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Ocean warming around Antarctica has the potential to trigger marine ice‐sheet instabilities. It has been suggested that abrupt and irreversible cold‐to‐warm ocean tipping points may exist, with possible domino effect from ocean to ice‐sheet tipping points. A 1/4° ocean model configuration of the Amundsen Sea sector is used to investigate the existence of ocean tipping points, their drivers, and their potential impact on ice‐shelf basal melting. We apply idealized atmospheric perturbations of either heat, freshwater, or momentum fluxes, and we characterize the key physical processes at play in warm‐to‐cold and cold‐to‐warm climate transitions. Relatively weak perturbations of any of these fluxes are able to switch the Amundsen Sea to an intermittent or permanent cold state, that is, with ocean temperatures close to the surface freezing point and very low ice‐shelf melt rate. The transitions are reversible, that is, canceling the atmospheric perturbation brings the ocean system back to its unperturbed state within a few decades. All the transitions are primarily driven by changes in surface buoyancy fluxes resulting from the freshwater flux perturbation or from modified net sea‐ice production due to either heat flux or sea‐ice advection anomalies. These changes affect the vertical ocean stratification over the continental shelf and thereby the eastward undercurrent at the shelf break, which both impact ice‐shelf melting. As sea‐ice induced deep convection is already quite limited in present‐day conditions, surface buoyancy gain in a warmer climate has relatively little effect on deep ocean properties compared to colder climate conditions. Plain Language Summary: The West Antarctic Ice Sheet is under the threat of a partial collapse, which would induce rapid global sea level rise. This threat is partly related to the thinning of floating ice shelves, and the consequent retreat of the grounding line, which is a self‐sustained ice dynamics process. It is triggered by increased basal melting of the ice shelves, which results from enhanced flow of relatively warm waters onto the continental shelf. It has been suggested that self‐sustained ocean processes may lead to abrupt changes in the flow of warm water into ice‐shelf cavities, which could facilitate the tipping to a marine ice‐sheet instability. Here, we analyze whether such abrupt ocean changes can occur under cold‐to‐warm or warm‐to‐cold transitions in the Amundsen Sea, West Antarctica. We use a regional ocean model with a set of idealized local atmospheric perturbations to characterize the thresholds and reversibility of ocean abrupt changes.We find that the currently warm Amundsen Sea could switch intermittently or permanently to a cold state for relatively weak atmospheric perturbations and could be slightly warmer in the future. All transitions are reversible. The main mechanism involved on decadal scale is related to a change in the surface buoyancy fluxes. Key Points: The currently warm ice‐shelf cavities of the Amundsen sector could become or have been cold for slightly colder climatic conditionsThe transitions are reversible: canceling the atmospheric perturbation brings the ocean back to its unperturbed state within a few decadesAll the transitions are primarily driven, at multi‐decadal scale, by changes in surface buoyancy fluxes over the continental shelf [ABSTRACT FROM AUTHOR]

Antarctic Ice Sheet projections show the highest sensitivity to increased basal melting in the Amundsen Sea. However, little is known about the processes that control future increase in melt rates. We build an ensemble of three ocean–sea‐ice–ice‐shelf simulations for both the recent decades and the late 21st century, constrained by regional atmosphere simulations and the multi‐model mean climate change of the fifth Climate Model Intercomparison Project under the RCP8.5 scenario. The ice‐shelf melt rates are typically multiplied by 1.4–2.2 from present day to future, for a total basal mass loss increased by 346 Gt yr−1 on average. This is equally explained by advection of warmer water from remote locations and regional changes in Ekman downwelling and in the ice‐shelf melt‐induced circulation, while increased iceberg melt plays no significant role. Our simulations suggest that high‐end melt projections previously used to constrain recent sea level projections may have been significantly overestimated. Plain Language Summary: Future sea level rise highly depends on how fast the ocean will melt the floating ice shelves in Antarctica, which modulates the ice flow from the ice sheet into the ocean. This is particularly true for the Amundsen Sea sector where the ice flow into the ocean is very sensitive to ocean‐induced melting. Here, we use a numerical model that represents the evolution of the Amundsen Sea, including under the floating ice shelves. Under a high‐end greenhouse‐gases concentration pathway, our simulations indicate that melt rates beneath the ice shelves may increase by 40%–120%. This is explained by both warmer seawater coming from distant regions and changes in the local wind stress. Our simulations suggest that high‐end melt projections previously used to constrain recent sea level projections may have been overestimated. Key Points: We present 1/12° ocean‐sea‐ice‐ice‐shelf projections at the end of the 21st century under the RCP8.5 scenarioIce shelf melt rates in the Amundsen Sea are typically multiplied by 1.4–2.2 from 1989–2009 to 2080–2100Advection of warmer water from remote locations and reduced Ekman downwelling are the main drivers of changes in ice shelf melt rates [ABSTRACT FROM AUTHOR]

