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During atmospheric river (AR) landfalls on the Antarctic ice sheet, the high waviness of the circumpolar polar jet stream allows for subtropical air masses to be advected toward the Antarctic coastline. These rare but high‐impact AR events are highly consequential for the Antarctic mass balance; yet little is known about the various atmospheric dynamical components determining their life cycle. By using an AR detection algorithm to retrieve AR landfalls at Dumont d'Urville and non‐AR analogs based on 700 hPa geopotential height, we examined what makes AR landfalls unique and studied the complete life cycle of ARs reaching Dumont d'Urville. ARs form in the mid‐latitudes/subtropics in areas of high surface evaporation, likely in response to tropical deep convection anomalies. These convection anomalies likely lead to Rossby wave trains that help amplify the upper‐tropospheric flow pattern. As the AR approaches Antarctica, condensation of isentropically lifted moisture causes latent heat release that—in conjunction with poleward warm air advection—induces geopotential height rises and anticyclonic upper‐level potential vorticity tendencies downstream. As evidenced by a blocking index, these tendencies lead to enhanced ridging/blocking that persist beyond the AR landfall time, sustaining warm air advection onto the ice sheet. Finally, we demonstrate a connection between tropopause polar vortices and mid‐latitude cyclogenesis in an AR case study. Overall, the non‐AR analogs reveal that the amplified jet pattern observed during AR landfalls is a result of enhanced poleward moisture transport and associated diabatic heating which is likely impossible to replicate without strong moisture transport. When the polar jet stream that surrounds Antarctica is highly wavy, air masses from the subtropics that are warm and humid are often transported over the ice sheet in the form of atmospheric rivers (ARs). When ARs reach Antarctica, they often bring extreme weather conditions that have large consequences for ice sheet snowfall and surface melt. Here we studied the full life cycle of ARs that reached Dumont d'Urville in East Antarctica and compared these ARs against events with similar profiles of atmospheric circulation. ARs typically form in areas of unusually high surface evaporation and thunderstorm convection in the subtropics. This convection sends Rossby waves toward the Antarctic coastline which help make the polar jet wavier. The amplitude of the polar jet is further enhanced when the moisture that accompanies the ARs condenses over the cooler seas around Antarctica and creates large latent heating. The higher amplitude of the polar jet often results in atmospheric blocks that transport further warm, moist air over the ice sheet even after the AR has made landfall and dissipated. Therefore, extreme weather events over Antarctica like ARs are sensitive to climate changes far from the continent over the subtropical regions. Atmospheric rivers have lower‐latitude moisture sources than extratropical cyclones and are likely influenced by tropopause polar vorticesLarge latent heat release from atmospheric river related moisture transport leads to downstream anticyclonic potential vorticity tendenciesThe resultant diabatic heating helps maintain atmospheric blocking after an atmospheric river has dissipated Atmospheric rivers have lower‐latitude moisture sources than extratropical cyclones and are likely influenced by tropopause polar vortices Large latent heat release from atmospheric river related moisture transport leads to downstream anticyclonic potential vorticity tendencies The resultant diabatic heating helps maintain atmospheric blocking after an atmospheric river has dissipated more...

Recent major melting events in West Antarctica have raised concerns about a potential hydrofracturing and ice shelf instability. These events often share common forcings of surface melt-like anomalous radiative fluxes, turbulent heat fluxes and föhn winds. Using an atmospheric river detection algorithm developed for Antarctica together with surface melt datasets, we produced a climatology of atmospheric river-related surface melting around Antarctica and show that atmospheric rivers are associated with a large percentage of these surface melt events. Despite their rarity (around 12 events per year in West Antarctica), atmospheric rivers are associated with around 40% of the total summer meltwater generated across the Ross Ice Shelf to nearly 100% in the higher elevation Marie Byrd Land and 40–80% of the total winter meltwater generated on the Wilkins, Bach, George IV and Larsen B and C ice shelves. These events were all related to high-pressure blocking ridges that directed anomalous poleward moisture transport towards the continent. Major melt events in the West Antarctic Ice Sheet only occur about a couple times per decade, but a 1–2 °C warming and continued increase in atmospheric river activity could increase the melt frequency with consequences for ice shelf stability. more...

