Shallow meridional circulations in monsoons: observed seasonal cycle and advective drying tendency

Although monsoons have traditionally been thought of as deep overturning circulations in which precipitating ascent extends through the entire depth of the troposphere, recent work has shown that shallow, dry circulations are superimposed on those deep circulations in nearly all monsoons. Near-surface flow in these shallow meridional circulations (SMCs) converges over hot deserts poleward of monsoonal precipitation maxima, and flow diverges around 700 hPa and flows toward the equator. Previous studies have suggested that SMCs might influence monsoon precipitation by advecting near-surface moisture poleward toward the desert and by advecting hot and dry air into the moist monsoon region at mid levels, but the interaction of SMCs with moist monsoon flow is in general poorly understood. Here we document the time-mean structure and climatological seasonal cycle of SMCs over Australia, West Africa, South Asia, and southern Africa using both in-situ measurements and reanalysis datasets. We present composites of the evolution of SMCs relative to monsoon onset in each region. And to improve understanding of the effect of SMCs on monsoon precipitation, we analyze the advective drying effect of SMCs on the monsoon moisture budget in the lower troposphere. This work provides a more comprehensive picture of the space-time structure of SMCs and their interactions with deep, precipitating flow in monsoons.