Effect of late 1970's climate shift on tropospheric biennial oscillation—role of local Indian Ocean processes on Asian summer monsoon

Abstract

AbstractThe tropical climate has undergone noticeable changes on interdecadal time scales. The climate shift that occurred in the late 1970s attained enormous attention owing to its global‐scale variations in ocean temperature, heat content and El Nino Southern Oscillation (ENSO) properties. Earlier studies presented the effect of this shift on ENSO and the Asian summer monsoon–ENSO relationship. The present study is an attempt to investigate the effect of late 1970's climate shift on tropospheric biennial oscillation (TBO), which is an important tropical phenomenon that includes both air–sea processes in the tropical Indian and Pacific Ocean regions. TBO is the tendency for the Asian–Australian monsoon system to alternate between relatively strong and weak years. The study comprises a detailed analysis of the TBO cycle in the time periods before (1951–1975) and after (1978–2002) the climate shift in 1976 with the help of National Centers for Environmental Prediction (NCEP)/National Center for Atmospheric Research (NCAR) data sets of 200‐hPa velocity potential; the Indian Ocean sea surface temperature (SST) and circulation are more obvious after the shift, although they were significant in the Pacific Ocean before 1976. The effect of ENSO in the biennial cycle is reduced with climate shift. The persistence of Asian‐to‐Australian summer monsoon has weakened in recent decades, as it is controlled by ENSO. Local oceanic processes in the Indian Ocean and local monsoon Hadley circulation have an increased role in the biennial oscillation of the Asian summer monsoon after 1976. Copyright © 2009 Royal Meteorological Society