The Antarctic ice sheet (AIS) is sensitive to short‐term extreme meteorological events that can leave long‐term impacts on the continent's surface mass balance (SMB). We investigate the impacts of atmospheric rivers (ARs) on the AIS precipitation budget using an AR detection algorithm and a regional climate model (Modèle Atmosphérique Régional) from 1980 to 2018. While ARs and their associated extreme vapor transport are relatively rare events over Antarctic coastal regions (∼3 days per year), they have a significant impact on the precipitation climatology. ARs are responsible for at least 10% of total accumulated snowfall across East Antarctica (localized areas reaching 20%) and a majority of extreme precipitation events. Trends in AR annual frequency since 1980 are observed across parts of AIS, most notably an increasing trend in Dronning Maud Land; however, interannual variability in AR frequency is much larger. This AR behavior appears to drive a significant portion of annual snowfall trends across East Antarctica, while controlling the interannual variability of precipitation across most of the AIS. AR landfalls are most likely when the circumpolar jet is highly amplified during blocking conditions in the Southern Ocean. There is a fingerprint of the Southern Annular Mode (SAM) on AR variability in West Antarctica with SAM+ (SAM−) favoring increased AR frequency in the Antarctic Peninsula (Amundsen‐Ross Sea coastline). Given the relatively large influence ARs have on precipitation across the continent, it is advantageous for future studies of moisture transport to Antarctica to consider an AR framework especially when considering future SMB changes. Plain Language Summary: The Antarctic continent, like many deserts in the world, receives a large percentage of its yearly precipitation from just a few intense precipitation events. Atmospheric rivers (ARs), narrow corridors of intense moisture transporting moisture from low to high latitudes, are commonly associated with heavy rain and snowfall in the midlatitudes like the west coasts of North/South America and Europe. In Antarctica, ARs are rarer with most near‐coastal regions in Antarctica experiencing AR conditions a few days per year but still have a major influence on the surface mass balance of the ice sheet. ARs are responsible for 10%–20% of the total snowfall across East Antarctica. Although a modest percentage, this contribution to the snowfall budget is the component that has been driving parts of the positive annual snowfall trends in Dronning Maud and negative trends in Wilkes Land. Also, ARs control the year‐to‐year variability of precipitation across most of the ice sheet. Given the link between ARs and snowfall accumulation trends, increased future AR activity would result in higher snowfall accumulation on the Antarctic continent and possibly offset some sea‐level rise from dynamic ice loss, but this must be considered in balance with increased melting frequency already documented with ARs. Key Points: Atmospheric rivers in Antarctica are rare events but are a key contributor to the ice sheet's surface mass balanceTheir impact on precipitation is most pronounced in East Antarctica where they are responsible for a majority of extreme precipitation eventsAtmospheric rivers are contributing to modern snowfall trends and controlling overall precipitation variability across Antarctica [ABSTRACT FROM AUTHOR]

Recent work on the Filchner-Ronne Ice Shelf (FRIS) system has shown that a redirection of the coastal current in the southeastern Weddell Sea could lead to a regime change in which an intrusion of warm Modified Circumpolar Deep Water results in large increases in the basal melt rate. Work to date has mostly focused on how increases in the Modified Circumpolar Deep Water crossing the continental shelf break leads directly to heat driven changes in melting in the ice-shelf cavity. In this study, we introduce a Weddell Sea regional ocean model configuration with static ice shelves. We evaluate a reference simulation against radar observations of melting, and find good agreement between the simulated and observed mean melt rates. We analyze 28 sensitivity experiments that simulate the influence of changes in remote water properties of the Antarctic Slope Current on basal melting in the FRIS. We find that remote changes in salinity quasi-linearly modulate the mean FRIS net melt rate. Changes in remote temperature quadratically vary the FRIS net melt rate. In both salinity and temperature perturbations, the response is rapid and transient, with a recovery time-scale of 5-15 years dependent on the size/type of perturbation. We show that the two types of perturbations lead to different changes on the continental shelf, and that ultimately different factors modulate the melt rates in the FRIS cavity. We discuss how these results, are relevant for ocean hindcast simulations, sea level, and melt rate projections of the FRIS. Plain Language Summary Ocean water temperature or the speed of ocean currents can directly influence the melt rate of the floating ice shelves at the edge of the Antarctic Ice Sheet. Ice sheet modelers use simple relationships between ocean temperature and ice-shelf melt rates to create projections of sea level rise, whereby changes in salinity are not directly considered. Here, using a model of the ocean and ice shelf for the Weddell Sea region, we re-visit these relationships in a simulated environment with realistic settings. We find that changes in far field salinity and temperature in the Antarctic Slope Current can cause changes in the Filchner-Ronne Ice Shelf melt rate; this is because both far field temperature and salinity variations have the capacity to change the strength of the ocean circulation in the ice shelf cavity. Our results suggest that future numerical simulations of this region may need to consider changes in both far field temperature and salinity. [ABSTRACT FROM AUTHOR]

