Co-variability of components of poleward atmospheric energy transports on seasonal and interannual timescales.

Kevin E. Trenberth abd David P. Stepaniak

National Center for Atmospheric Research P. O. Box 3000 Boulder, CO 80307

email: trenbert@cgd.ucar.edu voice: (303) 497 1318 fax: (303) 497 1333

A detailed vertically-integrated atmospheric heat and energy budget is presented along with estimated heat budgets at the surface and top-of-atmosphere for the subtropics. It is shown that the total energy transports are remarkably seamless in spite of greatly varying mechanisms. From the Tropics to about 31 degrees latitude, the primary transport mechanism is the Hadley and Walker overturning circulations. These are driven by latent heating in the upward branch that results from convergence of moisture by the circulation itself. Hence large poleward transports of dry static energy are compensated by equatorward transports of latent energy resulting in a modest poleward transport of moist static energy. The downward branch is driven by cooling in the subtropics that mainly arises from energy transport to higher latitudes by transient baroclinic eddies that are stronger in the winter hemisphere. The storm tracks of baroclinic eddies and the quasistationary waves in the extratropics covary in a symbiotic way as the location and activity in storm tracks are determined by, and in turn help maintain, through eddy transports, the quasistationary flow. Effectively, the radiation to space is distributed over middle and high latitudes and is not limited to the clear dry regions in the subtropics. Further, some of the radiative cooling in the subtropics is a consequence of the circulation. Hence the cooling by transient eddies in the subtropics is the fundamental driver of the Hadley circulation and realizes the seamless transport from Tropics to extratropics, while tropical sea surface temperatures determine where the upward branch is located. The relatively clear skies in the subtropics further provide for ample absorption of solar radiation at the surface where it feeds strong evaporation, that exceeds precipitation, and supplies the equatorward flow of latent energy into the upward branch of the Hadley circulation as well as the poleward transports into mid-latitude storm tracks. The evaporation is sufficiently strong that it also is compensated by a subsurface ocean heat tranport that in turn is driven by the Hadley circulation surface winds.