The drying tendency of shallow meridional circulations in monsoons

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.