Rapid warming of the Antarctic Peninsula over the past several decades has led to extensive surface melting on its eastern side, and the disintegration of the Prince Gustav, Larsen A, and Larsen B ice shelves. The warming trend has been attributed to strengthening of circumpolar westerlies resulting from a positive trend in the Southern Annular Mode (SAM), which is thought to promote more frequent warm, dry, downsloping foehn winds along the lee, or eastern side, of the peninsula. We examined variability in foehn frequency and its relationship to temperature and patterns of synoptic-scale circulation using a multidecadal meteorological record from the Argentine station Matienzo, located between the Larsen A and B embayments. This record was further augmented with a network of six weather stations installed under the U.S. NSF LARsen Ice Shelf System, Antarctica, project. Significant warming was observed in all seasons at Matienzo, with the largest seasonal increase occurring in austral winter (+3.71 ∘ C between 1962-1972 and 1999-2010). Frequency and duration of foehn events were found to strongly influence regional temperature variability over hourly to seasonal time scales. Surface temperature and foehn winds were also sensitive to climate variability, with both variables exhibiting strong, positive correlations with the SAM index. Concomitant positive trends in foehn frequency, temperature, and SAM are present during austral summer, with sustained foehn events consistently associated with surface melting across the ice sheet and ice shelves. These observations support the notion that increased foehn frequency played a critical role in precipitating the collapse of the Larsen B ice shelf.

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Ice flow acceleration has played a crucial role in the recent rapid retreat of calving glaciers in Alaska 1,2 , as well as in Greenland and Antarctica 3,4 . Fast flow of such glaciers is due primarily to basal ice motion 5 , but its mechanism is poorly understood because subglacial observations are scarce in calving glaciers. Here we show high-frequency ice speed and basal water pressure measurements performed in Glaciar Perito Moreno, a fast-flowing calving glacier in Patagonia. The water pressure was measured in a borehole drilled through the 515±5 m thick glacier at a site where more than 60% of ice is below the proglacial lake level. We found that mean basal water pressure reached 94-96% of the ice overburden pressure, and that a few percent of pressure changes were driving nearly 40% of ice speed variations. The ice speed was strongly correlated to air temperature, suggesting the glacier motion was modulated by water pressure 1 under the influence of changing meltwater input. Our observations demonstrate the great importance of basal water pressure in the calving glacier dynamics and its close connection to climate conditions. It is thus crucial to take into account the elevated basal water pressure for predicting future evolution of calving glaciers.Acceleration of fast-flowing calving glaciers is the focus of attention as it is responsible for the rapid retreat of large tidewater glaciers in Alaska 1,2 as well as the recent wastage of Greenland and the Antarctic ice sheets 3,4 . Calving glaciers flow much faster than those terminating on land as a result of basal ice motion enhanced by high basal water pressure 5 . A commonly used basal flow law stateswhere u b is the basal ice speed, τ b is the basal shear stress, P i and P w are ice overburden and basal water pressures, and k, p and q are empirical parameters 6,7 . Because τ b is primarily controlled by ice thickness and surface slope, changes in basal water pressure play a critical role in short-term ice speed variations. Observations in mountain glaciers have shown rapid acceleration as basal water pressure approaches ice overburden pressure 8−10 , which confirms the non-linear dependence of the basal ice speed on the effective pressure defined by P e = P i − P w .The hydraulic head within a calving glacier is expected to be higher than the surface level of the proglacial water body, which maintains basal water pressure closer to ice overburden.According to the inverse proportionality of u b to P e , small perturbations in P w near P i result in large ice speed variations. Moreover, changes in P i due to glacier thinning or thickening have a great impact on the ice speed as well. These characteristics make calving glacier dynamics more susceptible to external forcing than land terminating glaciers. Studying the response of ice speed to the changes in P e is thus crucial for predicting the future evolution of calving glaciers. In the austral summer 2008/09 and 2010, we operated three GPS (Global Positioning System) receivers on GPM at hourly intervals ...

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