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. Ice shelf instability is one of the main sources of uncertainty inAntarctica's contribution to future sea level rise. Calving events play acrucial role in ice shelf weakening but remain unpredictable, and theirgoverning processes are still poorly understood. In this study, we analyzethe unexpected September 2019 calving event from the Amery Ice Shelf, thelargest since 1963 and which occurred almost a decade earlier than expected,to better understand the role of the atmosphere in calving. We find thatatmospheric extremes provided a deterministic role in this event. A seriesof anomalously deep and stationary explosive twin polar cyclones over theCooperation and Davis seas generated tides and wind-driven ocean slope,leading to fracture amplification along the pre-existing rift andultimately calving of the massive iceberg. The calving was triggered by highoceanward sea surface slopes produced by the storms. The observedrecord-anomalous atmospheric conditions were promoted by blocking ridges andAntarctic-wide anomalous poleward transport of heat and moisture. Blockinghighs helped in (i) directing moist and warm air masses towards the iceshelf and (ii) maintaining the observed extreme cyclones stationary atthe front of the ice shelf for several days. Accumulation of cold air overthe ice sheet, due to the blocking highs, led to the formation of an intensecold high pressure over the ice sheet, which helped fuel sustainedanomalously deep cyclones via increased baroclinicity. Our results stressthe importance of atmospheric extremes in ice shelf dynamics via tides andsea surface slope and its need to be accounted for when consideringAntarctic ice shelf variability and contribution to sea level, especiallygiven that more of these extremes are predicted under a warmer climate.

Atmospheric extremes caused high oceanward sea surface slope triggering the biggest calving event in more than 50 years at the Amery Ice Shelf