Large-scale Variations in Summer Rainfall over South and East Asia and Their Relations to Anomalies of the Sea Level Pressure Field in the Tropics

The relationships between the summer (June to September) rainfall variability in South and East Asia and the sea level pressure (SLP) field in the tropics are clarified both by principal component analysis (PCA) and by singular value decomposition analysis (SVDA). First, we applied PCA to summer rainfall for detecting temporal and spatial variations in this area. Then, components of common variations between summer rainfall and the SLP field in the tropics from the Indian Ocean to the Pacific were revealed by SVDA. We found that the first 2 components of summer rainfall variation derived from PCA and SVDA have the same spatial patterns and temporal variations. The first SVD mode (SVD 1) has large signals of rainfall variations in India and north China, and its time coefficients show the relation to the warm event of ENSO. The corresponding spatial pattern of the SLP field indicates variations in the east-west circulation pattern between the areas from the Indian Ocean to the western Pacific and the eastern Pacific in the tropics. Time series of the first principal component (PC I) for summer rainfall and SVD 1 have decadal scale variations with a remarkable change in the latter half of the 1970 s. The second SVD mode (SVD 2) shows a variation in summer rainfall in central China associated with variation in the SLP field and convective activities in the western tropical Pacific. We discuss the relationships between the variation in summer rainfall and the SLP field for the first two SVD modes in terms of sea surface wind anomalies accompanied by variation in the SLP field. The rainfall anomalies over India in SVD I are explained by wind anomalies of meridional components in the southwestern monsoon over the Arabian Sea. These wind anomalies are accompanied by large SLP anomalies over the Arabian Sea near the Eurasian continent. On the other hand, the variation in summer rainfall in north China appears to be closely connected to the formation of a relative pressure anomaly field over the western tropical Pacific in the Northern Hemisphere. Positive (negative) SLP anomalies both in south China and in the western equatorial Pacific develop relative cyclonic (anticyclonic) circulations. This cyclonic (anticyclonic) circulation weakens (strengthens) the southerly wind component in the middle latitudes over East Asia. As a result, the water vapor transport to north China seems to be inhibited (promoted). Concerning SVD 2, SLP variations in the northwestern tropical Pacific are accompanied by the convergence or divergence of surface wind and variation in convective activities in the corresponding region. The variation in summer rainfall in central China is considered to be related to the impact of convective activities in- the northwestern tropical Pacific. Variation in the sea surface temperature (SST) accompanied by these variations in the SLP field were investigated using correlation analysis. The variation in SLP for SVD 1 is correlated significantly with those in the SST in the eastern tropical Pacific and the tropical Indian Ocean. The variation in east-west circulation from the Indian Ocean to the Pacific is considered to be caused by the movement of tropical convective regions with the variation in SST over the eastern tropical Pacific.