Tropical 45 Day Oscillations during the 1979 Northern Hemisphere Summer

Abstract

Based on FGGE 1,cvel IIIb data, the structural features of 45 day perturbations over a tropical belt (15°N-15°S) during the 1979 summer are detailed. At the equator, 45 day perturbations which are primarily associated with the zonal wind components of wavenumber 1, propagate eastward (8° of longitude per day) and upward (0.7 km per day), probably indicating downward energy flux. In the Southern Hemisphere tropics (0°?15°S), the 45 day zonal mean wind perturbations propagate downward with an approximate phase speed of 0.8 km per day. In the Northern Hemisphere tropics, they are largely of standing character with the maximum amplitude (3 m s?1) near 200 mb at 15°N. There exists a strong association between monsoon activity over South Asia and changes in the intensity of the equatorial Walker circulation. When active monsoons occur over South Asia, the Walker circulation becomes stronger than usual with prominent 850 mb easterlies (200 mb easterlies) over the eastern Pacific east of the date line and above normal 850 mb westerlies (200 mb easterlies) over the Indian Ocean and the western Pacific west of the date line. Equatorial convective activity appears to be above normal near the date line, as evident by abnormally strong ascending motions. During the break monsoon phase, the equatorial Walker circulation is depressed below normal. There are two bridges (one in the Eastern Hemisphere from approximately 50° to 150°E and another in the Western Hemisphere between 170° and 70°W,through which the 45 day perturbations of the Southern Hemisphere tropics at 200 mb interact with those in the Northern Hemisphere tropics. In comparison, the central Pacific between approximately 150° and 120° is the only favorable channel for interhemispheric interaction due to transient disturbances with time scales shorter than about 30 days. This interhemispheric interaction, due to short-period transient disturbances at 200 mb, is abnormally enhanced when the Asiatic monsoon is active.