Climatological study of abrupt surface air temperature changes in the northern hemisphere

Abstract

Climatological characteristics of abrupt temperature change (ATC) events in the northern hemisphere are examined using NCEP-NCAR reanalysis data for 1948-2001. An ATC event is defined by a daily mean surface air temperature difference exceeding 20 K within 3 days at each grid point. ATC events are frequently observed in the interior of continents at high latitudes: western Siberia, Alaska, northwestern Canada, and southeastern Canada. Most ATC events occur in the cold season (from November to February), but there are some seasonal dependencies in each region. In western Siberia, ATC events are frequently observed during early winter (November and December). On the other hand, the peak of ATC frequency is delayed by about 1 month (December and January) over North America. The frequency of ATC events decreases by 15-25% in warm anomaly years compared with cold anomaly years, especially in western Siberia and northwestern Canada. The typical synoptic evolutions of ATC events are described using composite charts in the above two regions. In the cooling case over northwestern Canada, an intense blocking anticyclone was formed over Alaska prior to the polar air outbreaks into northwestern Canada. In the warming case over western Siberia, the Siberian high was intensified accompanied by the southeastward passage of cold air from the West Siberian Plain to East Asia. This pattern resembles typical polar air outbreaks over East Asia. These results suggest that ATC events decrease when global warming is intensified.