Collaborative Research: The Role of Ocean Dynamical Feedback and Air-Sea Interaction in the Climate Response to Global Warming

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This project considers changes in regional climate occurring primarily in and over the tropical oceans, due to greenhouse gas increases. Among the robust regional climate changes found in ensembles of climate model simulations, three are addressed here: a pattern of "El Nino-like" sea surface temperature (SST) changes in the tropical Pacific; a minimum in the warming of SSTs in the tropical North Atlantic area which is the main development region (MDR) for Atlantic tropical cyclones, accompanied by an increase in vertical wind shear in the region (which would presumably inhibit tropical cyclone formation); and the meridional shift in the intertropical convergence zone (ICTZ) which has been tentatively ascribed to the slowdown of the Atlantic meridional overturning circulation (AMOC) caused by global warming. These regional changes will be examined by decomposing local SST change into components forced separately by 1) the increase in local surface radiation; 2) redistribution of changes in near-surface heat ocean content by the mean ocean circulation; 3) redistribution of the mean near-surface heat content by changes in the wind-driven surface currents; and 4) changes in evaporative cooling due to changes in surface wind speed. The four terms, which come from a linearization of the perturbation surface energy balance equation, have been previously associated with specific mechanisms of regional SST change. In particular term 4 expresses the dynamics of the wind-evaporation-SST (WES) feedback, in which the quadratic dependence of evaporation on surface wind speed cause evaporative cooling to vary regionally according to the strength of the mean winds and their direction relative to changes in wind. Likewise term 2 encapsulates the ocean dynamical thermostat (ODT) mechanism, in which a uniform increase in surface energy input causes less local SST increase in a region of divergent surface currents as the additional heat is transported out of the region. This mechanism would be expected to yield a reduction in SST warming in the Eastern Pacific cold tongue region relative to the western Pacific, thereby acting to oppose the El Nino-like warming pattern found in climate change simulations. The methodology for determining the relative contributions of the terms involves numerical integrations of a climate model in configurations in which the terms are selectively enabled and disabled.Beyond its intrinsic scientific value, the work has broader impacts due to the consequences of changes in tropical ocean conditions have for fisheries, marine ecology, and the formation of tropical cyclones. The work will also help to determine the extent to which climate model simulations can be confidently used to anticipate climate change at regional scales.