The 1976 transition in precipitation over the Americas: the influence of tropical SST

Most major features of the interdecadal shift in boreal winter-spring precipitation over the American continents associated with the 1976-77 transition is reproduced in atmospheric general circulation model (GCM) simulations forced with observed sea surface temperature (SST). The GCM runs forced with global and tropical Pacific SSTs produce similar multidecadal changes in precipitation, indicating the dominant influence of tropical Pacific SST. Companion experiments indicate that the shift in mean conditions in the tropical Pacific is responsible for these changes. The observed and simulated "post- minus pre-1976" difference in January-May precipitation is wet over Mexico and the southwest U.S., dry over the Amazon, wet over sub-Amazonian South America, and dry over the southern tip of South America. This pattern is not dramatically different from a typical El Nino induced response in precipitation. Although the interdecadal (post- minus pre-1976) and interannual (El Nino minus La Nina) SST anomalies differ in detail, they produce a common tropics-wide tropospheric warmth that may explain the similarity in the precipitation anomaly patterns for these two time scales.

An analysis of local moisture budget shows that, except for Mexico and the southwest U.S. where the interdecadal shift in precipitation is balanced by evaporation, elsewhere over the Americas it is balanced by a shift in low-level moisture convergence. Moreover, the moisture convergence is due mainly to the change in low-level wind divergence that is linked to low-level ascent and descent.