The Arctic Oscillation as the driver of spring warmings

Changes are occurring in the Arctic which appear to have begun in the late 1960s and have increased in the 1990s. These include tropospheric warming, reduction in sea ice extent and increased variability in snow cover. Much scientific interest has focused on the Arctic's leading mode of variability, the Arctic Oscillation(AO). The AO represents a pan-Arctic increase in upper atmospheric winds and a decrease in sea level pressure. A paradox is that the main shift in the AO is seen in winter while many surface changes are seen in spring and summer.

We make use of the major new resource for Arctic research, the"Path-P" data set, which has TOVS temperature and moisture data at 10 levels on a 100 km grid. We also use NCEP reanalysis data. Both data sets show a major lower tropospheric warming of 3 K in April for the 1990s relative to the 1980s. This warming is associated with the AO structure at the end of the winter. We hypothsize the these low level anomalies are maintained throught summer by regional thermodymanic feedbacks. In summary, the spring/summer transition is occuring earlier.