Successive intense cyclones triggered the September 2019 Amery Ice Shelf calving through ocean slope forcing

Ice shelf instability is one of the main sources of uncertainty in Antarctica’s contribution to future sea level rise. Calving events play a crucial role in ice shelf weakening but remain unpredictable and their governing processes are still poorly understood. In this study, we analyze the unexpected September 2019 calving event from the Amery Ice Shelf, the largest since 1963 and which occurred almost a decade earlier than expected, to better understand the role of the atmosphere in calving. We find that atmospheric extremes provided a deterministic role in this event. The calving was triggered by the occurrence of a series of anomalously-deep and stationary explosive twin polar cyclones over the Cooperation and Davis Seas which generated tides and wind-driven ocean slope leading to fracture amplification along the pre-existing rift, and ultimately calving of the massive iceberg. The observed record-anomalous atmospheric conditions were promoted by blocking ridges and Antarctic-wide anomalous poleward transport of heat and moisture. Blocking highs helped in (i) directing moist and warm air masses towards the ice shelf and in (ii) maintaining stationary the observed extreme cyclones at the front of the ice shelf for several days. Accumulation of cold air over the ice sheet, due to the blocking highs, led to the formation of an intense cold-high pressure over the ice sheet, which helped fuel sustained anomalously-deep cyclones via increased baroclinicity. Our results stress the importance of atmospheric extremes in ice shelf dynamics and the need to be accounted for when considering Antarctic ice shelf variability and contribution to sea level, especially given that more of these extremes are predicted under a warmer climate.