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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. 

Abstract The Western Antarctic Peninsula is undergoing rapid environmental change. Regional warming is causing increased glacial meltwater discharge, but the ecological impact of this meltwater over large spatiotemporal scales is not well understood. Here, we leverage 20 years of remote sensing data, reanalysis products, and field observations to assess the effects of sea surface glacial meltwater on phytoplankton biomass and highlight its importance as a key environmental driver for this region’s productive ecosystem. We find a strong correlation between meltwater and phytoplankton chlorophyll-a across multiple time scales and datasets. We attribute this relationship to nutrient fertilization by glacial meltwater, with potential additional contribution from surface ocean stabilization associated with sea-ice presence. While high phytoplankton biomass typically follows prolonged winter sea-ice seasons and depends on the interplay between light and nutrient limitation, our results indicate that the positive effects of increased glacial meltwater on phytoplankton communities likely mitigate the negative impact of sea-ice loss in this region in recent years. Our findings underscore the critical need to consider glacial meltwater as a key ecological driver in polar coastal ecosystems. 

Abstract Southern Ocean mode waters act as a major sink for both anthropogenic heat and carbon, yet the mechanistic understanding of their variability remains incomplete. We use observation-based data to examine an oceanic pathway that begins at the Indonesian Throughflow (ITF), extends through the South Equatorial Current and Agulhas Current, and ultimately encircles the Southern Ocean via the Antarctic Circumpolar Current (ACC). Our analysis reveals that interannual temperature–salinity variations observed in the ITF region propagate into the Southern Ocean along this pathway, taking approximately 3 years to propagate from the Indonesian Archipelago to the Agulhas Retroflection region. Lagrangian particle tracking reveals that both Agulhas Current waters and waters from the Atlantic and further poleward shape the temperature–salinity characteristics of ACC waters in the western Indian Ocean, although the relative contributions of these oceanic pathways and atmospheric forcings have yet to be quantified. In the western Indian Ocean, mixed-layer depth (MLD) and Subantarctic Mode Water (SAMW) volume and temperature–salinity properties all show strong quasi-biennial variability. These anomalies are advected eastward with the ACC as vertically coherent structures extending over the top 1000 m, reemerging in the eastern Indian Ocean approximately 1 year later, reaching the southeast Pacific in 4–5 years and further to the east of Drake Passage a year afterward. The propagation speed of these anomalies is around 40°–55° longitude per year along the ACC, or equivalently 0.15–0.2 m s⁻¹, matching the advective speed of water particles driven by mean geostrophic currents in the ACC. In specific years, the eastward advection of anomalies can complete a full circumpolar circuit around the Southern Ocean. 

Dense, cold waters formed on Antarctic continental shelves descend along the Antarctic continental margin, where they mix with other Southern Ocean waters to form Antarctic Bottom Water (AABW). AABW then spreads into the deepest parts of all major ocean basins, isolating heat and carbon from the atmosphere for centuries. Despite AABW’s key role in regulating Earth’s climate on long time scales and in recording Southern Ocean conditions, AABW remains poorly observed. This lack of observational data is mostly due to two factors. First, AABW originates on the Antarctic continental shelf and slope wherein situmeasurements are limited and ocean observations by satellites are hampered by persistent sea ice cover and long periods of darkness in winter. Second, north of the Antarctic continental slope, AABW is found below approximately 2 km depth, wherein situobservations are also scarce and satellites cannot provide direct measurements. Here, we review progress made during the past decades in observing AABW. We describe 1) long-term monitoring obtained by moorings, by ship-based surveys, and beneath ice shelves through bore holes; 2) the recent development of autonomous observing tools in coastal Antarctic and deep ocean systems; and 3) alternative approaches including data assimilation models and satellite-derived proxies. The variety of approaches is beginning to transform our understanding of AABW, including its formation processes, temporal variability, and contribution to the lower limb of the global ocean meridional overturning circulation. In particular, these observations highlight the key role played by winds, sea ice, and the Antarctic Ice Sheet in AABW-related processes. We conclude by discussing future avenues for observing and understanding AABW, impressing the need for a sustained and coordinated observing system. 

Abstract Compared to the Arctic, seasonal predictions of Antarctic sea ice have received relatively little attention. In this work, we utilize three coupled dynamical prediction systems developed at the Geophysical Fluid Dynamics Laboratory to assess the seasonal prediction skill and predictability of Antarctic sea ice. These systems, based on the FLOR, SPEAR_LO, and SPEAR_MED dynamical models, differ in their coupled model components, initialization techniques, atmospheric resolution, and model biases. Using suites of retrospective initialized seasonal predictions spanning 1992–2018, we investigate the role of these factors in determining Antarctic sea ice prediction skill and examine the mechanisms of regional sea ice predictability. We find that each system is capable of skillfully predicting regional Antarctic sea ice extent (SIE) with skill that exceeds a persistence forecast. Winter SIE is skillfully predicted 11 months in advance in the Weddell, Amundsen and Bellingshausen, Indian, and West Pacific sectors, whereas winter skill is notably lower in the Ross sector. Zonally advected upper ocean heat content anomalies are found to provide the crucial source of prediction skill for the winter sea ice edge position. The recently-developed SPEAR systems are more skillful than FLOR for summer sea ice predictions, owing to improvements in sea ice concentration and sea ice thickness initialization. Summer Weddell SIE is skillfully predicted up to 9 months in advance in SPEAR_MED, due to the persistence and drift of initialized sea ice thickness anomalies from the previous winter. Overall, these results suggest a promising potential for providing operational Antarctic sea ice predictions on seasonal timescales.