Recent surface melting in West Antarctica: Comparison of Remote and in-situ observations

The onset of surface melting is a fundamental glaciological change. It indicates that some warmth threshold has been crossed, it reduces albedo and thus promotes further melting, it changes other energy fluxes, it affects physical, chemical, particulate, and isotopic records, and it marks the first step towards generation of runoff that contributes to ice-sheet mass balance in the polar regions. Occurrence, timing and extent of melting can also be used as climatic indicators to monitor regional temperature changes, or as paleoclimatic indicators of past conditions. We seek a reliable way to identify surface melting over polar ice sheets.

Here we have combined observations from three different methods (snow and firn stratigraphy, automatic weather station air temperature and satellite-based passive microwave.) Our aim in this study is to learn how these different measures of melting are related to each other. This will improve our ability to learn how melting and climate have changed and affected Antarctica processes in the past, and how they may change in the future. We are specifically investigating and comparing melt records at sites where we can combine these different techniques over the same extended periods of time, including during melt events. Our primary focus is on the Ross Ice Shelf and Siple Dome regions of West Antarctica, where these methods are available for the last decade or longer.

Our study of recent Ross Embayment melt during the instrumental period shows that melt layers can be detected reliably in cores as well as snow pits and that they form during warm summers. Focusing on four automatic weather station (AWS) sites on the Ross Ice Shelf and one AWS on the West Antarctic Ice Sheet (Siple Dome) we find stratigraphic firn core and snow pit evidence for melt events at some of the sites during the austral summers 1982/83 and 1991/92. Reliance on air temperature records exceeding a temperature threshold (such as 0°C) alone tends to overestimate melt occurrence. Melting occurred when a positive degree-day threshold was exceeded in AWS data. We develop a new passive microwave technique to identify melt, as well as a new calibration to improve the previously published cross-polarized gradient ratio (XPGR) technique for this region. Both these techniques are found to work well to identify Ross Embayment melt, and can be used to map past melt occurrence across the ice sheet as well as to monitor changes in melt extent in the future.