The maintenance of a zonally asymmetric subtropical jet

The subtropical jet dominates over specific longitudinal sectors during both winters. The major source of this zonal asymmetry is localized tropical convection. In particular, during austral winter, the wide and powerful convection over the Asian monsoon region and Maritime Continent drives a subtropical jet over the Indian Ocean, Australia and the west and central Pacific. Further downstream in the east Pacific the jet tilts poleward, gradually shifting towards eddy-driven jet characteristics, while in the Atlantic sector only an eddy-driven jet prevails.

In this study, we show that the upper tropospheric circulation pattern over the whole Southern Hemisphere during winter is similar to that in an idealized model simulation, where the only zonal asymmetry source is localized tropical convection in the summer hemisphere. A similar momentum budget is found for the observations and model simulation. The first-order momentum balance is the geostrophic balance associated with a stationary Rossby wave driven by tropical convection. The upstream part of the subtropical jet (the Indian Ocean jet) is associated with a high equatorward of it, and the downstream part (the Pacific jet) is associated with a low poleward of it. This demonstrates that the subtropical jet zonally asymmetric component is a manifestation of a stationary Rossby wave in the upper troposphere. The second-order momentum balance is associated with approximate absolute angular momentum conservation in the localized Hadley cell, as is the dominant balance in zonally symmetric models. The third-order momentum balance is between meridional advection of absolute angular momentum and zonal momentum advection. Transient eddy momentum fluxes are negligible in the maintenance of the subtropical jet zonal structure.