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 the El Nino and the

Southern Oscillation (ENSO). Although this teleconnection provides a

basis for the seasonal predictability of SAM, it is shown that

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 Nino3 SST anomalies. It is also shown that the land-atmosphere feedback over the SAM region as a result of air-sea coupling is more robust that results in augmenting the decorrelation time of the daily SAM precipitation.

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.