioned the Southern Ocean via increased upwelling of warm, saline circumpolar deep water (CDW). Second, strong winds in 2015–2016 enhanced the mixing of CDW into the upper ocean, thereby initiating sea ice loss, particularly in East Antarctica. Third, sustained mixing of CDW into the surface layer, combined with reduced equatorward sea ice–derived freshwater export, maintained an unprecedentedly low sea ice state. East Antarctic sea ice loss was primarily subsurface driven via enhanced upward CDW flux, whereas West Antarctic sea ice loss was also forced by longwave radiative flux anomalies. Our findings suggest that persistent upwelling-favorable conditions under anthropogenic forcing may push the Southern Ocean into a prolonged low sea ice state.

The North Atlantic subpolar gyre is a highly dynamic region where ocean–atmosphere interactions are shaped by variations in freshwater export from the Arctic, import of subtropical waters via the Gulf Stream, mixing, in addition to large-scale atmospheric circulation patterns such as the NAO and the strength and position of the North Atlantic jet. Building on our earlier analysis using the CANARI Large Ensemble (HadGEM3-GC3.1), we previously showed that winters with anomalously fresh surface waters systematically exhibit shallower mixed layers, cooler SSTs, and weaker surface heat loss. These conditions imply enhanced freshwater-driven stratification, reduced deep convection, and a tendency for heat to be trapped below the surface, features consistent with the structure and persistence of the North Atlantic Warming Hole (NAWH).In this study, we extend that framework to examine how this wintertime surface cooling and associated changes in the surface heat fluxes interact with the overlying atmosphere across a range of background circulation states. Using ensemble member–specific sea level pressure anomaly patterns and a regime-classification approach, we identify multiple atmospheric response modes that differ in the strength and latitude of the North Atlantic pressure gradient. These regimes reveal that the atmospheric response to subpolar cooling is not uniform. The background fields play a decisive role in determining how the surface cooling interacts with the large-scale atmospheric circulation.Together, our results highlight a dynamically consistent pathway linking freshwater import, ocean stratification changes, regional winter SST cooling, heat flux responses, and large-scale atmospheric circulation shifts. This work provides new insight into the range of possible atmosphere–ocean climate feedbacks associated with ongoing and future freshening of the subpolar North Atlantic.

As a gateway between the Arctic and subtropics, the subpolar North Atlantic plays a key role in large-scale climate variability. In recent decades, the region has experienced pronounced freshening. While much attention has been paid to the expected long-term weakening of the Atlantic Meridional Overturning Circulation, the impact of freshwater on surface fluxes and seasonal to decadal air-sea coupling remains poorly understood. Here, we explore the detailed mechanism by which the sea surface salinity in the subpolar gyre affects the local sea surface temperature for the first time with the CANARI Large Ensemble of the HadGEM3-GC3.1 climate model. Our analyses suggest that freshwater fluxes modulate upper ocean dynamics, which initiates local cooling in winter and can explain the North Atlantic warming hole without the need to invoke large-scale ocean circulation changes as a driver. Moreover, by shaping the local air-sea heat fluxes, the freshening can have further-reaching implications.

Open-ocean polynyas, areas with little or no sea ice, reappeared extensively in 2016 and 2017 over the Maud Rise in the Weddell Sea after a 40-year hiatus, raising a series of unresolved questions about the atmosphere-ice-ocean interactions in the Antarctic region. These major polynyas significantly influence moisture and heat exchange between the atmosphere and the ocean, impacting both regional and global climate dynamics, as well as ecosystem functioning and biogeochemical processes. Notably, they may play a crucial role in contributing to the formation of Antarctic Bottom Water and influencing global ocean circulation. In this Review, we synthesize current knowledge on the drivers and impacts of Weddell Sea polynyas. Recent occurrences have been linked to factors such as a strengthening Weddell Gyre, a negative Southern Annular Mode, extreme local atmospheric conditions (atmospheric rivers and cyclones), and subsurface ocean heat buildup which acts as a preconditioning factor. The associated deep ocean convection from these polynyas can enhance air-sea gas exchange and trigger earlier phytoplankton blooms due to the influx of iron and nutrients from the deep ocean. While advancements in observation and modeling techniques have significantly improved our understanding of polynyas, substantial uncertainties remain regarding their interaction with recent Antarctic sea ice loss, their sensitivity to ocean mixing schemes, their excessive size or frequency in climate simulations, and future projections. Therefore, future research should focus on developing comprehensive four-dimensional regional observatories and targeted, data-constrained coupled models that accurately capture atmosphere-ice-ocean interactions across various timescales.