The increase of atmospheric greenhouse gases is expected to affect the hydrological cycle and large‐scale precipitation patterns. In parallel, unforced natural variability on decadal‐to‐multidecadal timescales can also modulate forced changes at the regional scales. Based on multimember ensembles from a coupled General Circulation Model, we investigate the sensitivity of CO2‐forced changes in tropical precipitation and atmospheric circulation to fluctuations of the Atlantic Multidecadal Overturning Circulation (AMOC). We show that contrasted AMOC states yield considerable differences in equatorial Pacific precipitation forced changes, by impacting the direct (within a year) CO2‐induced weakening of the Walker circulation. We use global atmospheric energetics, as a theoretical backdrop, to explain the relationship between the tropical atmospheric circulation and the AMOC state. A physical mechanism is then proposed, relating the direct CO2‐forced weakening of the atmospheric tropical circulation to its climatological strength in unperturbed climate and indirectly to the AMOC state. The precipitation response to increased greenhouse gases atmospheric concentration is one of the most critical factors of the overall impact of climate change. The presence of natural climate variability on decadal‐to‐multidecadal timescales is expected to modulate the long‐term projections at the regional scales.This can be viewed as an irreducible source of uncertainties for the climate response to GHG forcing, which should be quantified and better understood. Based on an ensemble of experiments from a fully coupled General Circulation Model, we quantify and investigate the sensitivity of the CO2‐forced climate response in terms of tropical precipitation and circulation, to the internal fluctuation of the Atlantic Multidecadal Overturning Circulation (AMOC). We show that contrasted states of AMOC, used as initial conditions in increased‐CO2experiments, yield considerable differences in the tropical precipitation forced changes over Indonesia, Southeast Asia, and the equatorial Pacific Ocean. We provide evidence that the AMOC‐related influence on precipitation changes is essentially due to the fast and direct (within a year) CO2‐induced weakening of the Walker circulation. Our findings are further supported by physical understanding based on large‐scale atmospheric energetics. Internal multidecadal variability is an important modulator of the tropical climate response to CO2forcingBy impacting global atmospheric energetics, AMOC fluctuations affect the strength of the mean atmospheric tropical circulationThe dependence of the CO2‐forced circulation response to AMOC‐related background state follows a strong‐gets‐weakerpattern more...

A common fault of atmospheric general circulation models (AGCMs) is an overestimation oforographic precipitation. One basic reason is that water vapour advection schemes do not useinformation about the local temperature. When water vapour is advected from a warm gridpoint to a colder one, supersaturation may occur on the way, and the water vapour advectedmay partly precipitate before reaching the latter. This process is particularly important whenmoisture is advected upward mountain slopes along terrain-following coordinates. Not takingit in account, i.e., letting all the moisture reach the colder point, leads to artificial drying of thewindward valleys and foothills, and to overestimation of rainfall over summits and plateaux.This spurious behaviour is amplified by the resulting biases in the circulation, due to misplacementof the moisture convergence. It is a general bias, although its magnitude may be reduced, for instance when σ-coordinates are replaced by hybrid coordinates, or increased by highlydiffusive schemes such as the upstream finite differencing. A simple way of correcting this biasis to test the advected water vapour with respect to saturation values, and redistribute itaccordingly over the grid points found along the advecting path. This method is tested on afinite difference model using σ-coordinates and an upstream advection scheme. The effect onthe distribution of moisture and rainfall is dramatic: precipitation is displaced from summitsand plateaux to slopes and foothills, leading to much more realistic patterns, in particular forthe Indian and Amazonian monsoons. more...

Here, we define weather regimes in the East Antarctica—Southern Ocean sector based on daily anomalies of 700 hPa geopotential height derived from ERA5 reanalysis during 1979–2018. Most regimes and their preferred transitions depict synoptic‐scale disturbances propagating eastwards off the Antarctic coastline. While regime sequences are generally short, their interannual variability is strongly driven by the polarity of the Southern Annular Mode (SAM). Regime occurrences are then intersected with atmospheric rivers (ARs) detected over the same region and period. ARs are equiprobable throughout the year, but clearly concentrate during regimes associated with a strong atmospheric ridges/blockings on the eastern part of the domain, which act to channel meridional advection of heat and moisture from the lower latitudes towards Antarctica. Both regimes and ARs significantly shape climate variability in Antarctica. Regimes favorable to AR occurrences are associated with anomalously warm and humid conditions in coastal Antarctica and, to a lesser extent, the hinterland parts of the Antarctic plateau. These anomalies are strongly enhanced during AR events, with warmer anomalies and dramatically amplified snowfall amounts. Large‐scale conditions favoring AR development are finally explored. They show weak dependency to the SAM, but particularly strong atmospheric ridges/blockings over the Southern Ocean appear as the most favorable pattern, in which ARs can be embedded, and to which they contribute. Atmospheric rivers (ARs) reaching East Antarctica show strong association with synoptic weather regimesARs enhance the positive temperature and snowfall anomalies associated with the regimesARs occur when the synoptic configuration is anomalously strong over the Southern Ocean Atmospheric rivers (ARs) reaching East Antarctica show strong association with synoptic weather regimes ARs enhance the positive temperature and snowfall anomalies associated with the regimes ARs occur when the synoptic configuration is anomalously strong over the Southern Ocean more...

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. 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. 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 Atmospheric rivers in Antarctica are rare events but are a key contributor to the ice sheet's surface mass balance Their impact on precipitation is most pronounced in East Antarctica where they are responsible for a majority of extreme precipitation events Atmospheric rivers are contributing to modern snowfall trends and controlling overall precipitation variability across Antarctica more...