14th Symposium on Global Change and Climate Variations


Seamless poleward atmospheric energy transports

Kevin E. Trenberth, NCAR, Boulder, CO; and D. P. Stepaniak

Observations show that not only is the total poleward heat transport continuous with latitude, so too is the atmospheric transport. Yet the mechanisms for carrying out the transport vary greatly. The large-scale overturning Hadley circulation is dominant in low latitudes while the baroclinic transient eddies, assisted by the quasi-stationary planetary waves in the Northern Hemisphere winter, are dominant in mid-latitudes. So how is it that the poleward heat transports are so seamless?

Most theories have an abrupt cut off at about 30 deg latitude at the poleward edge of the Hadley circulation for the heat transport. The Hadley cell overturning is driven by heating in the deep tropics and cooling in the subtropics. It is widely assumed that the primary heat balance in the subsiding branch of the Hadley circulation is between the adiabatic warming from subsidence and the diabatic effects of infrared radiative cooling to space. Instead the continuity of the heat transports across latitude implies that other dynamical mechanisms are also playing key roles. We show, in a new analysis of the atmospheric heat budget, that the cooling in the subtropics also arises from heat transport to higher latitudes by quasi-horizontal air flow in the transient baroclinic eddies and quasi-stationary waves. 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 we argue that some of the radiative cooling in the subtropics is a consequence of the transient baroclinic eddies.

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Session 12, General Circulation and Teleconnections
Thursday, 13 February 2003, 11:00 AM-12:00 PM

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