Evidence for recent changes in a surface-air warming singularity in late winter over central North America

[1] Evidence for an earlier onset of spring in recent years is supported by daily temperature data from central North America. From 1976-1999, mean temperatures during February 15th through 24th were appreciably higher (5degreesC or more over large areas) than during 1952-1975. This is associated with a strengthened Aleutian Low for those calendar days during 1976-1999, with negative pressure anomalies extending well into North America. This apparently reflects a decrease in strong, cold, high-pressure systems moving from western Canada into the north central United States during late February. These changes appear to be related to conditions in and over the North Pacific Ocean, possibly in conjunction with global warming, although the latter signal is not clearly obvious. Comparison of time series of daily minimum temperatures for the earlier period (1952 1975) and the recent period (1976-1999) indicates other times of the year when an appreciable warming has occurred; such warming is largely confined to the first half of the calendar year.