Tropospheric biennial oscillation of the Indian summer monsoon with and without the El Nino-Southern Oscillation

Abstract

Tropospheric biennial oscillation (TBO) is the tendency for a relatively strong monsoon to be followed by a weaker one, and vice versa, for the Asian–Australian monsoon system. According to this definition, TBO years include most of the El nino-Southern Oscillation (ENSO)-onset years and some other years also. It is believed that the TBO years, with and without ENSO-onset years, have differences only in the magnitude of the signal. In the present paper, the process of coupled TBO involving the Indian monsoon and the Indian Ocean are studied through composites of relatively strong minus weak monsoon years, with and without the ENSO-onset years, using National Centers for Environmental Prediction/National Centers for Atmospheric Research (NCEP/NCAR) data sets for the 1950–2004 time period. Different seasonal evolutions of anomalies for convection, Sea Surface Temperature (SST) and wind in a biennial cycle have been identified with and without the ENSO-onset years. In both these cases, a perfect biennial cycle for convection has been noted. In the non-ENSO biennial cycle, a perfect biennial cycle is obtained in the areas close to the Indian subcontinent for SST and wind. But for the ENSO-only years, the seasonal evolutions are different. The Indian Ocean dipole, which is considered an inherent factor for TBO, is absent in the non-ENSO years. The development and propagation of anomalies are also different in these two cases. It seems that factors other than ENSO are contributing to the biennial cycle in the non-ENSO years. Copyright © 2007 Royal Meteorological Society