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Abstract. Landfast sea ice (fast ice) is an important though poorly-understood component of the cryosphere on the Antarctic continental shelf, where it plays a key role in atmosphere-ocean-ice sheet interaction and coupled ecological and biogeochemical processes. Here, we present a first in-depth baseline analysis of variability and change in circum-Antarctic fast-ice distribution (including its relationship to bathymetry), based on a new high-resolution satellite-derived time series for the period 2000 to 2018. This reveals a) an overall trend of −882 ± 824 km²/y (−0.19 ± 0.18 %/y); and b) eight distinct regions in terms of fast-ice coverage and modes of formation. Of these, four exhibit positive trends over the 18 y period and four negative. Positive trends are seen in East Antarctica and in the Bellingshausen sea, with this region claiming the largest positive trend of +1,198 ± 359 km²/y (+1.10 ± 0.35 %/y). The four negative trends predominantly occur in West Antarctica, with the largest negative trend of −1,206 ± 277 km²/y (−1.78 ± 0.41 %/y) occurring in the Victoria and Oates Lands region in the eastern Ross Sea. All trends are significant. This new baseline analysis represents a significant advance in our knowledge of the current state of both the global cryosphere and the complex Antarctic coastal system that is vulnerable to climate variability and change. It will also inform a wide range of other studies.

Abstract. Landfast sea ice (fast ice) is an important though poorly understood component of the cryosphere on the Antarctic continental shelf, where it plays a key role in atmosphere–ocean–ice-sheet interaction and coupled ecological and biogeochemical processes. Here, we present a first in-depth baseline analysis of variability and change in circum-Antarctic fast-ice distribution (including its relationship to bathymetry), based on a new high-resolution satellite-derived time series for the period 2000 to 2018. This reveals (a) an overall trend of -882±824 km2 yr−1 (-0.19±0.18 % yr−1) and (b) eight distinct regions in terms of fast-ice coverage and modes of formation. Of these, four exhibit positive trends over the 18-year period and four negative. Positive trends are seen in East Antarctica and in the Bellingshausen Sea, with this region claiming the largest positive trend of +1198±359 km2 yr−1 (+1.10±0.35 % yr−1). The four negative trends predominantly occur in West Antarctica, with the largest negative trend of -1206±277 km2 yr−1 (-1.78±0.41 % yr−1) occurring in the Victoria and Oates Land region in the western Ross Sea. All trends are significant. This new baseline analysis represents a significant advance in our knowledge of the current state of both the global cryosphere and the complex Antarctic coastal system, which are vulnerable to climate variability and change. It will also inform a wide range of other studies.

The Sea of Okhotsk is a marginal sea that plays a major role in the ventilation of the North Pacific, being the key location where Dense Shelf Water (DSW) forms at the surface and sinks to the intermediate layer. The Okhotsk Sea Intermediate Water (OSIW) is a key water mass because it includes large amounts of DSW, outflows to the North Pacific, and supplies the ocean with the micronutrient iron. OSIW has been warming over the past few decades, which is attributed to a decreasing trend in DSW production. The acquisition of numerous hydrographic data after 2000 in the Kuril Basin, especially dissolved oxygen from profiling floats, offers an opportunity to better quantify the water mass composition of OSIW, and the changes in OSIW properties and DSW volume. Here, we used all available hydrographic records and a mapping technique specially adapted to polar and sub‐polar regions to revisit the Sea of Okhotsk water properties and document their long‐term changes. Our analysis revealed that the volume of heavier DSW (potential density above 26.9 kg.m−3) has decreased over the past three decades by 3,600 km3, or 15% of the volume present before 1990. This decline is nearly entirely compensated for by an increase in lighter DSW. This shift toward lighter DSW is possibly a sign of the weakening of the intermediate overturning circulation starting in the Okhotsk Sea. Additionally, we found that dense Soya Current Water only accounts for about 1% of OSIW, against the 5% previously estimated. Plain Language Summary: The water of the Sea of Okhotsk is a mix of several water masses which includes the Dense Shelf Water (DSW), a water mass generated in winter via sea ice formation. When sea ice forms, the surface water salinity and density increase, and the cold DSW is generated as the surface water sinks to the bottom of the continental shelf while micronutrients, especially iron, are also incorporated. DSW mixes with other water masses and becomes the Okhotsk Sea Intermediate Water (OSIW) before being exported to the North Pacific where it plays an important role for both the climate and the biological productivity. In this study, we used all the temperature, salinity, and dissolved oxygen data acquired since 1930 to evaluate how global warming may have affected the composition of OSIW. We found that, over the past century, the amount of denser DSW has strongly decreased whereas that of lighter DSW has increased, which is consistent with the decline in sea ice production in the Sea of Okhotsk. This also suggests that the ventilation of the Sea of Okhotsk and the circulation of the intermediate layer of the North Pacific is weakening, which could indirectly affect the ocean's biological productivity. Key Points: The composition of the Okhotsk Sea Intermediate Water (OSIW) is reevaluated using an updated data set and a new mapping techniqueOSIW properties have changed in the long‐term (≥50 years) scale, with a decrease (increase) in the amount of heavy (light) Dense Shelf WaterThis suggests that the North Pacific intermediate overturning is weakening, which could affect the ocean's biological productivity [ABSTRACT FROM AUTHOR]

The Labrador Sea and contiguous Baffin Bay play an important role in the formation of the upper layer of the North Atlantic Deep Water, an essential component of the Atlantic Meridional Ocean Circulation. The hydrography of these two seas is strongly influenced by the melting of sea-ice and glacier-ice, which has likely been affected by long-term climate changes. In this study, we use historical data of ocean temperature and salinity from 1950 to 2022 to estimate the summer freshwater volume (SFV) in Baffin Bay and the Labrador Sea, establish climatologies, and assess the impact of multi-decadal climate change. The SFV climatology (1956 km3)and the summer freshwater budget (2286 km3)estimated from various components are in good agreement. Sea ice and glacial melt account for 37 % and 26 % of the freshwater budget, respectively. SFV climatologies before and after 1995 reveal an increase in Baffin Bay (+226 km3) because of enhanced glacier melting, and a decline (-112 km3) in the Labrador Sea because of recent sea ice volume decreases. The time series of Labrador Sea SFV and total freshwater content are uncorrelated at the multi-decadal scale possibly because the influx of freshwater from the Beaufort Sea dominates the long-term variability. [ABSTRACT FROM AUTHOR]