Characteristics of Surface “Melt Potential” over Antarctic Ice Shelves based on Regional Atmospheric Model Simulations of Summer Air Temperature Extremes from 1979/80 to 2018/19

Abstract

We calculate a regional surface “melt potential” index (MPI) over Antarctic ice shelves that describes the frequency (MPI-freq, %) and intensity (MPI-int, K) of daily maximum summer temperatures exceeding a melt threshold of 273.15 K. This is used to determine which ice shelves are vulnerable to melt-induced hydrofracture and is calculated using near-surface temperature output for each summer from 1979/80 to 2018/19 from two high-resolution regional atmospheric model hindcasts (using the MetUM and HIRHAM5). MPI is highest for Antarctic Peninsula ice shelves (MPI-freq 23-35%, MPI-int 1.2-2.1 K), lowest (2-3%, < 0 K) for Ronne-Filchner and Ross ice shelves, and around 10-24% and 0.6-1.7 K for the other West and East Antarctic ice shelves. Hotspots of MPI are apparent over many ice shelves, and they also show a decreasing trend in MPI-freq. The regional circulation patterns associated with high MPI values over West and East Antarctic ice shelves are remarkably consistent for their respective region but tied to different large-scale climate forcings. The West Antarctic circulation resembles the central Pacific El Niño pattern with a stationary Rossby wave and a strong anticyclone over the high-latitude South Pacific. By contrast, the East Antarctic circulation comprises a zonally symmetric negative Southern Annular Mode pattern with a strong regional anticyclone on the plateau and enhanced coastal easterlies/weakened Southern Ocean westerlies. Values of MPI are 3-4 times larger for a lower temperature/melt threshold of 271.15 K used in a sensitivity test, as melting can occur at temperatures lower than 273.15 K depending on snowpack properties.

How Does the Southern Annular Mode Control Surface Melt in East Antarctica?

“…Our findings support recent work highlighting the influence of the SAM on surface melt in East Antarctica by showing that strong SAM‐melt relationships exist for modeled meltwater fluxes as well as satellite‐derived melt metrics (Johnson et al., 2022) and melt proxies from climate models (Orr et al., 2023). Establishing the relationship between the SAM and surface melt fluxes is important because accurate melt flux estimates, and their magnitude relative to precipitation, are key to determining whether melt ponds will form (Pfeffer et al., 1991; van Wessem et al., 2023).…”

Continent-scale mapping reveals a rise in East Antarctic surface meltwater

“…For the EAIS, the SAM has a strong control on surface meltwater abundance, consistent with its influence on surface air temperature 32,33 . There is a statistically significant negative correlation between the detrended Summer SAM Index and detrended maximum annual surface meltwater area (r = -0.47, p = 0.02), with greater meltwater area generally associated with negative SAM years (Fig.…”

Extreme warm events in the South Orkney Islands, Southern Ocean: Compounding influence of atmospheric rivers and föhn conditions

“…To detect whether föhn conditions may have occurred at the stations during the warm events, Tables 2 and 3 also include the corresponding values of wind speed, relative humidity and temperature observed at the time of each of the events, as well as six hours before each event. This shows that three out of seven summer events and five out of nine winter events at Signy station show some indication of föhn conditions, that is, upstream flow impinging on orography from a northerly or northwesterly direction, as well as strengthening winds, warming, and drying at the leeside surface (Elvidge et al ., 2015, 2016; Elvidge and Renfrew, 2016; King et al ., 2017; Orr et al ., 2008, 2021, 2023; Gilbert et al ., 2022). Tables 2 and 3 show that at Orcadas station, four out of eight and four out of six events in summer and winter respectively exhibit some indication of föhn conditions using these criteria based on station data.…”

“…Extreme warm‐temperature events in the Antarctic region typically last for a few hours to a couple of days (Turner et al ., 2021; Maclennan et al ., 2022; Wille et al ., 2022; Orr et al ., 2023). Studying these events thus requires subdaily meteorological observations.…”