Open-ocean polynyas, openings in the sea ice, reappeared extensively in 2016 and 2017 over the Maud Rise in the Weddell Sea after a 40-year hiatus, raising a series of unresolved questions about the atmosphere-ice-ocean interactions in the Antarctic region. These major polynyas significantly influence moisture and heat exchange between the atmosphere and ocean, impacting both regional and global climate dynamics, as well as ecosystem functioning and biogeochemical processes. Notably, they could play a crucial role in contributing to the formation of Antarctic Bottom Water and influencing global ocean circulation. In this Review, we synthesize current knowledge on the drivers and impacts of Weddell Sea polynyas. Recent occurrences have been linked to factors such as a strengthening Weddell Gyre, a negative Southern Annular Mode, extreme local atmospheric conditions (atmosphere river and cyclones), and subsurface ocean heat buildup which acts as a preconditioning factor. The associated deep ocean convection from these polynyas can enhance air-sea gas exchange and trigger earlier phytoplankton blooms due to the influx of iron and nutrients from the deep ocean. While advancements in observation and modeling techniques have significantly improved our understanding of polynyas, substantial uncertainties remain regarding their interaction with recent Antarctic sea ice loss, their sensitivity to ocean mixing schemes and their excessive size or frequency in climate simulations, and future projections. Therefore, future research should focus on developing comprehensive four-dimensional regional observatories and targeted, assimilated coupled models that accurately capture atmosphere-ice-ocean interactions across various timescales.

Open-ocean polynyas, openings in the sea ice, reappeared extensively in 2016 and 2017 over the Maud Rise in the Weddell Sea after a 40-year hiatus, raising a series of unresolved questions about the atmosphere-ice-ocean interactions in the Antarctic region. These major polynyas significantly influence moisture and heat exchange between the atmosphere and ocean, impacting both regional and global climate dynamics, as well as ecosystem functioning and biogeochemical processes. Notably, they could play a crucial role in contributing to the formation of Antarctic Bottom Water and influencing global ocean circulation. In this Review, we synthesize current knowledge on the drivers and impacts of Weddell Sea polynyas. Recent occurrences have been linked to factors such as a strengthening Weddell Gyre, a negative Southern Annular Mode, extreme local atmospheric conditions (atmosphere river and cyclones), and subsurface ocean heat buildup which acts as a preconditioning factor. The associated deep ocean convection from these polynyas can enhance air-sea gas exchange and trigger earlier phytoplankton blooms due to the influx of iron and nutrients from the deep ocean. While advancements in observation and modeling techniques have significantly improved our understanding of polynyas, substantial uncertainties remain regarding their interaction with recent Antarctic sea ice loss, their sensitivity to ocean mixing schemes and their excessive size or frequency in climate simulations, and future projections. Therefore, future research should focus on developing comprehensive four-dimensional regional observatories and targeted, assimilated coupled models that accurately capture atmosphere-ice-ocean interactions across various timescales.

