Source: Dissertation Abstracts International, Volume: 79-12(E), Section: B.

Adviser: William R. Boos.

The objective of this work is to examine how monsoons change when deviating from the classic first-baroclinic overturning forced by an axisymmetric heating. To begin with, we explore the influences of zonal asymmetry in the forcing on monsoon circulations in both an idealized dry framework and a more realistic moist framework. We then study the influences by a monsoonal vertical structure deviating from the classical first-baroclinic profile.

To address the impacts of zonal asymmetry; our study begins with the sensitivity of monsoon circulation to changes in the amplitude of a zonally confined off-equatorial heating (Chapter 2). Observed nonlinearities in the seasonal evolution of monsoons have been previously explained using theories for Hadley circulations driven by zonally symmetric thermal forcings, even though monsoonal forcings deviate strongly from the assumption of zonal symmetry. Here, we use an idealized 3D model of a dry atmosphere to compare the circulation response to zonally symmetric and asymmetric off-equatorial thermal forcings. For symmetric forcings, the zonal-mean, cross-equatorial mass flux increases more rapidly with the amplitude of the forcing once the forcing becomes strong enough to reduce the upper-tropospheric absolute vorticity to near zero, consistent with previous studies of the transition to angular momentum conserving flow. For zonally asymmetric forcings, the zonal-mean cross-equatorial flow exhibits a similar dependence on forcing strength and a similar reduction of the zonal-mean upper-level vorticity. but asymmetric forcings also produce strong zonal overturnings with subsidence west of the heating. as in the well-known linear response to off-equatorial heatings. The mass flux in these zonal overturnings increases linearly with forcing strength until its rate of increase tapers off for the strongest forcings; the total upward mass flux (i.e., the zonal-mean plus zonally asymmetric components) increases linearly with the strength of zonally asymmetric forcings and exhibits no abrupt dependence on forcing amplitude. These results indicate the importance of considering the zonally asymmetric part of the divergent response to off-equatorial forcings and suggest that theories based on zonally symmetric forcings need further examination before they can be assumed to describe observed monsoons.

In order to promote the application of the conclusions presented in Chapter 2 to the real-world monsoons, we continue our examination in a more realistic setup (Chapter 3). In particular, we simulate the Hadley circulation in a moist atmosphere with seasonal solar insolation by prescribing a subtropical zonally asymmetric land with constant and dynamically dependent surface heat flux coefficient, respectively. We first find that in a moist, seasonally varying atmosphere, the zonal asymmetry in land distribution weakens the monsoonal circulation and precipitation strength. In addition, the circulations with a constant surface heat flux tend to have greater seasonal variation in circulation strength. Furthermore, unlike the circulations with constant forcing, circulations driven by seasonal heating have less localized zonally asymmetric features due to lack of time for the realization of steady state, resulting in a significantly smaller contribution by stationary eddy momentum flux to the zonal mean meridional circulation. From the perspective of regime transitions, upper-level Hadley circulation adopts angular momentum conservation regime dominantly, especially for the circulations with constant surface heat fluxes.

The second aspect of our study is motivated by recent findings about the deviation in monsoonal vertical structure from the first-baroclinic deep convective mode. Shallow meridional overturning circulations are superimposed on the deep circulations that produce precipitation in nearly all monsoon regions, and these shallow flows transport subtropical, mid-tropospheric dry air into the tropical monsoon precipitation maxima. Horizontal moisture advection produced by shallow meridional circulations is characterized in the monsoon regions of West Africa, South Asia; Australia, and southern Africa during local summer. Horizontal flow in the upper and lower branches of the shallow meridional circulations consistently dries and moistens air, respectively, in the continental precipitation maxima of each region. Advection of time-mean moisture by time-mean wind dominates horizontal moisture advection in South Asia and West Africa, while most horizontal moisture advection in Australia and southern Africa is produced by transient eddies. Much of the transient eddy advection can be accurately represented as a first-order horizontal diffusion with a constant, globally uniform diffusivity.

These results suggest that horizontal moisture advection in theoretical and conceptual models of seasonal mean monsoons can be adequately represented in terms of time-mean winds plus a horizontal moisture diffusion. Finally, interannual variations in the summer mean regional averages of monsoon precipitation and horizontal advective drying in the lower free troposphere are shown to be negatively correlated in most regions, consistent with the hypothesis that advective drying by shallow meridional circulations inhibits monsoon precipitation.

Keywords: monsoons. Hadley circulations, shallow meridional circulations, tropical dynamics, eddies.