Seamless Poleward Atmospheric Energy Transports and Implications for the Hadley Circulation

Abstract

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 31Њ latitude, the primary transport mechanisms are the Hadley and Walker overturning circulations. In the extratropics the energy transports are carried out by baroclinic eddies broadly organized into storm tracks and quasi-stationary waves that 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 quasi-stationary flow. In the upward branch of the Hadley cell, the predominant diabatic process is latent heating 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 subsidence warming in the downward branch is compensated 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. Effectively, the outgoing longwave 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 a fundamental driver of the observed Hadley circulation and realizes the seamless transport from Tropics to extratropics, while tropical sea surface temperatures over the oceans 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, which 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 midlatitude storm tracks. The evaporation is sufficiently strong that it is also compensated by a subsurface ocean heat transport that in turn is driven by the Hadley circulation surface winds.

Decomposing the meridional heat transport in the climate system

“…9b). It is well-established that the mean circulation dominates in the tropics and eddies dominates in the mid-high latitudes (Trenberth and Stepaniak 2003b). The eddy LE transport AHT ELE is always poleward.…”

Meridional atmospheric heat transport constrained by energetics and mediated by large-scale diffusion

“…1a). Remarkably, when AHT associated with each circulation component is summed together they blend 'seamlessly' to produce a net AHT without pronounced and abrupt meridional variations (Trenberth and Stepaniak 2003). Net AHT has a peak magnitude of about 4 PW at around 40 • latitude in both hemispheres and is poleward everywhere except in the deep tropics where energy is transported southward across the equator (Fig.…”

“…Large-scale atmospheric motions predominantly act to transport energy poleward -from the warm and moist tropics, where insolation is strong, to cold and dry polar regions, where insolation is weak (e.g., Trenberth and Caron 2001;Trenberth and Stepaniak 2003;Fasullo and Trenberth 2008;Donohoe and Battisti 2012). As a consequence of meridional atmospheric heat transport (AHT), Earth's climate is more temperate than it would otherwise be, exhibiting a weaker pole-to-equator temperature gradient (e.g., Hartmann 2016).…”

Influence of different convection parameterisations in a GCM

“…Even though substantial evaporation takes place in these regions the water is transported towards the equator at low altitude in the Hadley circulation (e.g. Trenberth and Stepaniak, 2003), while subsidence suppresses deep convection and substantial upward transport of moisture.…”