Open-ocean polynyas, openings in the sea ice, reappeared extensively in 2016 and 2017 over the Maud Rise in the Weddell Sea after a 40-year hiatus, raising a series of unresolved questions about the atmosphere-ice-ocean interactions in the Antarctic region. These major polynyas significantly influence moisture and heat exchange between the atmosphere and ocean, impacting both regional and global climate dynamics, as well as ecosystem functioning and biogeochemical processes. Notably, they could play a crucial role in contributing to the formation of Antarctic Bottom Water and influencing global ocean circulation. In this Review, we synthesize current knowledge on the drivers and impacts of Weddell Sea polynyas. Recent occurrences have been linked to factors such as a strengthening Weddell Gyre, a negative Southern Annular Mode, extreme local atmospheric conditions (atmosphere river and cyclones), and subsurface ocean heat buildup which acts as a preconditioning factor. The associated deep ocean convection from these polynyas can enhance air-sea gas exchange and trigger earlier phytoplankton blooms due to the influx of iron and nutrients from the deep ocean. While advancements in observation and modeling techniques have significantly improved our understanding of polynyas, substantial uncertainties remain regarding their interaction with recent Antarctic sea ice loss, their sensitivity to ocean mixing schemes and their excessive size or frequency in climate simulations, and future projections. Therefore, future research should focus on developing comprehensive four-dimensional regional observatories and targeted, assimilated coupled models that accurately capture atmosphere-ice-ocean interactions across various timescales.

Open-ocean polynyas, openings in the sea ice, reappeared extensively in 2016 and 2017 over the Maud Rise in the Weddell Sea after a 40-year hiatus, raising a series of unresolved questions about the atmosphere-ice-ocean interactions in the Antarctic region. These major polynyas significantly influence moisture and heat exchange between the atmosphere and ocean, impacting both regional and global climate dynamics, as well as ecosystem functioning and biogeochemical processes. Notably, they could play a crucial role in contributing to the formation of Antarctic Bottom Water and influencing global ocean circulation. In this Review, we synthesize current knowledge on the drivers and impacts of Weddell Sea polynyas. Recent occurrences have been linked to factors such as a strengthening Weddell Gyre, a negative Southern Annular Mode, extreme local atmospheric conditions (atmosphere river and cyclones), and subsurface ocean heat buildup which acts as a preconditioning factor. The associated deep ocean convection from these polynyas can enhance air-sea gas exchange and trigger earlier phytoplankton blooms due to the influx of iron and nutrients from the deep ocean. While advancements in observation and modeling techniques have significantly improved our understanding of polynyas, substantial uncertainties remain regarding their interaction with recent Antarctic sea ice loss, their sensitivity to ocean mixing schemes and their excessive size or frequency in climate simulations, and future projections. Therefore, future research should focus on developing comprehensive four-dimensional regional observatories and targeted, assimilated coupled models that accurately capture atmosphere-ice-ocean interactions across various timescales.

The recent decline in Antarctic sea-ice, notably the extreme low winter cover in 2023 and 2024 is a major source of concern. Some progress has been made towards determining the drivers of ice loss but uncertainty remains regarding its impacts, particularly for ocean-atmosphere interaction. Resolution of this uncertainty is important as ice decline can significantly modify surface heat loss, and thus the ocean and atmosphere. We show that the substantial failure of ice regrowth in winter 2023 provided a major new source of turbulent ocean heat loss to the atmosphere. Ice concentration in the Weddell, Bellingshausen and Ross Seas is reduced by up to 80% and is accompanied by an unprecedented doubling of mid-winter ocean heat loss. Furthermore, peak heat loss shifts from late April to mid-June with weaker than normal heat loss in austral autumn. The strengthening of winter surface heat loss is accompanied by changes on both sides of the ocean-atmosphere interface. These include a rise in frequency of atmospheric storms and greater surface heat loss driven dense water formation. The findings reveal that the record low winter 2023 Antarctic sea-ice cover substantially modified Southern Ocean-atmosphere interaction and motivate in-depth analysis of the wider climate system impacts. The subsequent evolution of low ice conditions together with their ocean-atmosphere impacts through to 2025 will also be considered.