Collaborative Research: Feedbacks between Marine Stratiform Cloud, Atmospheric Circulation and Temperature on Decadal Timescales and in Anthropogenic Change

海洋层云（MSC）是在大陆西海岸的亚热带地区形成的低云，由此产生的“层云甲板”通过将阳光反射回太空来冷却地球。在气候变暖的情况下，层云甲板的命运可能是决定地球气候敏感性的一个关键因素，或者说，在大气温室气体增加的情况下，地球会变暖多少。如果层云甲板扩大，或变得更加持续多云，反射阳光的增加将产生冷却效果，并抵消一些温室效应（“负反馈”）。但如果它们收缩或变得不那么多云，全球变暖可能会大大增强（一种“积极的反馈”）。目前，MSC反馈的符号是未知的，并且确定反馈的符号的物理机制也没有很好地理解。该研究项目将通过检查MSC，大气环流和温度之间的关系，在观测和模式模拟中发生的MSC反馈。主要研究人员的初步工作表明，在云数据集中可以检测到年代际波动。此外，这些信号揭示了云对与气候变化相关的时间尺度上的环境条件变化的敏感性，可以作为云变化模型模拟的重要测试。该研究将在前期工作的基础上进行：1）使用多个独立的云和气象数据集，检查观测到的地球仪周围亚热带地区的长期变化; 2）通过使用观测到的云-气象关系，评估MSC在全球气候模式中的模拟，并评估模式云反馈和预测云和气候变化的可靠性; 3）使用NCAR全球气候模式（GCM）的当前版本和即将推出的版本进行量化MSC反馈的实验。本文的工作旨在检验三个相互关联的假设：1）副热带海洋环流对海温和大气环流有一个局部和区域性的正反馈作用，2）这种反馈作用对局部和区域海气系统的年代际变率有显著贡献; 3）长期人为增暖下亚热带MSC的变化主要与动力过程有关（大气环流的变化）而不是物理过程（直减率的变化）。由于需要更好地估计世界因温室气体增加而变暖的程度，因此在赠款下进行的工作将引起广泛的兴趣。此外，该项目将通过促进迈阿密大学和加州大学圣地亚哥分校之间的研究合作，加强研究和教育基础设施。这项工作还将支持两名研究生。将在这两所大学开展外联活动，包括鼓励中学女生从事科学职业的活动（在迈阿密），以及在高中生暑期学校开展的活动（在圣地亚哥）。

Marine stratus clouds (MSC) are low clouds that form in the subtropics off the west coast of continents, and the resulting "stratus decks" cool the earth by reflecting sunlight back to space. The fate of the stratus decks in a warming climate could be a key factor in determining the earth's climate sensitivity, or how much the planet will warm for a given increase in atmospheric greenhouse gases. If the stratus decks expand, or become more persistently cloudy, the resulting increase in reflected sunlight would have a cooling effect and counteract some of the greenhouse warming (a "negative feedback"). But if they contract or become less cloudy global warming could become considerably enhanced (a "positive feedback"). At present the sign of the MSC feeback is not known and the physical mechanisms which determine the sign of the feedback are not well understood. This research project will address the MSC feedback through an examination of relationships between MSC, atmospheric circulation, and temperature, occuring both in observations and in model simulations. Preliminary work by the principal investigators has shown that decadal fluctuations are detectable in cloud datasets. Furthermore, these signals reveal the sensitivity of the clouds to shifting environmental conditions on timescales relevant for climate change, and can serve as a important test of model simulations of cloud variability. Research under this grant will build on the preliminary work by 1) examining observed long-term variability in subtropical regions around the globe using multiple, independent cloud and meteorological datasets; 2) evaluating the simulation of MSC in global climate models through the use of observed cloud-meteorology relationships, and assessing the reliability of model cloud feedbacks and projected cloud and climate changes; and 3) using the current and upcoming versions of the NCAR global climate model (GCM) to carry out experiments quantifying MSC feedbacks. The work is designed to test three linked hypotheses: 1) Subtropical MSC exerts a local and regional positive feedback on SST and atmospheric circulation; 2) This feedback substantially contributes to decadal variability in the local and regional atmosphere-ocean system; and 3) Changes in subtropical MSC under long-term anthropogenic warming will be primarily associated with dynamical processes (changes in atmospheric circulation) rather than thermodynamical processes (changes in lapse rate). The work performed under the grant will be of broad interest because of the need for better estimates of the degree to which the world will warm in response to greenhouse gas increases. In addition, the project will enhance infrastructure for research and education by fostering a research collaboration between the University of Miami and the University of California, San Diego. The work will also support two graduate students. Outreach activities will be conducted at both universities, including activities to encourage middle-school girls to pursue careers in science (in Miami), and activities at a summer school for high school students (in San Diego).