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Abstract Thwaites Glacier is one of the fastest-changing ice–ocean systems in Antarctica 1–3 . Much of the ice sheet within the catchment of Thwaites Glacier is grounded below sea level on bedrock that deepens inland 4 , making it susceptible to rapid and irreversible ice loss that could raise the global sea level by more than half a metre 2,3,5 . The rate and extent of ice loss, and whether it proceeds irreversibly, are set by the ocean conditions and basal melting within the grounding-zone region where Thwaites Glacier first goes afloat 3,6 , both of which are largely unknown. Here we show—using observations from a hot-water-drilled access hole—that the grounding zone of Thwaites Eastern Ice Shelf (TEIS) is characterized by a warm and highly stable water column with temperatures substantially higher than the in situ freezing point. Despite these warm conditions, low current speeds and strong density stratification in the ice–ocean boundary layer actively restrict the vertical mixing of heat towards the ice base 7,8 , resulting in strongly suppressed basal melting. Our results demonstrate that the canonical model of ice-shelf basal melting used to generate sea-level projections cannot reproduce observed melt rates beneath this critically important glacier, and that rapid and possibly unstable grounding-line retreat may be associated with relatively modest basal melt rates. 

On 1 September 2021, the remnants of Hurricane Ida transformed into a lethal variant of tropical cyclone in which unprecedented short‐duration rainfall from clusters of supercells produced catastrophic flooding in watersheds of the Northeastern US. Short‐duration rainfall extremes from Ida are examined through analyses of polarimetric radar fields and rain gauge observations. Rainfall estimates are constructed from a polarimetric rainfall algorithm that is grounded in specific differential phase shift (K_DP) fields. Rainfall accumulations at multiple locations exceed 1000‐year values for 1–3 hr time scales. Radar observations show that supercells are the principal agents of rainfall extremes. Record flood peaks occurred throughout the eastern Pennsylvania—New Jersey region; the peak discharge of the Elizabeth River is one of the most extreme in the eastern US, based on the ratio of the peak discharge to the sample 10‐year flood at the gaging station. As with other tropical cyclones that have produced record flooding in the Northeastern US, Extratropical Transition was a key element of extreme rainfall and flooding from Ida. Tropical and extratropical elements of the storm system contributed to extremes of atmospheric water balance variables and Convective Available Potential Energy, providing the environment for extreme short‐duration rainfall from supercells.

 

Abstract. Ocean-driven ice loss from the West Antarctic Ice Sheet is asignificant contributor to sea-level rise. Recent ocean variability in theAmundsen Sea is controlled by near-surface winds. We combine palaeoclimatereconstructions and climate model simulations to understand past and futureinfluences on Amundsen Sea winds from anthropogenic forcing and internalclimate variability. The reconstructions show strong historical wind trends.External forcing from greenhouse gases and stratospheric ozone depletiondrove zonally uniform westerly wind trends centred over the deep SouthernOcean. Internally generated trends resemble a South Pacific Rossby wavetrain and were highly influential over the Amundsen Sea continental shelf.There was strong interannual and interdecadal variability over the AmundsenSea, with periods of anticyclonic wind anomalies in the 1940s and 1990s,when rapid ice-sheet loss was initiated. Similar anticyclonic anomaliesprobably occurred prior to the 20th century but without causing the presentice loss. This suggests that ice loss may have been triggered naturally inthe 1940s but failed to recover subsequently due to the increasingimportance of anthropogenic forcing from greenhouse gases (since the 1960s)and ozone depletion (since the 1980s). Future projections also featurestrong wind trends. Emissions mitigation influences wind trends over thedeep Southern Ocean but has less influence on winds over the Amundsen Seashelf, where internal variability creates a large and irreducibleuncertainty. This suggests that strong emissions mitigation is needed tominimise ice loss this century but that the uncontrollable future influenceof internal climate variability could be equally important. 

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.

 

In this study, we examine extremes of atmospheric water balance components through analyses of annual maxima of precipitable water and water vapor transport. Our analyses are grounded in Extreme Value Theory, using the Generalized Extreme Value (GEV) distribution as a platform for assessing water balance extremes. Annual maxima of atmospheric water balance terms are computed from North American Regional Reanalysis (NARR) fields for the 40‐year period extending from 1979 to 2018 on a grid of approximately 0.3‐degree resolution. We assess nonstationarities in the annual maximum time series through tests for monotonic trends. Estimates of the location, scale, and shape parameters for the Generalized Extreme Value (GEV) distribution are used to examine the spatial variability of water balance extremes. We focus on estimates of the GEV shape parameter, which dictates the “thickness” of the upper tail of the distribution. Estimates of the GEV shape parameter for precipitable water generally point to bounded distributions, but clusters of unbounded, thick‐tailed distributions are linked to exceptionally large record values of precipitable water associated with tropical cyclones in the Gulf of Mexico and Atlantic. Larger regions of “thick‐tailed” distributions are found for integrated water vapor transport (IVT). Non‐stationary GEV models are used to examine the impacts of trends on extremes of the atmospheric water balance. Mixtures of rare events associated with tropical cyclones and extratropical cyclones play a central role in analyses of water balance extremes.

 

Recent climate modeling studies point to an increase in tropical cyclone rainfall rates in response to climate warming. These studies indicate that the percentage increase in tropical cyclone rainfall rates often outpaces the increase in saturation specific humidity expected from the Clausius-Clapeyron relation (~7% °C⁻¹). We explore the change in tropical cyclone rainfall rates over all oceans under global warming using a high-resolution climate model with the ability to simulate the entire intensity spectrum of tropical cyclones. Consistent with previous results, we find a robust increase of tropical cyclone rainfall rates. The percentage increase for inner-core tropical cyclone rainfall rates in our model is markedly larger than the Clausius-Clapeyron rate. However, when the impact of storm intensity is excluded, the rainfall rate increase shows a much better match with the Clausius-Clapeyron rate, suggesting that the “super Clausius-Clapeyron” scaling of rainfall rates with temperature increase is due to the warming-induced increase of tropical cyclone intensity. The increase of tropical cyclone intensity and environmental water vapor, in combination, explain the tropical cyclone rainfall rate increase under global warming.

 

Abstract. Catchment-scale hydrological studies on drylands are lacking because of thescarcity of consistent data: observations are often available at the plotscale, but their relevance for the catchment scale remains unclear. Adatabase of 24 years of stream gauge discharge and homogeneoushigh-resolution radar data over the eastern Mediterranean allows us to describe the properties of floods over catchments spanning from desert toMediterranean climates, and we note that the data set is mostly of moderateintensity floods. Comparing two climatic regions, desert and Mediterranean,we can better identify specific rainfall-runoff properties. Despite the large differences in rainfall forcing between the two regions, the resulting unitpeak discharges and runoff coefficients are comparable. Rain depth andantecedent conditions are the most important properties to shape floodresponse in Mediterranean areas. In deserts, instead, storm core propertiesdisplay a strong correlation with unit peak discharge and, to a lesser extent,with runoff coefficient. In this region, an inverse correlation with meancatchment annual precipitation suggests also a strong influence of localsurface properties. Preliminary analyses suggest that floods in catchmentswith wet headwater and dry lower section are more similar to desertcatchments, with a strong influence of storm core properties on runoffgeneration.