Interdecadal Variations of Extratropical Cyclone Activities and Storm Tracks in the Northern Hemisphere
北半球温带气旋活动和风暴路径的年代际变化

Interdecadal variations of extratropical cyclone activities and storm tracks in different seasons for the years of 1958-2001 in the Northern Hemisphere, as well as the possible reasons, are analyzed by applying the Lagrange and Euler methods to the sea-level pressure and height fields from the European Center for Medium-Range Weather Forecasts reanalysis data ERA40. With the Lagrange method based on detecting and tracking extratropical cyclones, two main cyclone activity centers are got, i.e., the North Pacific and the North Atlantic/North America. At the same time, with the Euler method based on the synoptic scale variance for 500 hPa geopotential height, two storm tracks are obtained, which are in accordance with two main cyclone activity centers. The result shows that extratropical cyclones in the region of the North Atlantic/North America in the 44 years moved northward and intensified, especially in spring. Also, there was a feature with a poleward shift and increasing strength for the storm track. High correlation was observed between the movements of extratropical cyclones and the storm track. The North Atlantic/North America is a typical region, where cyclone activities reflected the northward movement of the storm track and the poleward expansion of temperate zone. However, it is interesting to find that the situation in the North Pacific is just the opposite, i.e., extratropical cyclones and the storm track migrated to the lower latitudes with enhanced intensity, especially in spring. For this conclusion, two methods are also statistically correlated. The storm track over the whole Northern Hemisphere had a poleward shift by some studies, but obvious differences were found in specific areas. Besides, the space distributions and time series of the first empirical orthogonal function (EOF) mode of the maximum Eady growth rate and cyclone activity frequency are similar. There may be a close relationship between the opposite trends of storm tracks in the North Pacific and the North Atlantic and the same phase variations of atmospheric baroclinity. This also can, on the other hand, support the analysis of interdecadal variations about extratropical cyclones and storm paths in this paper.