Principal Modes of Rainfall–SST Variability of the Asian Summer Monsoon: A Reassessment of the Monsoon–ENSO Relationship

Abstract

Using global rainfall and sea surface temperature (SST) data for the past two decades (1979?98), the covariability of the Asian summer monsoon (ASM) and El Niño?Southern Oscillation (ENSO) was investigated. The findings suggest three recurring rainfall?SST coupled modes. Characterized by a pronounced biennial variability, the first mode is associated with generally depressed rainfall over the western Pacific and the ?Maritime Continent,? stemming from the eastward shift of the Walker circulation during the growth phase of El Niño. The associated SST pattern consists of an east?west SST seesaw across the Pacific and another seesaw with opposite polarity over the Indian Ocean. The second mode is associated with a growing La Niña, comprising mixed, regional, and basin-scale rainfall and SST variability with abnormally warm water in the vicinity of the Maritime Continent and western Pacific. It possesses a pronounced low-level west Pacific anticyclone (WPA) near the Philippines and exhibits large subseasonal-scale variability. The third mode is associated with regional coupled ocean?atmosphere processes in the ASM region, having spatial and temporal variabilities that suggest extratropical linkages and interhemispheric interactions occurring on decadal timescales. Results indicate the importance of regional processes in affecting ASM rainfall variability. On the average, and over the ASM region as a whole, ENSO-related basin-scale SSTs can account for about 30% of the variability, and regional processes can account for an additional 20%. In individual years and over subregions, the percentages can be much higher or lower. In addition to the shift in the Walker circulation, it is found that the regional excitation of the WPA is important in determining the rainfall variability over south Asia and east Asia. Based on the results, a hypothesis is proposed that anomalous wind forcings derived from the WPA may be instrumental in inducing a biennial modulation to natural ENSO cycles. The causes of the 1997 and 1998 rainfall anomalies over the ASM subregions are discussed in the context of these results and in light of recent observations of long-term changes in the monsoon?ENSO relationship.