The Generation of Global Rotational Flow by Steady Idealized Tropical Divergence

Abstract

Tropical convective heating is balanced on the large scale by the adiabatic cooling of ascent. The horizontal divergence of the wind above this heating may be viewed as driving the upper tropospheric rotational wind field. A vorticity equation model is used to diagnose this relationship. It is shown that because of the advection of vorticity by the divergent component of the flow, the Rossby wave source can be very different from the simple −fD source often used. In particular, an equatorial region of divergence situated in easterly winds can lead to a Rossby wave source in the subtropical westerlies where it is extremely effective. This part of the source can be relatively insensitive to the longitudinal position of the equatorial divergence. A divergence field which is asymmetric about the equator can lead to a quite symmetric Rossby wave source. For a steady frictionless flow the Rossby wave source averaged over regions within closed streamfunction or absolute vorticity contours is, under certain simplifying assumptions, nearly zero. It is very important to recognize the correct nature of the Rossby wave source associated with a region of tropical heating: The predicted atmospheric response can be dramatically in error if this source is misrepresented. Several features of related observational and GCM studies that have proved difficult to explain with simpler models can be understood in this light. Our analysis emphasizes the crucial role played by the tropical heating in the general circulation of the troposphere, and points to the importance of modeling the horizontal and vertical structure of this heating accurately.