Collaborative Research: Dynamics of Subtropical Humidity

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The water vapor feedback is the most important mechanism for amplifying the climatic response to increased anthropogenic greenhouse gases. The dry regions of the subtropical free troposphere play an especially important role in determining the strength of this feedback. The humidity in the dry subtropics depends on the temperature at which saturation was last encountered by air parcels at remote locations along the ensemble of trajectories that terminate in the subtropics at a given time. Changes in the water vapor feedback depend on how these statistics of last saturation may change as the climate warms. A tracer-based technique is developed for diagnosing the statistics of last saturation in Global Climate Models (GCMs) and reanalysis data. The research objective of this work is to use these diagnostic techniques to better understand the processes that control the humidity of the subtropics in modern and future climates. Intellectual merit: Completion of this work will result in (1) development of the statistics of last saturation within the modern climate of the subtropics, as recorded in reanalysis data and in GCM simulations, with a focus on seasonal and interannual variability and on the roles of extratropical eddies and the Hadley and Walker circulations in controlling subtropical humidity; (2) further understanding of how the last saturation statistics may change under global warming scenarios; (3) examination of the mechanisms controlling subtropical humidity at a more basic, process-oriented level using idealized GCMs. Broader impacts: The research will provide training to two graduate students in diagnostic analysis of tropical moisture variability and impacts on present and future global climates. The results will aid the broader climate change assessment efforts and will be of potential benefit for societal and policy considerations.