The meridional overturning circulation has two regimes. Under weak insolation and fast rotation, the circulation is dominantly driven by momentum forcing (including friction, eddy momentum transport etc.), and is thermally-indirect in most of the low latitudes (mechanism). Under strong insolation and slow rotation, the circulation is dominantly driven by thermal forcing (including diabatic heating, eddy heat transport etc.), and is thermally-direct.

Panel (a) shows a measure of the contribution of momentum-related processes (blue) and thermal-related processes (red) to the total meridional overturning circulation. Panel (b-g) shows the circulation decomposition for a weak insolation case (top) and a strong insolation case (bottom). 

As circulation transforms from a thermally indirect circulation to a thermally direct one, the upper air starts to be filled with low angular momentum air from the extratropics (panel c versus panel b), meanwhile, the low latitudes start to be dominated by downward motion, largely reducing the cloud fraction (panel f versus panel e).

The observables for the regime transition in OBL80-S. (a-c) are for the 10 mb zonal wind, and (d-f) are for the vertically-integrated cloud cover. (a) and (d) show the whole progression of the zonal wind and cloud as insolation increases. While, (b,e) and (c,f) zoom in and show the seasonal cycle at the beginning and the end of the OBL80-S simulation, which corresponds to 1250 W/m2 and 1750 W/m2 insolation. Zero wind speed is denoted with black contours in (a-c).


Kang. W. 2019, Regime transition between eddy-driven and moist-driven circulation on High Obliquity Planets Astrophysical Journal (accepted)


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