Observational Evaluation of a Convective Quasi-Equilibrium View of Monsoons

Abstract

Idealized dynamical theories that employ a convective quasi-equilibrium (QE) treatment for the diabatic effects of moist convection have been used to explain the location, intensity, and intraseasonal evolution of monsoons. This paper examines whether observations of the earth's regional monsoons are consistent with the assumption of QE. It is shown here that in local summer climatologies based on reanalysis data, maxima of free-tropospheric temperature are, indeed, nearly collocated with maxima of subcloud equivalent potential temperature, u eb , in all monsoon regions except the North and South American monsoons. Free-tropospheric temperatures over North Africa also exhibit a strong remote influence from the South Asian monsoon. Consistent with idealized dynamical theories, peak precipitation falls slightly equatorward of the maxima in u eb and free-tropospheric temperature in regions where QE seems to hold.Vertical structures of temperature and wind reveal two types of monsoon circulations. One is the deep, moist baroclinic circulation clearly seen in the South Asian monsoon. The other is of mixed type, with the deep moist circulation superimposed on a shallow dry circulation closely associated with boundary layer temperature gradients. While the existence of a shallow dry circulation has been documented extensively in the North African monsoon, here it is shown to also exist in Australia and southern Africa during the local summer. Analogous to moist QE theories for the deep circulation, the shallow circulation can be viewed in a dry QE framework in which shallow ascent occurs just equatorward of the peak boundary layer potential temperature, u b , providing a unified system where the poleward extents of deep and shallow circulations are bounded by maxima in u eb and u b , respectively.

Hadley Circulation Response to Orbital Precession. Part II: Subtropical Continent

“…8). The situation is similar to that over several of Earth's continents, which have (dry) potential temperature maxima and shallow circulations (Nie et al 2010). The factors governing the interactions between the thermodynamic state of the atmosphere and shallow and deep circulations have not been fully laid bare, but the arguments of Privé and Plumb assume that quasi-equilibrium holds and, implicitly, the vertical structure of the circulation is deep.…”

Effects of Rotation Rate and Seasonal Forcing on the ITCZ Extent in Planetary Atmospheres

“…When used in conjunction with the ''weak temperature gradient'' (WTG) approximation (Sobel et al 2001), some such theories successfully reconstruct tropical convergence and tend to predict maximum convergence and rainfall over the warmest surface temperatures (Neelin and Held 1987;Raymond 2000;Sobel and Bretherton 2000). When applied to convergence zones associated with zonally averaged overturning circulations that conserve angular momentum (Lindzen and Hou 1988) and coupled to quasiequilibrium theories of moist convection (Arakawa and Schubert 1974;Emanuel et al 1994;Nie et al 2010), such theories argue that the ITCZ, defined here and throughout this paper as the latitude of maximum zonal-mean precipitation, lies just equatorward of the maximum low-level moist static energy (MSE) (Emanuel 1995;Privé and Plumb 2007;Bordoni and Schneider 2008).…”

Signatures of Tibetan Plateau heating on Indian summer monsoon rainfall variability

“…Second, the maximum temperature aloft should overlie the region of maximum subcloud moist entropy or, virtually equivalently, the maximum subcloud moist static energy [e.g., Clift and Plumb, 2008, Chapter 1;Emanuel, 1991;Emanuel et al, 1994;Neelin, 2007;Plumb, 2007]. Both modern data [Bordoni and Schneider, 2008;Hurley and Boos, 2013;Nie et al, 2010] and numerical calculations [Bordoni and Schneider, 2008;Plumb, 2007a, 2007b] corroborate these relationships among subcloud moist entropy or moist static energy, maximum temperature aloft, and the poleward edge of the cross-equatorial circulation. Thus, the observed maximum in moist static energy over northern India at the monsoon onset [Bordoni and Schneider, 2008] and its relation to the large-scale circulation would seem to make the heating over Tibet unimportant, and perhaps not necessary.…”