The Impact of Intra-Americas Sea TD-Wave Activity on Historical and Projected Regional Rainfall in Select IPCC AR5 Models

The tracks of westward propagating synoptic disturbances across the Intra-Americas Sea (IAS), known as easterly waves or tropical depression (TD) waves, are an important feature of the region’s climate. They are associated with heavy rainfall and seed the majority of tropical cyclones across the region. Results of the first part of this study suggests that CMIP5 models have biases in TD wave track density and that these biases are well correlated with model biases in sea surface temperature and mid-level moisture.  In addition, results confirm a projected southward shift in TD track density across the IAS for both the RCP 4.5 and RCP 8.5 warming scenarios, similar to what was seen in the CMIP3 models, whereas changes in TD strength are less clear. The southward shift of the TD track density is most correlated with a shift in the genesis potential index, with changes in deep wind shear and mid-level moisture having the greatest impact on the change in the geographic distribution of the index in future scenarios. The present study examines the ability of CMIP5 models to capture the synoptic variability in precipitation associated with TD wave activity over Mesoamerica, including northwestern Colombia, Central America and Mexico, in association with the seasonal migration of the ITCZ and American monsoon systems.  Analyses of the projected change in synoptic scale precipitation in a warming climate are also examined for these regions.