Collaborative Research: Linking the Long-Term Congo Drought to Changes in Walker-Type Circulations Affecting Equatorial Africa

刚果河流域是世界第二大热带雨林的所在地，它养活着不断增长的人口和丰富的动植物物种。尽管雨林的面积、人口和作为生物多样性热点的重要性，但在气候研究文献中并没有得到很好的代表，特别是与面积更大但人口较少的亚马逊雨林相比。特别是，最近亚马逊的短期干旱引起了全世界对热带森林脆弱性的关注，但刚果盆地几十年来一直处于干旱状态。刚果盆地的降雨量比亚马逊流域少，古气候数据表明，在之前的干旱时期，刚果盆地经历了严重的森林损失。因此，刚果雨林的重要性和潜在的脆弱性促使进一步研究调节该区域气候和降雨的机制。这里开展的工作旨在确定该区域降雨不足的原因，特别是邻近海洋的海洋表面温度(SST)升高在多大程度上造成了降雨不足。近赤道海温变化可以通过影响被称为沃克环流的大尺度大气翻转环流来影响大陆降水。研究小组将检验这一假设，即干旱主要是由于印度洋和西太平洋海温上升，导致沃克环流加强并向西移动，进而导致刚果上空下沉，减少水分输送。这项研究使用了观测数据和模型模拟相结合的方法。该小组将利用刚果及其周边地区1921年至2014年基于雨量计的降水量数据集，该数据集是在之前国家科学基金的支持下由其中一家PIS汇编的。模式模拟使用了一个完全耦合的大气-海洋气候模式，一个只有大气的模式，有规定的海温，以及一种“起搏器”配置，其中在特定的海洋区域(比如赤道印度洋)指定了SST，但耦合的模式确定了该区域以外的自己的SST。如上所述，由于刚果盆地雨林的重要性和脆弱性，这项工作具有更广泛的影响。一个特别令人担忧的问题是，印度洋和西太平洋海温的上升可能是人为温室气体变暖造成的，在这种情况下，降雨量的减少不太可能逆转。结果被传达给非洲气候社区和非营利组织，用于气候影响评估和制定适应战略。此外，该项目为两名研究生和一名本科生提供支持和培训。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

The Basin of the Congo river is home to the second largest rainforest in the world, which sustains a growing population and an abundance of plant and animal species. Despite its size, population, and importance as a biodiversity hotspots, the rainforest is not well represented in the climate research literature, particularly compared to the larger but less populous Amazon rainforest. In particular, recent short-term Amazon droughts have drawn worldwide attention to the vulnerability of tropical forests, but the Congo Basin has been in drought for several decades. The Congo Basin receives less rainfall than the Amazon, and paleoclimate data suggests that it has experienced dramatic forest loss during previous dry periods. The importance and potential vulnerability of the Congo rainforest thus motivate further study of the mechanisms which regulate climate and rainfall in the region.Work performed here seeks to determine the causes of the rainfall deficits in the region, in particular the extent to which they are driven by increases in the sea surface temperature (SST) of the adjacent oceans. Near-equatorial SST changes can affect continental precipitation through their effects on the large-scale atmospheric overturning circulations known as Walker cells. The research team will test the hypothesis that the drought is primarily due to SST increases in the Indian and Western Pacific oceans, which result in an intensified and westward-shifted Walker cell, which in turn induces subsidence over the Congo and reduces moisture transport into it. The research is conducted using a combination of observational data and model simulations. The team will take advantage of a rain gauge-based precipitation dataset for the Congo and surrounding regions from 1921 to 2014, compiled by one of the PIs under previous NSF support. Model simulations are performed using a fully-coupled atmosphere-ocean climate model, an atmosphere-only model with prescribed SSTs, and a "pacemaker" configuration in which SST is specified over a particular ocean region (say, the equatorial Indian ocean), but the coupled model determines its own SST outside the region.The work has broader impacts due to the importance and vulnerability of the Congo Basin rainforest, as noted above. A particular concern is that the SST increases in the Indian and Western Pacific Oceans may be due to anthropogenic greenhouse warming, in which case the rainfall reductions are unlikely to reverse. Results are communicated to African climate communities and non-profit organizations for use in climate impact assessments and the formulation of adaptation strategies. In addition, the project provides support and training to two graduate students and one undergraduate.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.