Collaborative Research: The Effect of Variations in Cloud Versus CO2 Radiative Forcing on Tropical SST Gradients, Atmospheric Circulation and Rainfall Patterns

Yale University, New Haven CT

Investigators

Abstract

Simulations of the response of tropical climate to greenhouse gas increases show a prominent role for clouds in determining how much the tropics warm, how the warming is distributed geographically, and how it affects other aspects of the climate. Many aspects of tropical climate, including the distribution of rainfall and surface winds, are closely related to the east-west sea surface temperature (SST) gradient along the equatorial Pacific, between the warm pool of the western Pacific and the cold tongue extending from the eastern boundary. Previous work by the PIs shows a strong relationship between the SST gradient in the equatorial Pacific and the contrast in brightness, or albedo, between tropical and extratropical clouds. The PIs have developed a theory to explain this connection, in which extratropical clouds with higher albedo imply less sunlight absorbed by the ocean at higher latitudes, which in turn requires greater heat transport from the tropics, which is satisfied by greater oceanic heat uptake in the cold tongue and subsequent oceanic transport, which requires colder temperatures in the cold tongue and thus a greater temperature contrast between the cold tongue and the warm pool. Motivated by this theoretical framework, the PIs test the hypothesis that latitudinal variations in the response of cloud albedo to greenhouse warming play a key role in driving changes in tropical climate associated with the east-west SST contrast across the Pacific. The PIs perform greenhouse warming simulations with the Community Earth System Model (CESM) in n fully coupled, slab ocean, and prescribed SST configurations, in which cloud feedbacks and their effects on the equatorial Pacific SST contrast are assessed. Complimentary analysis of greenhouse warming simulations from the archive of the Coupled Model Intercomparison Project version 5 (CMIP5) is also performed. As ocean heat transport plays an important role, a water parcel tracking scheme is applied to identify the location and time of last ventilation for water in the equatorial mixed layer, which can then be related to cloud radiative effects felt by the parcel during ventilation. The research has broader societal impacts as it addresses fundamental issues in tropical climate which are essential for anticipating future climate changes in a populous region. The work has broader scientific impacts because the results will likely be helpful for understanding warm climates of the past, in particular the Pliocene, a period 3-5 million years ago when carbon dioxide levels were as high as today and the equatorial Pacific SST gradient is believed to have been weaker. The work has additional educational broader impacts through the participation of one of the PIs in National Center for Science Education "Scientists in the Classroom" pilot project, in which she will work with a middle school teachers and students. In addition, the project provides support and training to a graduate student and a postdoc, thereby providing for the future workforce in this research area.