Abstract

Previously unknown foehn jets have been identified to the east of the Antarctic Peninsula above the Larsen C Ice Shelf. These jets and associated foehn warming have major implications for the east coast of the AP, a region of rapid climatic warming and where two large sections of ice shelf have collapsed in recent years.

During three foehn events across the Antarctic Peninsula, leeside warming and drying is seen in new aircraft observations and simulated well by the Met Office Unified Model (MetUM) at ~1.5 km grid spacing. In case A weak southwesterly flow and an elevated upwind inversion characterise a highly non-linear flow regime with upwind flow blocking. The consequent strongly-accelerated downslope flow leads to high amplitude warming (associated with ice shelf melt) in the immediate lee of the Peninsula. However, the warming diminishes rapidly downwind, due to upward ascent of the foehn flow via a hydraulic jump. In case C strong northwesterly winds characterise a relatively linear case with little upwind flow blocking. There is no hydraulic jump and strong foehn winds reach right across the ice shelf at low level, mechanically mixing the near-surface levels, and delivering large sensible heat fluxes to the ice shelf resulting in high melt rates. Case B resides somewhere between cases A and C in flow regime linearity and leeside response.

The foehn jets – apparent in aircraft observations where available and MetUM simulations of all three cases – are mesoscale features (up to 60 km in width) originating from the mouths of leeside inlets. Through back trajectory analysis they are identified as a type of gap flow. They have cool, moist signatures due to a weaker foehn effect than in neighbouring regions of calmer flow.