Downslope föhn winds over the Antarctic Peninsula and their effect on the Larsen ice shelves

Abstract

Abstract.Mesoscale model simulations are presented of a westerly föhn event over the Antarctic Peninsula mountain ridge and onto the Larsen C ice shelf, just south of the recently collapsed Larsen B ice shelf. Aircraft observations showed the presence of föhn jets descending near the ice shelf surface with maximum wind speeds at 250-350 m in height. Surface flux measurements suggested that melting was occurring. Simulated profiles of wind speed, temperature and wind direction were very similar to the observations. However, the good match only occurred at a model time corresponding to ∼ 9 h before the aircraft observations were made since the model föhn jets died down after this. This was despite the fact that the model was nudged towards analysis for heights greater than ∼1.15 km above the surface.Timing issues aside, the otherwise good comparison between the model and observations gave confidence that the model flow structure was similar to that in reality. Details of the model jet structure are explored and discussed and are found to have ramifications for the placement of automatic weather station (AWS) stations on the ice shelf in order to detect föhn flow. Cross sections of the flow are also examined and were found to compare well to the aircraft measurements. Gravity wave breaking above the mountain crest likely created a situation similar to hydraulic flow and allowed föhn flow and ice shelf surface warming to occur despite strong upwind blocking, which in previous studies of this region has generally not been considered. Our results therefore suggest that reduced upwind blocking, due to wind speed increases or stability decreases, might not result in an increased likelihood of föhn events over the Antarctic Peninsula, as previously suggested.The surface energy budget of the model during the melting periods showed that the net downwelling short-wave surface flux was the largest contributor to the melting energy, indicating that the cloud clearing effect of föhn events is likely to be the most important factor for increased melting relative to non-föhn days. The results also indicate that the warmth of the föhn jets through sensible heat flux ("SH") may not be critical in causing melting beyond boundary layer stabilisation effects (which may help to prevent cloud cover and suppress loss of heat by convection) and are actually cancelled by latent heat flux ("LH") effects (snow ablation). It was found that ground heat flux ("GRD") was likely to be an important factor when considering the changing surface energy budget for the southern regions of the ice shelf as the climate warms.

Föhn winds on South Georgia and their impact on regional climate

“…Recent research includes the documentation of föhn events in the Smoky Mountains (Gaffin, 2002), Japan (Takane & Kusaka, 2011), the Alps , the Appalachian Mountains (Gaffin, 2007;, the American Rockies (Oard, 1993), the Antarctic Dry Valleys (Speirs et al, 2010;Steinhoff et al, 2013; and the Antarctic Peninsula (Elvidge et al, 2014a;2014b;Grosvenor et al, 2014). Being found in such widely diverging geographical locations has resulted in a number of local names for the föhn wind, including the Chinook (American Rockies), the berg wind (South Africa), Santa Ana winds (California), the Zonda (Andes) and the Nor'wester (New Zealand) (among many others, see Richner & Hächler, 2013).…”

“…Climate of the Antarctic Peninsula (OFCAP) project (Elvidge et al, 2014a;2014b) and have been shown to have major implications on regional warming and nearsurface energy and mass balances of ice sheets (Kuipers Munneke et al, 2012;Grosvenor et al, 2014;Luckman et al, 2014). The displacement of the circumpolar westerlies is also thought to be responsible for the recent cooling found on the interior of the Antarctic continent (Comiso, 2000).…”

Identification of Air Masses Responsible for Warm Events on the East Antarctic Coast

“…The isentropic drawdown of this warm air could therefore explain a large part of the foehn warming observed. In Antarctica, this type of foehn warming (nonlinear foehn event) has been observed on the east side of the AP (Elvidge et al 2015Cape et al 2015;Grosvenor et al 2014) and in the McMurdo dry valleys of EA (Speirs et al 2010;Steinhoff et al 2013Steinhoff et al , 2014. We cannot, however, ignore the contribution of the latent heating of precipitation.…”

“…The summer warming of the AP over the last half of the 20th century, for example, is consistent with strengthened westerly marine air advection driven by changes in the Southern Hemisphere Annular Mode (SAM) (e.g., Marshall et al 2006;Turner et al 2016). This warm westerly flow travels over mountain ranges on the AP, causing additional temperature increase by adiabatic warming as the air descends on the east side of the AP that is called a foehn warming (e.g., Elvidge et al 2015Cape et al 2015;Grosvenor et al 2014). The increase in poleward marine air advection is also known to have contributed to summer warming at Byrd Station through most of the late 1980s, though it can not solely explain long-term trends in West Antarctica (Bromwich et al 2012).…”

Estimación del efecto <em>Venturi</em> como factor desencadenante de la pluviometría en la Sierra de Grazalema

“…Como es bien conocido, el efecto föhn no produce lluvias per sé (Grosvenor et al, 2014;Nastos et al, 2017;van Drooge & Ballesta, 2010). El término föhn se refiere estrictamente al ejemplo de los Alpes europeos, aunque tiene diferentes nombres en distintas regiones del mundo: en Málaga y Cádiz se denomina terral (aunque en Málaga sea de componente noroeste y en Cádiz de noreste).…”