Abstract
Seasonal and interannual changes of cloud amount over the western Pacific were studied using GMS cloud-amount data from April in 1978 to January in 1985, paying attention to the change in the variation with periods of about 30 days. The following results were attained.
The variation with period of about 30 days shows a large amplitude in every season in the neighborhood of the tropical cloudy regions and in spring to autumn near large cloud-amount regions in the northern middle latitudes. The large amplitude regions are enlarged and extended to the middle latitudes in the summer Hemisphere, besides the seasonal northward or southward shift of large-amplitude regions, while the amplitude is small even in the subtropical and tropical regions in the winter Hemisphere. The contribution of 30-day period variation to total variation in the subtropics is larger in summer than in winter. The seasonal changes of cloud amount and the amplitude of 30-day period variations show an out-of-phase relationship in the middle latitude region and an in-phase relationship in the subtropics. interannual change in the amplitude of 30-day period variations, as well as cloud amount, is much influenced by the El Nino event. In the El Nino year 1982 to 1983 the amplitude was smaller than in other years in the Northern Hemisphere, while it was larger around (l5°S, 180°). In early 1980 the amplitude was large over a very wide region, in strong contrast to the small amplitude in early 1983. Out-of-phase or in-phase relationships between cloud amount and the amplitude can be seen in their interannual changes, too.
