Abstract:Various methods defining the storm track as well as its intensity and location are reviewed and summarized, and each index is reappraised using 44-year (1958-2001) ERA40 geopotential height data in winter. It has been found all these indices are consistent in describing the intensity of storm track, but they are quite different in describing the position. According to the main features of storm track over the North Pacific in winter, using 500-hPa synoptic-scale geopotential height data over the North Pacific and its surrounding areas (30°N-60°N, 120°E-120°W), a method which better defines the storm track indices is proposed, that is, the values at the grid points with variance greater than 20 dagpm2 are averaged as the winter North Pacific storm track intensity index (NII), the average of the longitudes at the grid points is defined as the longitude index (NXI), and the average of the latitudes is defined as the latitude index (NYI). It is proved that the newly defined indices can better reflect the variation of the storm track in this study. It has also revealed that during the period of 1958/1959-2001/2002, the storm track over the North Pacific was slightly strengthened and tended to move southward and westward. The relationship between the strength and location was complicated before the mid-1970s, and it became quasi-consistent after the mid-1970s, with the location northward and eastward in the strong storm track stage, and vice versa. Moreover, such a long-term variation is related to the low-frequency variability of the Pacific sea-air systems, and the causes are sophisticated.