At monsoon onset, the meridional overturning circulation zonally averaged over the Asian and Australian sectors rearranges itself from an equinox pattern, with two cells almost symmetric about the equator, to a monsoon pattern dominated by a single cross-equatorial cell, with ascent in the subtropics in the summer hemisphere and descent in the winter hemisphere. In the equinox regime, eddy momentum fluxes of midlatitude origin extend into the upper branches of the equinox cells, forcing them to deviate from angular-momentum conservation and strongly influencing their strength. In the monsoon regime, eddy momentum fluxes are confined to the descending branch of the cross-equatorial cell, allowing it to more closely conserve angular momentum and respond more directly to the thermal forcing. An analysis of the upper tropospheric zonal momentum budget will be presented, to quantify the nature of monsoon transitions as shifts in the dominant zonal momentum balance at the center of the overturning circulations.
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