Abstract/Summary

In Part 1 of this study, the annual cycle of the Northern Hemisphere storm-tracks was investigated using feature tracking and Eulerian variance based diagnostics applied on both vorticity and meridional wind. Results were presented and discussed for the four seasons at both upper (250hPa) and lower (850hPa) tropospheric levels. Here, using the meridional wind diagnostics, the annual cycles of the North Pacific and North Atlantic storm-tracks are examined in detail. This is done using monthly and 20° longitudinal sector averages. Many sectors have been considered, but the focus is on sectors equally spaced in the two main oceanic storm-tracks situated at their western, central and eastern regions, the western ones being mainly over the upstream continents. The annual cycles of the upper and lower tropospheric storm-tracks in the central and eastern Pacific, and western and central Atlantic sectors all have rather similar structures. In amplitude, each sector at both levels has a summer minimum and a relatively uniform strength from October to April, despite the strong winter maxima in the westerly jets. However, high intensity storms occur over a much wider latitudinal band in winter. The storm-track in each sector moves poleward from May to August and returns equatorward from October to December, and there is a marked asymmetry between spring and autumn. There are many differences between the North Pacific and North Atlantic storm-tracks, and some of these seem to have their origin in the behaviour over the upstream East Asian and North American continents, suggesting the importance of seeding from these regions. The East Asian storm-track near 48°N has marked spring and autumn maxima and weak amplitude in winter and summer. The 33°N track is strong only in the first half of the year. In contrast, the eastern North American storm-track is well-organised all year, around the baroclinicity that moves latitudinally with the seasons. The signatures associated with these features are found to gradually decrease downstream in each case. In particular, there is very little latitudinal movement in the storm-track in the Eastern Atlantic.