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Abstract The mixed layer of polynyas is vital for local climate as it determines the exchange of properties and energy between ocean, sea ice, and atmosphere. However, its evolution is poorly understood, as it is controlled by complex interactions among these components, yet highly undersampled, especially outside summer. Here, we present a 2-month, high vertical-resolution, full-depth hydrographic dataset from the southeastern Amundsen Sea polynya in austral autumn (from mid-February to mid-April 2014) collected by a recovered seal tag. This novel dataset quantifies the changes in upper-ocean temperature and salinity stratification in this previously unobserved season. Our seal-tag measurements reveal that the mixed layer experiences deepening, salinification, and intense heat loss through surface fluxes. Heat and salt budgets suggest a sea ice formation rate of ∼3 cm per day. We use a one-dimensional model to reproduce the mixed layer evolution and further identify key controls on its characteristics. Our experiments with a range of reduced or amplified air–sea fluxes show that heat loss to the atmosphere and related sea ice formation are the principal determinants of stratification evolution. Additionally, our modeling demonstrates that horizontal advection is required to fully explain the mixed layer evolution, underlining the importance of the ice-covered neighboring region for determining sea ice formation rates in the Amundsen Sea polynya. Our findings suggest that the potential overestimation of sea ice production by satellite-based methods, due to the absence of oceanic heat flux, could be offset by horizontal advection inhibiting mixed layer deepening and sustaining sea ice formation. 

Abstract Rift propagation, rather than basal melt, drives the destabilization and disintegration of the Thwaites Eastern Ice Shelf. Since 2016, rifts have episodically advanced throughout the central ice-shelf area, with rapid propagation events occurring during austral spring. The ice shelf's speed has increased by ~70% during this period, transitioning from a rate of 1.65 m d⁻¹in 2019 to 2.85 m d⁻¹by early 2023 in the central area. The increase in longitudinal strain rates near the grounding zone has led to full-thickness rifts and melange-filled gaps since 2020. A recent sea-ice break out has accelerated retreat at the western calving front, effectively separating the ice shelf from what remained of its northwestern pinning point. Meanwhile, a distributed set of phase-sensitive radar measurements indicates that the basal melting rate is generally small, likely due to a widespread robust ocean stratification beneath the ice–ocean interface that suppresses basal melt despite the presence of substantial oceanic heat at depth. These observations in combination with damage modeling show that, while ocean forcing is responsible for triggering the current West Antarctic ice retreat, the Thwaites Eastern Ice Shelf is experiencing dynamic feedbacks over decadal timescales that are driving ice-shelf disintegration, now independent of basal melt. 

Abstract Atmospheric rivers (ARs) are efficient mechanisms for transporting atmospheric moisture from low latitudes to the Antarctic Ice Sheet (AIS). While AR events occur infrequently, they can lead to extreme precipitation and surface melt events on the AIS. Here we estimate the contribution of ARs to total Antarctic precipitation, by combining precipitation from atmospheric reanalyses and a polar‐specific AR detection algorithm. We show that ARs contribute substantially to Antarctic precipitation, especially in East Antarctica at elevations below 3,000 m. ARs contribute substantially to year‐to‐year variability in Antarctic precipitation. Our results highlight that ARs are an important component for understanding present and future Antarctic mass balance trends and variability. 

Abstract Understanding the recent history of Thwaites Glacier, and the processes controlling its ongoing retreat, is key to projecting Antarctic contributions to future sea-level rise. Of particular concern is how the glacier grounding zone might evolve over coming decades where it is stabilized by sea-floor bathymetric highs. Here we use geophysical data from an autonomous underwater vehicle deployed at the Thwaites Glacier ice front, to document the ocean-floor imprint of past retreat from a sea-bed promontory. We show patterns of back-stepping sedimentary ridges formed daily by a mechanism of tidal lifting and settling at the grounding line at a time when Thwaites Glacier was more advanced than it is today. Over a duration of 5.5 months, Thwaites grounding zone retreated at a rate of >2.1 km per year—twice the rate observed by satellite at the fastest retreating part of the grounding zone between 2011 and 2019. Our results suggest that sustained pulses of rapid retreat have occurred at Thwaites Glacier in the past two centuries. Similar rapid retreat pulses are likely to occur in the near future when the grounding zone migrates back off stabilizing high points on the sea floor. 

