AN ABRUPT SPRING AIR-TEMPERATURE RISE OVER THE GREENLAND ICE CAP

We report on an abrupt springtime temperature rise (ASTR) occurring ov er the Greenland ice cap. The abrupt 12 degrees-15 degrees C temperatu re rise typically occurs in May over a 24-36 hour period and is accomp anied by a 16-22 hPa pressure rise that begins 2-3 days earlier. The G reenland ASTR is examined using composites of pressure and temperature data from Automatic Weather Stations at ''Cathy'' and ''Kenton'' for the years 1987 through 1990 and 1992 and 1993. Comparative analysis of the ASTR at coastal weather stations is made and contemporaneous atmo spheric circulation variations are examined with gridded hemispheric s ea level pressure and 500 hPa data. At the ice cap summit the ASTR is accompanied by an average 360 m rise in 500 hPa geopotential heights, strong surface winds, and high relative humidities. During an ASTR, co ncomitant air temperatures along the western coast of Greenland and on Baffin Island also rise, but with a magnitude only half that at the i ce cap summit. The contemporaneous ASTR pressure rise at coastal stati ons is of equivalent magnitude to that at the summit and is most promi nent at Greenland stations and on Iceland. Evidence is presented that the ASTR is linked to a seasonal readjustment in the regional Atlantic atmospheric circulation in which the westerlies become weakest in May and migrate northward into the Arctic. A meridional flow regime is es tablished with southwesterly flow advecting mild air over Greenland. B locking highs form over the Atlantic and the surface mean Icelandic lo w is weak, or replaced by mean high pressure. The nature and timing of the ASTR is important for planning field activities on the Greenland ice sheet. Further study of the ASTR should lead to a better understan ding of boundary layer processes over the ice sheet.