Coupled Air, Sea, and Land Interactions of the South American Monsoon

The dominant interannual variation of the austral summer South American monsoon season (SAM) is associated with El Nino-Southern Oscillation (ENSO). Although this teleconnection provides a basis for the seasonal predictability of SAM, it is shown that the conventional tier-2 modeling approach of prescribing observed sea surface temperature (SST) is inappropriate to capture this teleconnection. Furthermore, such a forced atmospheric general circulation model (AGCM) simulation leads to degradation of the SAM precipitation variability. However, when the same AGCM is coupled to an ocean general circulation model to allow for coupled air-sea interactions, then this ENSO-SAM teleconnection is reasonably well simulated. This is attributed to the role of air-sea coupling in modulating the large-scale east-west circulation, especially associated with Nino-3 SST anomalies. It is also shown that the land-atmosphere feedback in the SAM domain as a result of the inclusion of air-sea coupling is more robust. As a consequence of this stronger land-atmosphere feedback the decorrelation time of the daily rainfall in the SAM region is prolonged to match more closely with the observed behavior. A subtle difference in the austral summer seasonal precipitation anomalies between that over the Amazon River basin (ARB) and the SAM core region is also drawn from this study in reference to the influence of the air-sea interaction. It is shown that the dominant interannual precipitation variability over the ARB is simulated both by the uncoupled and coupled (to OGCM) AGCM in contrast to that over the SAM core region in southeastern Brazil.