The North American summer monsoon circulation exhibits substantial interannual variability. Establishing clear links between this variability and the slowly varying boundary forcing (sea surface temperatures - SSTs - and land surface conditions) has proven difficult. For example, no clear relationship has been found between SST anomalies associated with El Nino/La Nina events and monsoon rainfall. Recent observational and regional modeling studies have suggested that anomalies in snow cover/soil moisture in the Rocky Mountains can have an impact on the monsoon. However, there remains the possibility that a substantial portion of the variability is due to dynamical processes internal to the atmosphere. Determining the relative roles of internal and lower boundary forcing processes in producing interannual variations in the monsooon is a major objective of our current research. Our methodology is based on the analysis of simulations performed with the UCLA atmospheric general circulation model. Currently, our focus is on the influence of SSTs on monsoon variability, therefore we deliberately suppress the interaction/influence of land processes. Two 20-year long simulations have been completed, a CONTROL using climatological SSTs and an AMIP-type simulation using observed SSTs for the period 1979-1998.

We have compared the simulated and observed interannual variability via differences between composites of 'wet' and 'dry' years in the southwest U.S. In the CONTROL simulation, the composite 'wet' - 'dry' precipitation difference maps show a pattern and magnitude that is similar to the observed in the core monsoon region. These results suggest that internal atmospheric processes alone can produce variability comparable in magnitude to that observed. In this regard, 'wet' - 'dry' composite differences in global sea level pressure and zonal wind suggest a possible connection (in both the CONTROL and observations) between monsoon variability and Arctic oscillation-type variability.

In the AMIP simulation we do not find a systematic impact of SST anomalies on the monsoon. There is little correspondence between observed 'wet' and 'dry' monsoon years and the 'wet' and 'dry' years in the AMIP simulation. In fact, three of the four dry monsoon years in the simulation were observed to be wetter than normal. There are several possible explanations for this result: 1) atmospheric circulation anomalies are forcing the extratropical SST anomalies rather than the SST anomalies forcing atmospheric anomalies, 2) the single realization of each year is not sufficient to distinguish the SST-forced signal from the internal variability (noise), 3) land surface processes play a significant mediating role in the relationship between SSTs and monsoon strength. To address 2) we have performed a twenty-member ensemble of integrations for the summer forced by the global SST anomaly field from an observed composite of 'wet' minus 'dry' monsoon years. A preliminary analysis of this ensemble indicates that there is no systematic impact of the SST anomalies on the atmospheric circulation over North America, suggesting that 2) cannot explain the AMIP results. A complete analysis of this ensemble will be presented at the meeting, along with results from an ensemble of simulations using a mixed-layer ocean designed to investigate explanation 1).