30–50 Day Oscillation of 200-mb Temperature and 850-mb Height during the 1979 Northern Summer

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Abstract
A coherent seesaw fluctuation of 200-mb temperature T(200 mb) and 850-mb height Z(850 mb) between the Tibet-India region and the North Pacific is analyzed with the ECMWF FGGE III-b data for the northern summer (May-September 1979). Both T(200 mb) and Z(850 mb) oscillate with an opposite phase between these two regions and with a period of about 30–50 days. Furthermore, T (200 mb) and Z (850 mb) of the same location also oscillate with an opposite phase relationship. It was demonstrated from the empirical orthogonal function (EOF) analysis that this coherent seesaw oscillation of these two variables between the Tibet-India region and the North Pacific can be attributed to the eastward propagation of the 30–50 day oscillation. The eastward propagation of this low-frequency oscillation may exert a profound impact on the regional atmospheric circulation downstream. The thermal energy pumped up from the ocean by cumulus convections over the moonson regions might be propagated out by this 30–50 day oscillation to warm up the cold center of the North Pacific. Moreover, the oceanic anticyclone over the North Pacific would be intensified periodically by the eastward propagation of this low-frequency oscillation, which would affect the moisture transport over East Asia and even North America. As for the former region, the Mei-yu trough is deepened and the onset of the Mei-yu regime is triggered by the eastward propagation of the low center of the Indian monsoon trough associated with the 30–50 day oscillation.

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