Abstract/Summary

In this paper and Part 2 a comprehensive picture of the annual cycle of the Northern Hemisphere storm-tracks is presented and discussed for the first time. It is based on both feature tracking and Eulerian based diagnostics, applied to vorticity and meridional wind in the upper and lower troposphere. Here, the storm-tracks, as diagnosed using both variables and both diagnostic techniques, are presented for the four seasons for each of the two levels. The oceanic storm-tracks retain much of their winter mean intensity in spring with only a small change in their latitude. In the summer they are much weaker, particularly in the Pacific and are generally further poleward. In autumn the intensities are larger again, comparable with those in spring, but the latitude is still nearer to that of summer. However, in the lower troposphere in the eastern ocean basins the tracking metrics show northern and southern tracks that change little with latitude through the year. The Pacific mid-winter minimum is seen in upper troposphere standard deviation diagnostics, but a richer picture is obtained using tracking. In winter there are high intensities over a wide range of latitudes in the central and eastern Pacific, and the west Pacific has high track density but weak intensity. In the lower troposphere all the diagnostics show that the strength of the Pacific and Atlantic storm-tracks are generally quite uniform over the autumn-winter-spring period. There is a close relationship between the upper tropospheric storm-track, particularly that based on vorticity, and tropopause level winds and temperature gradients. In the lower troposphere, in winter the oceanic storm-tracks are in the region of the strong meridional SST gradients, but in summer they are located in regions of small or even reversed SST gradients. However, over North America the lower tropospheric baroclinicity and the upstream portion of the Atlantic storm-track stay together throughout the year.