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

海洋层状云(MSC)是在大陆西海岸附近的亚热带形成的低层云，由此产生的“层状云”通过将阳光反射回太空来冷却地球。在气候变暖的情况下，层状甲板的命运可能是决定地球气候敏感度的关键因素，也就是大气温室气体每增加一次，地球将变暖多少。如果层状层扩大，或变得更加多云，由此产生的反射太阳光的增加将产生降温效应，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加剧(这是一个“积极的反馈”)。目前MSC反馈的信号尚不清楚，决定MSC反馈信号的物理机制也不是很清楚，本研究项目将通过研究MSC、大气环流和温度之间的关系来解决MSC反馈问题，这些关系发生在观测和模式模拟中。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，这些信号揭示了云在与气候变化相关的时间尺度上对不断变化的环境条件的敏感性，并可作为云可变性模型模拟的重要测试。这笔赠款下的研究将建立在以下前期工作的基础上：1)使用多个独立的云和气象数据集检查全球亚热带地区观测到的长期可变性；2)通过使用观测到的云与气象的关系，评估全球气候模式中MSC的模拟，并评估模式云反馈和预测的云和气候变化的可靠性；以及3)使用NCAR全球气候模式(GCM)的当前和即将推出的版本来进行实验，对MSC反馈进行量化。这项工作旨在检验三个相互关联的假设：1)副热带MSC对SST和大气环流施加局地和区域正反馈；2)这种反馈对当地和区域大气-海洋系统的年代际变化有很大贡献；3)在长期人为变暖下，副热带MSC的变化将主要与动力过程(大气环流的变化)有关，而不是与热力学过程(衰减率的变化)有关。在赠款下开展的工作将引起广泛的兴趣，因为需要更好地估计全球变暖的程度，以应对温室气体的增加。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，加强研究和教育的基础设施。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)，以及高中生暑期学校的活动(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).