UPDATED INFORMATION ON*

CHANGES IN ANTARCTIC SNOWFALL

for

The International Antarctic Weather Forecasting Handbook:

IPY 2007-08 Supplement

by

Andrew Monaghan

Polar Meteorology Group

Byrd Polar Research Center

The Ohio State University

monaghan.11@osu.edu

Submitted September 2008

CHANGES IN ANTARCTIC SNOWFALL

Precipitation, overwhelmingly in the form of snowfall, mostly falls along the steep coastal margins of Antarctica (up to ~2000 mm y^-1) due to orographic lifting of relatively warm, moist air associated with the many transient, synoptic-scale cyclones that encircle the continent.  The influence of synoptic activity decreases inward from the coast, and over the highest, coldest reaches of the continent the primary mode of precipitation is due to cooling of moist air just above the surface-based temperature inversion.   This extremely cold air has little capacity to hold moisture, and thus the interior of Antarctica is a polar desert.  Figure 1 is a map of long-term annual snowfall accumulation in Antarctica.

The annual snowfall accumulation averaged over Antarctica for the past few decades is about 150-170 mm y^-1, which is the global sea level equivalent of 6-to-7 mm.  It is clear in light of the current global sea level rise of about 3 mm y^-1 that assessing the long-term trends of Antarctic snowfall is a key research priority. Even small trends of Antarctic snowfall will have important implications for sea level rise.  The latest global and regional atmospheric model estimates indicate that snowfall has not changed much over the continent as a whole since about 1980, although there have been some important regional trends.  It has been problematic to assess model precipitation trends prior to ~1980 due to artificial trends or jumps in model fields related to the change in the volume and type of data available at the onset of the modern satellite era in 1979. 

The snowfall record has been extended back to the 1950s by using atmospheric model precipitation fields to extrapolate a suite of ice core snowfall accumulation records.  The 50-year trends in each glacial basin in Antarctica are shown in Figure 2.  Most trends are not statistically significant from zero, emphasizing the large year-to-year snowfall variability that makes the basin-averages noisy. Positive and negative trends tend to cancel out at the continental scale, and thus there is no indication of large scale precipitation trends over the past 50 years that would cause a net contribution to (or mitigation of) sea level rise.  The alternating positive/negative pattern of trends that encircles the continent suggests that the mean upper level wave position has possibly shifted over the past several decades.  The positive snowfall trends on the western side of the Antarctic Peninsula have been corroborated in other recent observational studies, and have been attributed to a deepening of the circumpolar pressure trough, which has enhanced ascent in the region.  Regionally declining sea ice extent and a shorter sea ice season in the Bellingshausen Sea have also enhanced local moisture availability. 

Over large regions and decadal time scales, Antarctic snowfall variability is consistent with temperature variability, meaning that if Antarctic temperatures increase snowfall will too.  This is an important relationship because it allows long-term snowfall trends to be estimated from the more easily measured temperature trends. If global climate model projections of 2-to-3.5 Celsius temperature increases over Antarctica by the end of this century are accurate, a ~10-20% increase in snowfall might be expected by the end of this century.  A 15% increase of Antarctic snowfall would mitigate an additional ~1 mm y^-1 of global sea level in 2100 compared to today.