Ehsan Erfani, David Mitchell

The North American Monsoon is a seasonal shift in the large-scale circulation that supplies 60-80% of annual rainfall in northwestern Mexico and 30-40% in the US southwest. Regional climate models have shown that summer precipitation prediction over North America is the poorest in the Monsoon region. Most climate models do not account for a crucial mechanism of Monsoon: the boundary layer inversion over the Gulf of California controls the low-level moisture transport. To investigate this mechanism, a set of carefully designed simulations of a regional climate model is used to investigate the dependence of Monsoon precipitation on sea surface temperature (SST) in the Gulf. The results are consistent with enhanced observations from a field campaign and show that warmer Gulf SSTs tend to weaken boundary layer inversion and enhance low-level moisture flux, and as a result, more Monsoon precipitation occurs. This highlights the necessity for climate models to implement the mentioned mechanism.