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

Abstract

Abstract. 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. A series of anomalously deep and stationary explosive twin polar cyclones over the Cooperation and Davis seas generated tides and wind-driven ocean slope, leading to fracture amplification along the pre-existing rift and ultimately calving of the massive iceberg. The calving was triggered by high oceanward sea surface slopes produced by the storms. 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 (ii) maintaining the observed extreme cyclones stationary 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 via tides and sea surface slope and its 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.

Climate Change and Weather Extremes in the Eastern Mediterranean and Middle East

“…Amid the ongoing energy uptake of the global ocean, this trend has high inertia and will continue in the future. Indications of Antarctic land ice destabilization are steadily gaining support (e.g., Francis, Mattingly, et al., 2021), strengthening the likelihood of the significant acceleration of sea‐level rise (SLR) during the present and following centuries. This will potentially affect coastal settlements, infrastructure, agriculture and cultural heritage sites.…”

State of the Climate in 2020

“…6.5c). There were no major calving events from the large ice shelves in 2020, unlike 2019 with the calving of iceberg D-28 from the Amery Ice Shelf (Walker et al 2021;Francis et al 2021). However, the ice front of Pine Island Ice Shelf, a rapidly thinning ice shelf in the Amundsen Sea sector (Paolo et al 2015), retreated to its most landward location observed in recent decades following a small calving event (~200 km 2 ) in February 2020 (Lhermitte et al 2020).…”

Antarctica and the Southern Ocean

“…There were no major calving events from the large ice shelves in 2020, unlike 2019 with the calving of iceberg D-28 from the Amery Ice Shelf (Walker et al 2021;Francis et al 2021). However, the ice front of Pine Island Ice Shelf, a rapidly thinning ice shelf in the Amundsen Sea sector (Paolo et al 2015), retreated to its most landward location observed in recent decades following a small calving event (~200 km 2 ) in February 2020 (Lhermitte et al 2020).…”