Spatial and Temporal Variations in the Eastern North Pacific Storm Track
北太平洋东部风暴轴的时空演变特征

On the basis of daily reanalysis data from the European Center for Medium-Range Weather Forecasting (ERA-40), the filtered 500-hPa geopotential height variance between 2.5 and 6 days is first used for a statistical analysis of the number of maxima centers and the position of the strongest center for 528 monthly mean storm tracks over the North Pacific ocean (30°N-60°N, 120°E-120°W) from December 1957 to November 2001. Then, after a comparison with other subregions, the temporal and spatial variation in the eastern part of the North Pacific storm track is revealed. The results are summarized as follows: (1) As viewed from month to month, a polycentric distribution, most typically with two or three centers, is common, with a probability as high as 94.7% over the entire Pacific storm track. This distribution appeared most often in spring and least often in autumn and winter, whereas a polycentric distribution with four or more centers appeared most often in summer. (2) The eastern pattern storm track, which is defined as having the strongest center located east of 160°W, generally with a 33% probability, appeared most often in summer, especially in July, and least often in winter, especially in January and February. (3) In terms of the vertical structure, the track intensity can be strongest over the eastern part of the North Pacific storm track, but not the baroclinic transport of energy poleward and upward by transient eddies. (4) The empirical orthogonal function (EOF) result shows that some differences do exist under different conditions when seasonal, regional, and track type effects are considered, but a few unique features still appear in the variation in the eastern part of the North Pacific storm track, e.g., the major EOF modes are not always in a one-to-one correspondence with their counterparts over the entire North Pacific storm track. These results suggest that the mechanism of the variation in the eastern Pacific storm track is worthy of further investigation.