The American Midsummer Drought: Causal Mechanisms and Seasonal-to-Interannual Predictability

The Intra–Americas Sea (IAS) region– including the northeastern tropical Pacific Ocean, Caribbean Sea, Gulf of Mexico, western tropical Atlantic Ocean, and all adjacent landforms–represents a fascinating natural climate laboratory due to a confluence of diverse oceanic, orographic, atmospheric, and remote influences. The IAS region is also home to a large portion of humanity whose livelihood depends critically upon the spatiotemporal variability of precipitation. Throughout most of the IAS region, the rainy season spans roughly May through October with a break in precipitation in July–August known as the midsummer drought (MSD). This feature of the rainfall climatology is highly unique to the IAS region, and is particularly evident over Central America and the adjacent northeastern tropical Pacific Ocean. Indeed, the MSD is such a pervasive phenomenon that crop insurance programs incorporate what little information is known of the MSD in pricing and triggering policies in Central America. Since the recognition of the MSD as a regular climatological feature in the early 1960s, much effort has been directed toward characterizing and understanding the MSD. Both local processes (e.g., SST–convection–radiation feedback) and aspects of the general circulation (e.g., the North Atlantic subtropical high) have been shown to influence the MSD. To date, however, a unifying explanation for the very existence of the MSD has yet to emerge. As a result, our understanding of the interannual variability and– most importantly– predictability of the MSD is only in a nascent stage. Seasonal–to–interannual climate predictions for the IAS region would benefit greatly from an understanding of the causal mechanisms for the existence and variability of the MSD. We propose to first focus on analysis of observations: satellite and in situ measurements, as well as state–of–the–art global and regional reanalyses to diagnose the dominant mechanisms of the MSD in the IAS region. Secondly, we will use state–of–the–art general circulation models to test specific hypotheses regarding the dominant mechanisms of the MSD and its variability. This approach will allow us to thoroughly examine and identify the features of the global atmospheric circulation and, especially, the role of the ocean, that are crucial for predicting seasonal hydroclimate variability in the IAS region.