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

加勒比地区的雨季呈现出双峰结构，两个最大值(5-6月和9-10月)被所谓的“仲夏”干旱(7-8月)隔开。降雨量的减少可能会很明显，7月底和8月初的降雨量减少了40%。该地区仲夏干旱的原因在某种程度上是一个谜，很少有研究探讨这种差异的原因。这一合作研究项目将调查仲夏干旱的开始、持续时间和强度的空间变异性，并将这种空间模式与嵌入在半球或全球背景下的潜在区域大气过程联系起来，这些过程有助于仲夏降水的减少。这个项目的方法论旨在检验两个假设。第一种假设认为，加勒比仲夏干旱是由三个地方到区域机制共同造成的。首先，阴影效应(初夏几个月的对流产生的云量和降雨阻挡了到来的太阳辐射)导致了负反馈，因为地表水域冷却，抑制了进一步的对流。太阳辐射减少之后，低层风的反气旋环流引起的辐散风异常，最后，信风的加强增加了风的搅动导致的蒸发。第二种假设认为，大西洋副热带高压的加强和扩大导致更强的信风、较冷的海面温度和普遍的下沉，从而导致7月和8月加勒比海地区的降雨量减少。这些假设将通过(1)用月度数据评估加勒比仲夏干旱的空间格局，(2)用每日地面观测以及卫星降水和风数据缩小所确定的模式的尺度，(3)确定与该现象的开始、持续时间和强度有关的潜在大气过程，以及(4)确定全球气候变化和变化对加勒比仲夏干旱的发生、持续时间和强度的潜在影响。此外，全球气候变化情景表明，加勒比地区未来将经历更大的干旱压力。该项目的成果将通过回答以下问题来扩大加勒比气候学和水文学的知识：为什么加勒比地区的降雨量在预计降雨量会增加的夏季减少？拟议研究产生的更广泛影响包括为加勒比资源管理者提供信息，以制定更准确的供水和干旱预报，这是许多岛屿经济活动所必需的。此外，研究结果将为气候变化科学家提供加勒比区域尺度气候过程的信息，可用于改进海洋-大气环流模型和构建分级尺度模型。这类模型更有可能提供可供加勒比小岛屿国家用来构建应对气候变化情景的现实对策的信息。最后，该项目将通过本科院校研究计划为相当数量的本科生提供服务。

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.