Abstract High snowfall events on Thwaites Glacier (TG, West Antarctica) are a key influencer of its mass balance, and can act to mitigate sea level rise due to ocean warming‐induced ice loss. We use the output of a high‐resolution regional climate model, RACMO2, in conjunction with MERRA‐2 and ERA5 atmospheric reanalyses for the period 1980–2015 and show that there is a pronounced seasonal cycle in snowfall over TG, driven by the Amundsen Sea Low (ASL). We find that the total annual snowfall does not correlate significantly with the Southern Annular Mode or El Niño Southern Oscillation, but it does relate to the zonal wave three pattern over Antarctica through the coupling of the ASL with a blocking high over the Antarctic Peninsula during high snowfall events. Our results highlight that atmospheric circulation and consequent high snowfall events on TG are highly variable, and recognizing their future change will aid to improve predictions of mass balance. 

Abstract Determining the injection of glacial meltwater into polar oceans is crucial for quantifying the climate system response to ice sheet mass loss. However, meltwater is poorly observed and its pathways poorly known, especially in winter. Here we present winter meltwater distribution near Pine Island Glacier using data collected by tagged seals, revealing a highly variable meltwater distribution with two meltwater-rich layers in the upper 250 m and at around 450 m, connected by scattered meltwater-rich columns. We show that the hydrographic signature of meltwater is clearest in winter, when its presence can be unambiguously mapped. We argue that the buoyant meltwater provides near-surface heat that helps to maintain polynyas close to ice shelves. The meltwater feedback onto polynyas and air-sea heat fluxes demonstrates that although the processes determining the distribution of meltwater are small-scale, they are important to represent in Earth system models. 

Abstract Widespread ice shelf thinning has been recorded in the Amundsen Sea in recent decades, driven by basal melting and intrusions of relatively warm Circumpolar Deep Water (CDW) onto the continental shelf. The Dotson Ice Shelf (DIS) is located to the south of the Amundsen Sea polynya, and has a high basal melting rate because modified CDW (mCDW) fills the Dotson‐Getz Trough (DGT) and reaches the base of the ice shelf. Here, hydrographic data in the DGT obtained during seven oceanographic surveys from 2007 to 2018 were used to study the interannual variation in mCDW volume and properties and their causes. Although mCDW volume showed relatively weak interannual variations at the continental shelf break, these variations intensified southward and reached a maximum in front of the DIS. There, the mCDW volume was ∼8,000 km³in 2007, rapidly decreased to 4,700 km³in 2014 before rebounding to 7,300 km³in 2018. We find that such interannual variability is coherent with local Ekman pumping integrated along the DGT modulated by the presence of sea ice, and complementing earlier theories involving shelf break winds only. The interannual variability in strength of the dominant south‐southeast coastal wind modulates the amplitude of Ekman upwelling along the eastern boundary of the Amundsen Sea polynya during the austral summers of the surveyed years, apparently leading to change in the volume of mCDW along the DGT. We note a strong correlation between the wind variability and the longitudinal location of the Amundsen Sea Low. 

Abstract. In coastal polynyas, where sea-ice formation and melting occur, it is crucial to have accurate estimates of heat fluxes in order to predict future sea-ice dynamics. The Amundsen Sea Polynya is a coastal polynya in Antarctica that remains poorly observed by in situ observations because of its remoteness. Consequently, we rely on models and reanalysis that are un-validated against observations to study the effect of atmospheric forcing on polynya dynamics. We use austral summer 2022 shipboard data to understand the turbulent heat flux dynamics in the Amundsen Sea Polynya and evaluate our ability to represent these dynamics in ERA5. We show that cold- and dry-air outbreaks from Antarctica enhance air–sea temperature and humidity gradients, triggering episodic heat loss events. The ocean heat loss is larger along the ice-shelf front, and it is also where the ERA5 turbulent heat flux exhibits the largest biases, underestimating the flux by up to 141 W m−2 due to its coarse resolution. By reconstructing a turbulent heat flux product from ERA5 variables using a nearest-neighbor approach to obtain sea surface temperature, we decrease the bias to 107 W m−2. Using a 1D model, we show that the mean co-located ERA5 heat loss underestimation of 28 W m−2 led to an overestimation of the summer evolution of sea surface temperature (heat content) by +0.76 °C (+8.2×107 J) over 35 d. By obtaining the reconstructed flux, the reduced heat loss bias (12 W m−2) reduced the seasonal bias in sea surface temperature (heat content) to −0.17 °C (−3.30 × 107 J) over the 35 d. This study shows that caution should be applied when retrieving ERA5 turbulent flux along the ice shelves and that a reconstructed flux using ERA5 variables shows better accuracy.