Planetary Circulations: 3. The Terrestrial Quasi-Geostrophic Regime

Abstract

The characteristics of the two-level quasi-geostrophic model are evaluated for a wide range of parameters in the terrestrial domain. Flow form is determined primarily by ? (the Coriolis gradient) and by τD, (the time scale of the surface drag), acting through the influence of Rhines' transitional wave-number k? = (?/2U)1/2 where U2 is the barotropic energy level. Two extreme types of circulation occur: jets when k? is large, and gyres when wave propagation and drag are negligible. The present terrestrial circulation, in its quasi-geostrophic representation, is extremely efficient: the system can cope with increased heating rates without a significant rise in the pole-to-equator temperature differential. Although each hemisphere is, on occasion, near to transforming into a double-jet state, multi-jet circulations?corresponding to those in the Jovian regime?occur more readily at higher rotation rates. For the existing circulation to switch to a gyre form requires a large, unrealizable drop in surface drag.