Understanding CMIP5 Projected Drying over Meso-America and the Intra-America Seas

One of the most robust climate change signals in CMIP3 models is a projected future drying over the Meso-America and the Intra-American Seas region, including Central America, southern Mexico, and the Caribbean. In the scenario simulations, precipitation decreases by 25%, with the largest decreases in June, effectively lengthening the Mid-Summer Drought in much of the region. This late spring and early summer drying is associated with a southward displacement of the eastern Pacific ITCZ, stronger low-level easterlies and an intensified Caribbean low-level jet (LLJ), and reduced warming of sea surface temperatures (SSTs) overall in the tropical North Atlantic compared to the zonal mean.

We examine the CMIP5 results for a variety of scenarios (RCP85, RCP45, and RCP26) to determine if the same drying signal is present, and to further understand forcing factors. In RCP85 and RCP45, precipitation decreases in spring and early summer and increases in fall, indicating a shift and intensification of the annual cycle. These precipitation changes reflect overall decreases in subtropical precipitation, but regional circulation features such as an early westward expansion of the North Atlantic subtropical high in late spring and a stronger Caribbean LLJ associated with a stronger inter-basin pressure gradient regionally enhance drying. Upper-level circulation changes suggest the same baroclinic response to reduced SST warming in the tropical North Atlantic as in CMIP3. In contrast to RCP85 and RCP45, precipitation increases in the low emissions scenario (RCP26), so projected precipitation changes differ not only in intensity with scenario but also in sign.