Tropical large-scale circulations: asymptotically non-divergent?

Abstract

Although the large-scale tropical atmospheric circulations are often considered as primarily divergent, a simple scale analysis, originally presented by Charney (1963), suggests otherwise—a dominance of vorticity over divergence. The present paper quantitatively documents the asymptotic non-divergence of the large-scale tropical atmosphere, in association with Madden–Julian oscillations, with use of the Tropical Ocean Global Atmosphere—Coupled Ocean Atmosphere Response Experiment, Large-Scale Array (TOGA–COARE LSA) data set. The vorticity is larger than the divergence at the majority (70%–80%) of points at any instant for the levels between 850 and 250 hPa, and the vorticity is more than 10 times stronger than the divergence both at 850 and 500 hPa more than half of the time. The root mean square (rms) ratio between the transient components of divergence and vorticity, which is defined as the deviation from the mean for the whole data period, decreases substantially with increasing horizontal scales from 100 to 2000 km, over an intraseasonal timescale (20–100 d). The analysis suggests that the Madden–Julian oscillations are dominated more by vorticity than divergence and more so than at the smaller scales. The analysis as a whole suggests the feasibility of constructing an asymptotically non-divergent theory for large-scale tropical circulations. A brief sketch of the formulation is presented.