The influence of föhn winds on annual and seasonal surface melt on the Larsen C Ice Shelf, Antarctica

Abstract

Abstract. Warm, dry föhn winds are observed over the Larsen C Ice shelf year-round and are thought to contribute to the continuing weakening and collapse of ice shelves on the eastern Antarctic Peninsula. We use a surface energy balance (SEB) model, driven by observations from two locations on the Larsen C ice shelf and one on the remnants of Larsen B, in combination with output from the Antarctic Mesoscale Prediction System (AMPS), to investigate the year-round impact of föhn winds on the SEB and melt from 2009–2012. Föhn winds have an impact on the individual components of the surface energy balance in all seasons, and lead to an increase in surface melt in spring, summer and autumn up to 100 km away from the foot of the AP. When föhn winds occur in spring they increase surface melt, extend the melt season and increase the number of melt days within a year. Whilst AMPS is able to simulate the percentage of melt days associated with föhn with high skill, it overestimates the total amount of melting during föhn events and non-föhn events. This study extends previous attempts at quantifying the impact of föhn on the Larsen C ice shelf by including a four-year study period and a wider area of interest and provides evidence for föhn-related melting on both Larsen C and Larsen B ice shelves.

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

“…Föhn events influence glacier ablation mainly through an increase in shortwave radiation and an increased sensible heat flux [14]. For the glaciers of the SPI, the föhn case of a linear type can be seen as less harmful since the spatial extent of the föhn response into the lee is larger, but the strength is weaker.…”

“…The strongest positive sensible heat flux anomaly at CE occurs during the transition case, which likely impacts the higher elevated regions of the SPI. We also include the turbulent latent heat flux in Table 5 which shows that the predominantly negative anomalies indicate a tendency for sublimation to increase during föhn events, which has been observed before over the Antarctic Peninsula [12,14]. This response seems plausible, as field measurements support that windy and dry conditions favor sublimation [78].…”

“…The impact of föhn winds on the surface energy and mass balances varies by location and individual events, as shown in the Antarctic and Subantarctic Islands [12,13,22], USA [73] and Japan [74], since the strength of a föhn is influenced by several factors such as the topography, mountain height, stability, flow regime or wind direction. Simultaneously, the strength of a föhn impacts the response of the atmospheric variables that dictate the radiative and turbulent heat fluxes between the glacier surface and atmospheric surface layer, such as the wind speed, temperature and relative humidity [14].…”

“…In Patagonia, it was shown that the surface energy balance of glaciers is dominated by net radiation and sensible heat flux whereas latent heat flux plays a minor role [41,76]. Föhn winds particularly influence two aspects of the surface energy balance, which have the largest impact on melting: (i) increased shortwave radiation due to the "föhn-clearance" and (ii) increased sensible heat flux between the lower atmosphere and the ice surface due to the warm and windy conditions [14]. It was found that föhn winds over the Antarctic Peninsula have the ability to increase the amount of melting [48], to extend the melt season [12,14], and to initiate melting also in winter [77], when sensible heat flux is the dominant control over melt.…”

“…Therefore, to quantify melt rates and their variation in space and time, knowledge of the occurrence and on the characteristics of föhn events is required. The significance of such events for the mass balance of glacier surfaces has been proven in many regions of the world, such as Antarctica or South Georgia [12][13][14]. Sauter and Galos (2016) demonstrated that local wind systems cause small-scale sensible heat flux variations on an Alpine mountain glacier [15].…”