RUI: Collaborative Research: Physical Mechanisms Behind the Caribbean Mid-Summer Drought

University Of North Carolina At Wilmington, Wilmington NC

Investigators

Abstract

The rainy season in the Caribbean region exhibits a double-peaked structure with two maxima (May-June and September-October) separated by what has been termed a "mid-summer" drought (July-August). The decrease in precipitation can be significant, with as much as a 40 percent decrease in late July and early August. The cause of the region's mid-summer drought has been somewhat of an enigma, with few studies examining the reasons for the variability. This collaborative research project will investigate the spatial variability in the onset, duration, and intensity of the mid-summer drought and link such spatial patterns with potential regional atmospheric processes imbedded within the hemispheric or global context that contribute to the decrease in mid-summer rainfall. The methodology for this project is designed to test two hypotheses. The first hypothesis states that the Caribbean mid-summer drought is created through a combination of three local to regional mechanisms. First, a shadowing effect (cloudiness and rainfall produced by convection in the early summer months block incoming solar radiation) leads to a negative feedback as surface waters cool inhibiting further convection. The decrease in solar radiation then is followed by divergent wind anomalies caused by an anticyclonic circulation of low-level winds, and finally, a strengthening of trade winds increases evaporation due to wind stirring. The second hypothesis states that an intensification and expansion of the Atlantic Ocean sub-tropical high pressure causes stronger trade winds, cooler sea surface temperatures, and general subsidence that result in less Caribbean rainfall in July and August. These hypotheses will be tested through (1) assessment of Caribbean mid-summer drought spatial patterns with monthly data, (2) downscaling of identified patterns with daily surface observations and satellite precipitation and wind data, (3) identification of potential atmospheric processes linked to the onset, duration, and intensity of the phenomenon, and (4) identification of the potential influence of global climate variability and change, particularly over the Pacific (El Nino) and Atlantic upon the onset, duration, and intensity of the Caribbean mid-summer drought. The nations of the Caribbean are prone to natural disasters from hydrologic drought. Further, global climate change scenarios suggest that the Caribbean will experience greater drought stress in the future. The results from this project will expand knowledge of Caribbean climatology and hydrology by answering the question, why does Caribbean rainfall decrease in the summer, a time when precipitation is expected to increase? Broader impacts resulting from the proposed research include providing information for Caribbean resource managers to create more accurate water supply and drought forecasts, a necessity for many island economic activities. In addition, results will provide climate change scientists with information of regional scale climatic processes in the Caribbean that can be used to refine ocean-atmosphere general circulation models and construct hierarchical scale-based models. Such models are more likely to provide information that can be used by Caribbean small island states to construct realistic responses to climate change scenarios. Finally, the project will serve a significant number of undergraduate students through the Research in Undergraduate Institutions program.