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

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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).