Like many western boundary currents, the East Australian Current (EAC) extension is projected to get stronger and warmer in the future. The CMIP5 multimodel mean (MMM) projection suggests up to 5°C of warming under an RCP85 scenario by 2100. Previous studies employed Sverdrup balance to associate a trend in basin wide zonally integrated wind stress curl (resulting from the multidecadal poleward intensification in the westerly winds over the Southern Ocean) with enhanced transport in the EAC extension. Possible regional drivers are yet to be considered. Here we introduce the NEMO-OASIS-WRF coupled regional climate model as a framework to improve our understanding of CMIP5 projections. We analyze a hierarchy of simulations in which the regional atmosphere and ocean circulations are allowed to freely evolve subject to boundary conditions that represent present-day and CMIP5 RCP8.5 climate change anomalies. Evaluation of the historical simulation shows an EAC extension that is stronger than similar ocean-only models and observations. This bias is not explained by a linear response to differences in wind stress. The climate change simulations show that regional atmospheric CMIP5 MMM anomalies drive 73% of the projected 12 Sv increase in EAC extension transport whereas the remote ocean boundary conditions and regional radiative forcing (greenhouse gases within the domain) play a smaller role. The importance of regional changes in wind stress curl in driving the enhanced EAC extension is consistent with linear theory where the NEMO-OASIS-WRF response is closer to linear transport estimates compared to the CMIP5 MMM. [ABSTRACT FROM AUTHOR]

Abstract: The East Australian Current (EAC) plays a major role in regional climate, circulation, and ecosystems, but predicting future changes is hampered by limited understanding of the factors controlling EAC separation. While there has been speculation that the presence of New Zealand may be important for the EAC separation, the prevailing view is that the time‐mean partial separation is set by the ocean's response to gradients in the wind stress curl. This study focuses on the role of New Zealand, and the associated adjacent bathymetry, in the partial separation of the EAC and ocean circulation in the Tasman Sea. Here utilizing an eddy‐permitting ocean model (NEMO), we find that the complete removal of the New Zealand plateau leads to a smaller fraction of EAC transport heading east and more heading south, with the mean separation latitude shifting >100 km southward. To examine the underlying dynamics, we remove New Zealand with two linear models: the Sverdrup/Godfrey Island Rule and NEMO in linear mode. We find that linear processes and deep bathymetry play a major role in the mean Tasman Front position, whereas nonlinear processes are crucial for the extent of the EAC retroflection. Contrary to past work, we find that meridional gradients in the basin‐wide wind stress curl are not the sole factor determining the latitude of EAC separation. We suggest that the Tasman Front location is set by either the maximum meridional gradient in the wind stress curl or the northern tip of New Zealand, whichever is furthest north. [ABSTRACT FROM AUTHOR]

Abstract: It has been suggested that the coastal Southern Ocean subsurface may warm over the 21st century in response to strengthening and poleward shifting winds, with potential adverse effects on West Antarctic glaciers. However, using a 1/12° ocean regional model that includes ice‐shelf cavities, we find a more complex response to changing winds in the Amundsen Sea. Simulated offshore subsurface waters get colder under strengthened and poleward shifted winds representative of the SAM projected trend. The buoyancy‐driven circulation induced by ice‐shelf melt transports this cold offshore anomaly onto the continental shelf, leading to cooling and decreased melt below 450 m. In the vicinity of ice‐shelf fronts, Ekman pumping contributes to raise the isotherms in response to changing winds. This effect overwhelms the horizontal transport of colder offshore waters at intermediate depths (between 200 and 450 m), and therefore increases melt rates in the upper part of the ice‐shelf cavities, which reinforces the buoyancy‐driven circulation and further contributes to raise the isotherms. Then, prescribing an extreme grounding line retreat projected for 2100, the total melt rates simulated underneath Thwaites and Pine Island are multiplied by 2.5. Such increase is explained by a larger ocean/ice interface exposed to CDW, which is then amplified by a stronger melt‐induced circulation along the ice draft. Our main conclusions are that (1) outputs from ocean models that do not represent ice shelf cavities (e.g., CMIP5 models) should not be directly used to predict the thermal forcing of future ice shelf cavities; (2) coupled ocean/ice sheet models with a velocity‐dependent melt formulation are needed for future projections of glaciers experiencing a significant grounding line retreat. [ABSTRACT FROM AUTHOR]

Abstract: The East Australian Current (EAC), like many other subtropical western boundary currents, is believed to be penetrating further poleward in recent decades. Previous observational and model studies have used steady state dynamics to relate changes in the westerly winds to changes in the separation behavior of the EAC. As yet, little work has been undertaken on the impact of forcing variability on the EAC and Tasman Sea circulation. Here using an eddy‐permitting regional ocean model, we present a suite of simulations forced by the same time‐mean fields, but with different atmospheric and remote ocean variability. These eddy‐permitting results demonstrate the nonlinear response of the EAC to variable, nonstationary inhomogeneous forcing. These simulations show an EAC with high intrinsic variability and stochastic eddy shedding. We show that wind stress variability on time scales shorter than 56 days leads to increases in eddy shedding rates and southward eddy propagation, producing an increased transport and southward reach of the mean EAC extension. We adopt an energetics framework that shows the EAC extension changes to be coincident with an increase in offshore, upstream eddy variance (via increased barotropic instability) and increase in subsurface mean kinetic energy along the length of the EAC. The response of EAC separation to regional variable wind stress has important implications for both past and future climate change studies. [ABSTRACT FROM AUTHOR]