Fluctuations in the northern hemispheric winter circulations

Abstract

Between 12 and 16 January 1971, exceptionally large cloudiness was observed over Malaysia and Indonesia. The 700 mb winds over the western North Pacific and south China Sea between about l5oN and the equator, normally easterlies, were replaced by somewhat strong westerlies. Concurrently, near Japan (South Australia) the jet stream at 700 mb diminished  (increased) substantially in speed. Below normal cloudiness was observed over eastern Asia, perhaps reflecting weak surface monsoonal surges over that region. At 200 mb, the E-W divergent circulations became well established over the equatorial western Pacific, presumably carrying excess energy supplied over the Indonesian region very far downstream to the equatorial central Pacific dry zone.

From 19 to 23 January 1971, below normal cloudiness existed over Malaysia and Indonesia in contrast to well above normal cloudiness over the south Indian Ocean and South Africa between about 5oS and l5oS. During that period, near equatorial westerlies at 700 mb disappeared from the Malaysian region, while they intensified over the equatorial Indian Ocean. At 200 mb, the jet stream near and to the east of Japan was exceptionally weak. Over the equatorial central Pacific, the E-W divergent circulation became ill defined, while N-S overturnings were more active than usual.