Scientists Examine Extensive Surface Melting Event in Antarctica During 2015-2016 El Niño
Research observations provide clues on atmospheric contributions to an Antarctic melt event.
West Antarctica is one of the most rapidly warming regions on Earth, and this warming is closely connected with global sea level rise. However, this warming has not yet been comprehensively explained. A study of a large-scale surface melt event in West Antarctica determined that the event was likely favored by the concurrent strong El Niño that transported warm air into the region. The liquid water clouds that accompanied this warm, moist air supported the surface warming by providing a thermal blanket that trapped more energy than was lost by clouds’ reflection of sunlight.
Previous research indicates that warm ocean water is melting the West Antarctic ice shelves from below, but this is one of the first studies documenting how warm air could cause large-scale melting from above. Other studies have predicted that more extreme and frequent El Niños could occur in the future, which could mean that such major surface melt events in West Antarctica become more common. Improved understanding of the complex physical processes that contribute to surface melt events will reduce uncertainty in projections of future changes in the Antarctic ice shelves.
A large-scale and prolonged surface melt event occurred in January 2016 over the Ross Ice Shelf region of West Antarctica. Analysis of passive microwave satellite data find the event to be near-record size for the Ross sector since satellite observations began in 1978. The unusual extent and duration of the melt event were likely favored by the concurrent strong El Niño by advecting warm air into the region, despite the counteracting influence of particularly strong circumpolar westerlies that act as a barrier to incursions of warm air into the region. The melt event occurred upwind from an Atmospheric Radiation Measurement (ARM) West Antarctic Radiation Experiment (AWARE) boundary site, located at the West Antarctic Ice Sheet (WAIS) divide ice camp drill site. The ARM instrumentation made the first-ever surface measurements of clouds and radiation at WAIS, and the first radiosonde-based atmospheric profiles of temperature and moisture in almost 50 years. The measurements were used to determine surface energy balance at WAIS. Results find a marked increase in the surface energy gain during the event, despite the surface temperature hovering just below freezing, and that the increase persisted afterward until the energy balance returned to normal. Thin liquid water clouds frequently had liquid water paths within the range where the cloud radiative enhancement occurs, as previously observed over Greenland. In this range, the clouds are thick enough to enhance the downwelling longwave radiation, but thin enough to also allow shortwave radiation to reach the surface. However, in contrast to Greenland, a significant frequency of thicker liquid water clouds were found and signify a more prominent role of thermal blanketing as a consequence of the warm air advection. While previous research indicates that warm ocean water is melting the West Antarctic ice shelves from below, this is one of the first studies documenting how warm air could cause large-scale melting from above. Other studies have predicted that, should planetary warming trends continue, more extreme and frequent El Niños could occur, which could mean that such major surface melt events become more common.
Brookhaven National Laboratory
J.P.N., A.B.W., and D.H.B. were supported by National Science Foundation (NSF) grants PLR-1443443 and PLR-1341695. A.M.V. is supported by the U.S. Department of Energy (DOE) under contract DE-SC0012704. R.C.S. was supported by National Aeronautics and Space Administration grant NNX15AN45H. M.P.C. is supported by the DOE under contract DE-AC02-06CH11357. J.V. was supported by NSF grant PLR-1443495. AWARE is supported by the DOE ARM Climate Research Facility and NSF Division of Polar Programs.
Nicolas, J. P., A. M. Vogelmann, R. C. Scott, A. B. Wilson, M. P. Cadeddu, D. H. Bromwich, J. Verlinde, D. Lubin, L. M. Russell, C. Jenkinson, H. H. Powers, M. Ryczek, G. Stone, and J. D. Wille. 2017. “January 2016 Extensive Summer Melt in West Antarctica Favoured by Strong El Nino,” Nature Communications 8, DOI: 10.1038/ncomms15799.