On the Role of the Divergent Motions for the Tropical Momentum Balance

Throughout the seasonal cycle, the momentum balance in the upper tropical troposphere reflects an approximate equilibrium between meridional advection of absolute momentum by the Hadley cell and the eddy momentum forcing. This eddy forcing accelerates the zonal flow over the summer hemisphere and decelerates it over the winter hemisphere. Previous studies have shown that the westerly eddy acceleration over the summer hemisphere is primarily associated with meridional convergence of the eddy momentum flux, associated with the propagation of Rossby waves generated by the asymmetric heating. However, we will show in this presentation that: (i) the vertical eddy momentum flux also has a sizable impact, balancing some of the upper-level meridional convergence and spreading the westerly acceleration downward; (ii) because the meridional momentum transport is performed by the divergent meridional velocity, it is associated with vertical momentum advection rather than with meridional vorticity fluxes. As a result, the observed westerly eddy acceleration is primarily forced by vertical rather than meridional advection (with the exception of the Northern monsoon season). We interpret this result using the upper-trosposphere vorticity budget and discuss its implications.