Flow Regimes and Föhn Types Characterize the Local Climate of Southern Patagonia

The local climate in Southern Patagonia is strongly influenced by the interaction between the topography and persistent westerlies, which can generate fohn events, dry and warm downslope winds. The upstream flow regime influences different fohn types which dictate the lee-side atmospheric response regarding the strength, spatial extent and phenomenology. We use a combination of observations from four automatic weather stations (AWSs) and high-resolution numerical modeling with the Weather Research and Forecasting (WRF) model for a region in Southern Patagonia (48° S–52° S, 72° W–76.5° W) including the Southern Patagonian Icefield (SPI). The application of a fohn identification algorithm to a 10-month study period (June 2018–March 2019) reveals 81 fohn events in total. A simulation of three events of differing flow regimes (supercritical, subcritical, transition) suggests that a supercritical flow regime leads to a linear fohn event with a large spatial extent but moderate intensity. In contrast, a spatially limited but locally strong fohn response is induced by a subcritical regime with upstream blocking and by a transition regime with a hydraulic jump present. Our results imply that the hydraulic jump-type fohn event (transition case) is the most critical for glacier mass balances since it shows the strongest warming, drying, wind velocities and solar radiation over the SPI. The consideration of flow regimes over the last 40 years shows that subcritical flow occurs most frequently (78%), however transitional flow occurs 14% of the time, implying the potential impact on Patagonian glaciers.