Indian Monsoon GCM Sensitivity Experiments Testing Tropospheric Biennial Oscillation Transition Conditions

Abstract

Observational studies have shown that the tropospheric biennial oscillation (TBO) involves transitions that occur from northern spring [March?April?May (MAM)] to the Indian monsoon season [June?July?August?September (JJAS)] such that a relatively strong monsoon the previous year is often followed by a relatively weak one, and vice versa. Several conditions involving anomalous land and ocean surface temperature anomalies in the Indo-Pacific region in MAM have been identified to be associated with TBO monsoon transitions. Though it is possible to quantify the relative contribution of each transition condition year by year in observations, they are interrelated and the question remains whether each condition by itself could cause a monsoon transition. Here, a series of GCM sensitivity experiments is performed to isolate the effects of each of the transition conditions to document their respective influences on the anomalous patterns of monsoon rainfall associated with TBO transitions. Three conditions postulated to contribute to these TBO transitions associated with Indian monsoon rainfall are 1) atmospheric circulation?related anomalous south Asian land temperatures and resulting meridional temperature gradients, 2) anomalous SSTs in the Indian Ocean, and 3) anomalous tropical Pacific SSTs. Sensitivity experiments with an atmospheric GCM (the NCAR CCM3) are performed to address these conditions by specifying 1) warmer land temperatures over Asia to produce a stronger meridional temperature gradient, 2) warm Indian Ocean SST anomalies, and 3) cold Pacific Ocean SST anomalies. The model results demonstrate that each of the transition conditions is associated with distinct physical processes and can contribute to a relative TBO transition in monsoon strength by themselves. The anomalous tropical Indian and Pacific Ocean SST anomalies produce a larger monsoon response in the model compared to the anomalous meridional temperature gradient over Asia indicating they are the dominant conditions associated with TBO transitions. The location of the SST anomalies over the tropical Indian Ocean is important, with warm SST anomalies throughout the tropical Indian Ocean producing enhanced rainfall over the ocean and south Asian land areas, and warm SST anomalies near the equatorial Indian Ocean producing increased rainfall locally with decreased rainfall over south Asian land areas. Case studies from observations illustrate that the various transition conditions are evident in the raw data in individual years. Several more GCM experiments are performed to show how some conditions can act cumulatively to produce monsoon transitions.