Long-Term Variations of Cloud Amount in the Western Pacific Region

Abstract

Long-term variations of heat sources and their spatial distributions in the western Pacific region are examined by using satellite-observed high-cloud amount data from 1978 to 1983. There exist large seasonal variations with maximum cloud amount in the summer hemispheric tropical regions. Spatial distributions of cloud amount are largely affected by summer and winter monsoons over Southeast Asia and Australia.
Large interannual variability of cloud amount is confined in the equatorial region during northern winter, but large variations are found not only in the equatorial region, but in the northern subtropics around 20°N and in the middle latitudes near 35°N during summer.
Two dominant cloud anomaly patterns are found by correlation computations and Empirical Orthogonal Function (EOF) analysis. The first anomaly pattern (El Nino pattern) is the east-west oscillation between the equatorial central Pacific and the western Pacific. This pattern was significantly amplified during 1982-83 years corresponding to the El Nibo event. The second anomaly pattern (South Japan pattern) is the north-south oscillation between northern subtropics and middle latitudes extending from East China to the dateline. This pattern becomes most evident during northern summer.
Cloud anomalies along the equator during 1982-83 show that positive anomalies near 150°E started to move eastward in May - June 1982, reached the central and eastern Pacific in late '82 and continued to stay in the eastern Pacific till next summer. The large portion of the western Pacific regions was occupied by intense negative cloud anomalies. During this period cloud amounts along the mid-latitude storm tracks from East China Sea to Northwest Pacific increased probably due to enhancement of extratropical cyclone activities.
During the summers of 1978 and 1981 convection centers in the tropical western Pacific are abnormally shifted northeastward from the normal position near Philippines to western Pacific subtropics around 20°N, 140°-150°E. Cloud amounts both in mid-latitude regions to the north of the convection centers and in the equatorial regions to the south are greatly reduced and Japan Islands have experienced hot summers during these years.
Relationships between variations of cloud amount and sea surface temperature (SST) in the tropical western Pacific are examined. EOF analysis of SST shows that SST variations are dominated by the first eigen mode with large amplitudes in the tropical western Pacific east of 140°E and from the equator to about 20°N. the amplitude of this mode significantly increases in 1982-83 indicating that large negative SST anomalies appear in the vast tropical western Pacific regions during the El Nino period. The South Japan cloud anomaly pattern develops during summers when large positive SST anomalies in the tropical weatern Pacific appear. Correlation computations also indicate close relationships between cloud amount and SST.