The drying tendency of shallow meridional circulations in monsoons

Abstract

Shallow meridional overturning circulations are superimposed on the deep circulations that produce precipitation in nearly all monsoon regions, and these shallow circulations transport subtropical, mid‐tropospheric dry air into the tropical monsoon precipitation maxima. Here 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. The peak drying by horizontal advection occurs at a lower altitude than peak winds in the upper branch of the shallow circulations, consistent with the small scale height of water vapour. 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 simple 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.

A Hierarchy of Idealized Monsoons in an Intermediate GCM

“…First, by assuming a gray atmosphere, the model neglects the interaction among water vapor, cloud, and radiation. Second, the relative coarse horizontal and vertical resolutions used here are not likely to well represent processes such as the shallow meridional circulation (Peyrille and Lafore 2007;Zhang et al 2008;Thorncroft et al 2011;Zhai and Boos 2017). Both mechanisms have certain impacts on the monsoon rainfall.…”

“…In response to an energy source that produces a tendency of vertically integrated moist static energy, the circulation would change accordingly to transport the anomalous energy out. For example, the zonal-mean Hadley circulation tends to shift toward the hemisphere that receives more energy, and the monsoon extent has been shown to be limited by the zonal and meridional ventilation of the lower moist static energy air from cooler oceans Privé and Plumb 2007b) and from the subtropics (Boos and Kuang 2010;Thorncroft et al 2011;Zhai and Boos 2017), respectively. The energetic perspective should be internally consistent with the surface θ e perspective.…”

The Dynamics of the Global Monsoon: Connecting Theory and Observations

“…& Cook, 2007;Nie et al, 2010;Trenberth, Stepaniak, & Caron, 2000;C. Zhang, Nolan, Thorncroft, & Nguyen, 2008); advective drying by this shallow circulation appears to suppress monsoon precipitation (Zhai & Boos, 2017). Accounting for this behavior requires some adjustments to the CQE assumption (Shekhar & Boos, 2016).…”

Transport pathways across the West African Monsoon as revealed by Lagrangian Coherent Structures

“…In the case of West Africa, further factors need to be considered: the development of the Atlantic cold tongue close to the Equator during boreal spring and summer, or the presence of the Saharan Heat low (see Fig. 1 a) that limits the northward extension of the monsoon 4 , 11 , 12 . These factors play a role in shaping the different flows involved in the WAM, which, in turn, are key for transporting moisture and energy across the monsoon.…”

“…1 d. Above it, another shallow meridional circulation develops related to the SHL. Its lower southerly branch transports moisture across the Sahel, while its upper northerly branch transports dry and warm air, limiting the northward extent of the monsoon 12 , 19 . Understanding transport across the monsoon features described below is the main aim of this document.…”