Tuesday, 15 January 2002: 9:00 AM
Evidence for a recent advance in the timing of a surface-air warming
The timing of the abrupt increase in daily temperatures over the north-central United States, which frequently occurs during the second half of February, has tended to occur about a week earlier during the 24 years of available data since 1975 (i.e., 1976-99) than in the previous period of equal length (1952-1975). The end of February coincides roughly with the beginning of rapid snowmelt in central North America, and also with a time of year identified by others as the "termination of winter" or the end of "hard winter." Spatial analysis suggests a tendency for changes in synoptic-scale phenomena during the 10 calendar days from February 15-24; specifically, for those calendar days, a synoptic-scale pattern of temperature change is centered in Northern Minnesota, where increases in daily minimum temperature from the earlier period (1952-75) to the later one (1976-99) average around 15F. These results provide additional evidence that a tendency for climatic warming is becoming evident in the annual cycle of synoptic events.