During the 1990s, pronounced changes occurred in major climatological fields worldwide. In the Northern Hemisphere, regarding the major shifts in the dry and wet patterns, the mid-1990s decadal change occurred more rapidly in winter than in summer. What accompanied by this decadal change were the strengthened lower-tropospheric anticyclones over the subtropical Pacific Ocean and a northward shift of the midlatitude westerly jet stream across the Afro–Asia–Pacific region. Increased trade winds resulted in considerable sea surface cooling over the central and eastern Pacific regions. These circulation changes induced the poleward expansion of two boreal winter systems: convection over the Maritime Continent and dry conditions in North Africa and West Asia. Moreover, the weakening of the Arctic upper-tropospheric circulation further modulated the seasonal shift of the midlatitude jet stream. In boreal summer, the more gradual decadal change was associated with the westward shift of Asian monsoon and northward penetration of African monsoon. What used to be a substantial vertical coupling of the westerly flows above these two monsoons was weakened during the 1990s. Judging from the effects of these circulation changes, it appears that the different rate of change in the jet streams may have exerted a dynamic effect on the pace of the decadal change in different seasons.
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