Intense Winter Surface Melt on an Antarctic Ice Shelf

Abstract

The occurrence of surface melt in Antarctica has hitherto been associated with the austral summer season, when the dominant source of melt energy is provided by solar radiation. We use in situ and satellite observations from a previously unsurveyed region to show that events of intense surface melt on Larsen C Ice Shelf occur frequently throughout the dark Antarctic winter, with peak intensities sometimes exceeding summertime values. A regional atmospheric model confirms that in the absence of solar radiation, these multiday melt events are driven by outbreaks of warm and dry föhn wind descending down the leeside of the Antarctic Peninsula mountain range, resulting in downward turbulent fluxes of sensible heat that drive sustained surface melt fluxes in excess of 200 W/m 2 . From 2015 to 2017 (including the extreme melt winter of 2016), ∼23% of the annual melt flux was produced in winter, and spaceborne observations of surface melt since 2000 show that wintertime melt is widespread in some years. Winter melt heats the firn layer to the melting point up to a depth of ∼3 m, thereby facilitating the formation of impenetrable ice layers and retarding or reversing autumn and winter cooling of the firn. While the absence of a trend in winter melt is consistent with insignificant changes in the observed Southern Hemisphere atmospheric circulation during winter, we anticipate an increase in winter melt as a response to increasing greenhouse gas concentration. Plain Language SummaryAround the coast of Antarctica, it gets warm enough in summer for snow to start melting, and the sun provides most of the energy for that melt. Almost all meltwater refreezes in the snowpack, but especially on floating glaciers in Antarctica, it has been observed that meltwater forms large ponds. The pressure exerted by these ponds may have led to ice shelves collapsing into numerous icebergs in recent decades. It is therefore important to understand how much meltwater is formed. To find out, we installed an automatic weather station on a glacier in Cabinet Inlet, in the Antarctic Peninsula in 2014. The station recorded temperatures well above the melting point even in winter. The occurrence of winter melt is confirmed by satellite images and by thermometers buried in the snow, which measured a warming of the snow even at 3 m depth. Between 2014 and 2017, about 23% of all melt in Cabinet Inlet occurred in winter. Winter melt is due to warm winds that descend from the mountains, known as föhn. We have not seen the amount of winter melt increasing since 2000. However, we expect winter melt to happen more frequently if greenhouse gas continues to accumulate in the atmosphere.

Decline in Surface Melt Duration on Larsen C Ice Shelf Revealed by The Advanced Scatterometer (ASCAT)

“…The small regions of increasing melt we observe are close to the mountains (Figure ) in the domain affected by föhn winds descending from the AP mountains (Elvidge et al, ; Luckman et al, ; Wiesenekker et al, ). The winds bring episodes of positive surface air temperatures and clear skies, even in the winter (Kuipers Munneke et al, ; Wiesenekker et al, ), and therefore, the melt trend is less likely to be driven by the summer circulation that led to cooling on the rest of the shelf.…”

“…A number of metrics have been used to report remotely sensed melt, including melt duration (the number of melt days per year), melt extent (the total area affected by melt in a given year), and melt intensity (the product of melt duration and melt extent, analogous to CMS). Also reported for some regions are melt onset and melt end; we do not report the latter metrics in this study as it is not uncommon on LCIS for there to be periods of melt outside of the continuous summer melt period that are associated with föhn wind events (Kuipers Munneke et al, 2018;Luckman et al, 2014)…”

The Effect of Foehn‐Induced Surface Melt on Firn Evolution Over the Northeast Antarctic Peninsula

“…Studies focusing on the 2010–2011 summer have calculated foehn frequencies of 20% in January to March (King et al, ) or 30% from November to March (Luckman et al, ). Observations in other seasons have found that foehn is more prevalent in winter and spring (Wiesenekker et al, ) and that 23% of the annual melt actually occurred in winter (Kuipers Munneke et al, ). Recent years (2015–2017) have shown a series of strong, late‐season foehn‐induced surface melt events (Bozkurt et al, ; Kuipers Munneke et al, ; Wiesenekker et al, ).…”

Strongest MJO on Record Triggers Extreme Atacama Rainfall and Warmth in Antarctica

“…Almost simultaneously, Esperanza station (6324 ′ S, 5659 ′ W, 13 m) located at the tip of the Antarctic Peninsula registered the highest temperatures ever recorded for continental Antarctica (Skansi et al, 2017) (17.5 • C on 24 March 2015, see Figure S1b). This extreme Antarctic heat wave triggered a significant melting on the Larsen C ice sheet (Munneke et al, 2018) and was ultimately traced to foehn-induced warming that resulted from a landfalling atmospheric river (Bozkurt et al, 2018). Floods and extreme precipitation were also reported during the second half of March 2015 along the semiarid coast of Northern Peru and Ecuador (Bendix et al, 2017) Given the rarity of these events, the extreme nature of these environments and the fact that they occurred simultaneously over a large region of the Eastern Pacific, we look for a common large-scale origin of this compound extreme event.…”