Atmosphere–Warm Ocean Interaction and Its Impacts on Asian–Australian Monsoon Variation*

Asian-Australian monsoon (A-AM) anomalies depend strongly on phases of El Nin o (La Nin a). Based on this distinctive feature, a method of extended singular value decomposition analysis was developed to analyze the changing characteristics of A-AM anomalies during El Nin o (La Nin a) from its development to decay. Two off-equatorial surface anticyclones dominate the A-AM anomalies during an El Nin oone over the south Indian Ocean (SIO) and the other over the western North Pacific (WNP). The SIO anticyclone, which affects climate conditions over the Indian Ocean, eastern Africa, and India, originates during the summer of a growing El Nin o, rapidly reaches its peak intensity in fall, and decays when El Nin o matures. The WNP anticyclone, on the other hand, forms in fall, attains maximum intensity after El Nin o matures, and persists through the subsequent spring and summer, providing a prolonged impact on the WNP and east Asian climate. The monsoon anomalies associated with a La Nin a resemble those during an El Nin o but with cyclonic anomalies. From the development summer to the decay summer of an El Nin o (La Nin a), the anomalous sea level pressure, low-level winds, and vertical motion tend to reverse their signs in the equatorial Indian and western Pacific Oceans (10S-20N, 40-160E). This suggests that the tropospheric biennial oscillation is intimately linked to the turnabouts of El Nin o and La Nin a.