The Precipitation Response to El Niño/Southern Oscillation (ENSO) over Tropical South America: Spatial and Temporal Heterogeneity and the Role of the Land Surface

众所周知，厄尔尼诺/南方涛动（ENSO）事件对全球天气和气候产生重大影响，包括南美洲热带大部分地区降雨量减少，并对水资源、农业以及其他人类和自然系统造成破坏。降水异常的最终原因是邻近赤道太平洋ENSO条件引起的大尺度大气环流的变化，但也可能受到南美洲上空发生的陆-气耦合的调制。这里的陆-气耦合是指地表条件影响降水的几种机制，其中之一是土壤水分通过蒸发和蒸腾作用成为水汽的来源，从而促进降水。这种“降水循环”可以延长和加强干旱期，因为缺乏雨水会使土壤干燥，减少蒸发蒸腾，导致降雨量进一步减少。另一方面，由于蒸发冷却较少，陆地表面干燥可能意味着白天陆地表面的加热更大，而陆地表面较热可能导致大气边界层的不稳定，从而增加对流降水的机会。陆地-大气耦合可能因土地覆盖和其他因素而变化很大，因此可能导致降雨对ENSO事件的响应具有比大尺度大气环流异常所期望的更大的空间变异性。在ENSO事件发生的一个季节或更长时间内，它还可能导致日和次日降雨事件的频率、强度和持续时间的变化。该项目的目标是确定陆地-大气耦合在多大程度上解释了南美洲热带地区对ENSO事件的降雨响应的空间异质性。这项研究的主要内容是对热带南美洲的降水、大气和陆地表面数据进行统计分析，这些数据来自卫星和地面观测以及再分析产品。本文采用平行分析方法对CMIP5全球陆地-大气耦合试验（GLACE-CMIP5）模式子集的模拟结果进行了分析，其中CMIP5全球陆地-大气耦合试验采用气候土壤湿度对模式进行整合，从而通过交互土壤湿度和固定土壤湿度模拟结果的比较来评估陆地-大气耦合。在统计评估的同时，还利用QTCM2模式进行了模式试验。QTCM2是一种简化模式，可以模拟南美洲热带地区降水响应的关键方面，其中土壤湿度、地表感热通量以及边界层与上覆自由对流层之间的热水汽交换等关键因素可以得到控制和检验。考虑到与enso相关的降水中断对该地区的重大影响，该项目下的工作除了具有科学价值外，还具有重要的更广泛的影响。这项研究的结果也有望揭示在类似地表条件普遍存在的热带其他地区，陆地-大气耦合的作用。这项工作还促进了国际合作，因为它涉及哥伦比亚两所大学的未获资助的合作者。除了研究的广泛影响外，该项目还通过科学与工程研究（RiSE）项目支持本科生研究助理，这是一个为期10周的暑期项目，重点关注传统上代表性不足的群体的学生。此外，该项目还为研究生提供支持和培训，从而为该研究领域的下一代科学工作者提供支持和培训。

El Nino/Southern Oscillation (ENSO) events are known to have significant impacts on weather and climate worldwide, including reductions in rainfall over much of tropical South America with associated disruptions to water resources, agriculture, and other human and natural systems. The rainfall anomalies are ultimately due to changes in the large-scale atmospheric circulation induced by ENSO conditions in the neighboring equatorial Pacific, but they may also be modulated by land-atmosphere coupling occurring over South America. Here land-atmosphere coupling refers to several mechanisms through which the condition of the land surface influences precipitation, one of which is that soil moisture serves as a source of water vapor through evaporation and transpiration, thereby promoting precipitation. This sort of "precipitation recycling" can prolong and enhance dry spells, as lack of rain dries the soil and reduces evapotranspiration, leading to further reductions in rainfall. On the other hand, a drier land surface can mean greater heating of the land surface during the day as there is less evaporative cooling, and a hotter land surface can lead to instability in the atmospheric boundary layer, which increases the chances of convective precipitation. Land-atmosphere coupling can be quite variable depending on land cover and other factors, and can thus cause the rainfall response to ENSO events to be more spatially variable that would be expected from the large-scale atmospheric circulation anomalies. It can also cause changes in the frequency, intensity, and duration of daily and sub-daily rainfall episodes within the period of a season or more during which an ENSO event takes place.The goal of this project is to determine the extent to which land-atmosphere coupling accounts for the spatial heterogeneity in the rainfall response to ENSO events over tropical South America. The research consists in large part of statistical analysis of precipitation and atmospheric and land surface data for tropical South America, taken from satellite and surface observations and reanalysis products. Parallel analysis is applied to model simulations from the Coupled Model Intercomparison Project version 5 (CMIP5), including simulations from the subset of models which contributed to the CMIP5 Global Land-Atmosphere Coupling Experiment (GLACE-CMIP5), in which models were integrated using climatological soil moisture so that land-atmosphere coupling could be assessed by comparison between simulations with interactive and fixed soil moisture. The statistical assessment is accompanied by model experiments using the quasi-equilibrium tropical circulation model version 2 (QTCM2), a simplified model which can simulate key aspects of the precipitation response over tropical South America, and in which key factors such as soil moisture, surface sensible heat flux, and the exchange of heat and water vapor between the boundary layer and the overlying free troposphere can be controlled and examined.Work under this project has important broader impacts in addition to its scientific merit, given the substantial consequences of ENSO-related precipitation disruptions in the region. The results of this study are also expected to shed light on the role of land-atmosphere coupling in other regions of the tropics where similar surface conditions prevail. The work also promotes international collaboration, as it involves unfunded collaborators in two Columbian universities. Aside from the broader impacts of the research, the project also supports undergraduate research assistants through the Research in Science and Engineering (RiSE) program, a 10-week summer program which focuses on students from traditionally underrepresented populations. In addition, the project provides support and training to a graduate student, thereby providing for the next generation of the scientific workforce in this research area.