The tropospheric biennial oscillation and Indian monsoon rainfall

An analysis of observed data shows that the tropospheric biennial oscillation (TBO, with roughly a 2-3 year period) encompasses most ENSO years (with their well-known biennial tendency) as well as additional years that contribute to biennial transitions in the Indian monsoon. Three conditions (related to anomalous surface temperatures) postulated to contribute to TBO transitions are examined for the northern spring season before the Indian monsoon: 1) anomalous tropical Pacific SSTs, 2) atmospheric circulation-related anomalous south Asian land temperatures and resulting meridional temperature gradients, and 3) anomalous SSTs in the Indian Ocean. We quantify these three interannual transition conditions on a year by year basis, thus taking into account intermittent influences and secular variations in the strength of any particular linkage in any given year. The more transition conditions that are included in the analysis, the greater the TBO amplitude in terms of spectral power in the TBO periods of 2-3 years.