Tropical and extratropical general circulation with a meridional reversed temperature gradient as expected in a high obliquity planet

Abstract

Planets with high obliquity receive more radiation in the polar regions than at low latitudes, and thus, assuming an ocean-covered surface with sufficiently high heat capacity, their meridional temperature gradient was shown to be reversed for the entire year. The objective of this work is to investigate the drastically different general circulations of such planets, with an emphasis on the tropical Hadley circulation and the mid-latitude baroclinic eddy structure.We use a 3D dry dynamic core model, accompanied by an eddy-free configuration and a generalized 2D Eady model. When the meridional temperature gradient is reversed, the Hadley cell becomes much weaker, shallower and thermally indirect, as seen in other studies, though not in the context of high obliquity planets. Regardless of whether the temperature is decreasing (normal) or increasing (reverse) poleward, surface friction and eddy momentum transport are shown to be the primary drivers of the Hadley cell. Because mid-latitude baroclinic eddies concentrate westerly momentum toward the mid-latitude baroclinic zone, the surface wind pattern is easterly-westerly-easterly from the equator to the pole in both normal and reverse cases. As a result, low-latitude air parcels near the surface gain westerly momentum through friction, and are redirected equatorward by the Coriolis force, forming thermally indirect (direct) circulation in the reverse (normal) case. The Hadley cell under a reverse temperature gradient configuration is shallow and weak, even when the magnitude of the gradient is the same as in the normal case. This shallow structure is a result of the bottom-heavy structure of the baroclinic eddies in the reverse case, and relatively weak wave activity. We propose a new mechanism to explain the mid-latitude eddy structure for both cases, and verify it using the generalized Eady model. With seasonal variations included, the annual mean circulation resembles that under perpetual annual mean setup. Approaching the solstices, a strong cross-equator Hadley cell forms in both cases, and about 2/3 of the Hadley circulation is driven by eddies, as shown by eddy-free simulations and using a decomposition of the Hadley cell.