The Tropospheric Biennial Oscillation of the Monsoon–ENSO System in an Interactive Ensemble Coupled GCM

Abstract

The processes for the coupled tropospheric biennial oscillation involving the Indian monsoon and El Niño?Southern Oscillation (ENSO) are studied through composites of sequential wet monsoon/La Niña year followed by dry monsoon/El Niño year using observations and outputs from the Center for Ocean?Land?Atmosphere Studies (COLA) interactive ensemble coupled general circulation model. The previous composites emphasize the role of ENSO in the monsoon transition but do not exclude the possible role of factors other than ENSO. It is found that the central eastern equatorial Pacific sea surface temperature (SST) anomalies can affect the Indian monsoon transition by two processes: (i) a shift of large-scale east?west circulation across the equatorial Indian?Pacific Oceans; and (ii) a Rossby wave?type response over the eastern north Indian Ocean?western North Pacific. The former provides the low-level anomalous moisture convergence (divergence) that preconditions the atmosphere for a wet (dry) Indian monsoon. The latter induces anomalous downward (upward) motion and low-level anomalous anticyclonic (cyclonic) circulation over the eastern north Indian Ocean?South China Sea. Associated shortwave radiation and surface evaporation changes favor the eastward shift of cold (warm) SST anomalies and in turn low-level easterly (westerly) anomalies through SST gradient changes. The moistening (drying) of the atmosphere before a wet (dry) Indian monsoon is due to the low-level anomalous moisture convergence (divergence) that is dominated by anomalous wind convergence of the mean humidity. A large increase (decrease) in surface evaporation is observed in conjunction with the transition to a wet (dry) monsoon. The Indian Ocean SST change is mainly a response to the monsoon and ENSO-induced surface heat flux changes. The Asian land surface condition anomalies are not necessary in this coupled biennial variability of the monsoon?ENSO system in the model.