8.2 Land-ocean surface warming contrast: Theory, idealized simulations, and CMIP5 simulations

Wednesday, 19 June 2013: 10:45 AM
Viking Salons ABC (The Hotel Viking)
Michael P. Byrne, ETH Zürich, Zürich, Switzerland; and P. A. O'Gorman

In simulations and observations of global warming, surface air temperatures over land are found to increase at a greater rate than those over ocean. Previous work has proposed that this land-ocean warming contrast is related to different changes in lapse rates over land and ocean due to limited moisture availability over land. A simple theory for the tropical land-ocean contrast is presented here in which lapse rates are determined by an assumption of convective quasi-equilibrium. The theory predicts that the difference between land and ocean temperatures increases as the climate warms or as the land becomes more arid. However, the ratio of differential warming over land and ocean varies non-monotonically with temperature for constant relative humidities and reaches a maximum at roughly 290K.

This theory is applied to idealized GCM simulations with a variety of continental configurations. The simulated warming contrast is confined to latitudes below 50 degrees when climate is varied by changes in longwave optical thickness, though a small temperature contrast may be induced at higher latitudes by imposing an arid land surface. The magnitude of the warming contrast is found to depend on temperature, the land aridity and the continental configuration and is generally well captured by the theory.

Analogous behavior is found in global warming simulations from the CMIP5 archive. The theory captures the amplitude of the multimodel mean land-ocean warming contrast and its latitudinal variation from the equator to approximately 40 degrees latitude. The intermodel scatter in the tropics is also well captured, with disparate changes in relative humidity over land being the primary cause of differences in the warming contrast amongst models. Possible extensions of the theory to account for extratropical land-ocean contrasts are discussed.

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