Interannual Variability of Mascarene High and Australian High and Their Influences on East Asian Summer Monsoon

Abstract

Based on the reanalysis data from NCEP/NCAR and other observational data, interannual variability of the Mascarene high (MH) and Australian high (AH) during boreal summer from 1970 to 1999 is examined. It is shown that interannual variability of MH is dominated by the Antarctic oscillation (AAO), and MH tends to be intensified with the development of the circumpolar lows in high southern latitudes. On the other hand, AH is correlated with AAO as well as El Nino and Southern Oscillation (ENSO), and tends to be intensified when El Nino occurs. Since AH, especially MH, is positively correlated with AAO, composite analysis on the difference between the positive and negative MH is performed to reveal the physical mechanism responsible for the East Asian summer monsoon anomalies associated with AAO. The result shows that, with the intensifi- cation of MH, the Somali jet, and Indian monsoon westerlies, tend to be strengthened. Accordingly, AH and the associated cross-equatorial flow, become stronger whereas the trade wind over the tropical western and middle Pacific become weaker. In association with the above changes, convective activities near the Philippine Sea are largely suppressed, as a consequence, exciting a negative convection anomaly, and a Rossby wave train from East Asia via the North Pacific to the western coast of North America (a negative Pacific-Japan pattern). Corresponding to the negative Pacific-Japan pattern, an anomalous rainfall pattern appears in East Asia. Correlation analysis between AAO and sea level pressure, 500 hPa geopotential height further indicates that AAO is a strong signal influencing the climate anomaly in both hemispheres, including East Asia. Due to the seasonal persistence, AAO and the related MH and AH in boreal spring, may provide some useful information for the East Asian summer monsoon prediction. With the intensification of MH during boreal spring through summer, the Meiyu/Baiu rainfall from the Yangtze River valley to the Japan Islands tends to increase, while less rainfall is found outside of this region. In contrast with MH, the effect of AH on summer rainfall is confined to southern China.