Temporal variations in the northern hemispheric summer circulations

Abstract

Changes in kinetic energy averaged over the tropical (5.0°S-19.6°N), subtropical (24.2°N-37.1 oN) and mid-latitude (41.0°N-48.1°N) belts were examined for the 92-day period from June through August 1970. Some-what systematic fluctuations (10-15 day period) were observed in the eddy and zonal mean kinetic energies of the 200 mb tropical belt. Mid-latitude periodicity was longer at about 20 days. Short period (~5 day) fluctuations were considerably stronger at 700 mb than that at 200 mb, over the tropical belt.

Abnormal transitions were observed to begin around 12 July with a simultaneous decrease in 200 mb eddy kinetic energy at all latitudes, 700 mb eddy kinetic energy over the tropical belt, and tropical baroclinic activity as determined from cloudiness; all reached minimum values around 20-24 July. On the other hand, tropical zonal mean kinetic energy increased to maximum values in the same period.

During 20-24 July, the tropospheric circulation displayed singularly different characteristics compared to normal, and break monsoon" conditions prevailed over India. The zones of cloudiness and upper divergence maxima over the monsoon region shifted northwards to the Tibetan area, while dry weather and abnormal upper convergent wind inflow was experienced over southeast Asia. A conspicuous and anomalous excitation of convective activity over 'tropical regions in the western North and South Pacific was associated with pronounced upper (lower) tropospheric anticyclonic (cyclonic) circulations. Consequently, 200 mb equatorial easterlies over the western and central Pacific were unusually strong. Concurrently the anticyclone over the Mexican region reached maximum intensity and was displaced to the north of its normal position