A quasi-geostrophic diagnosis of the zonal flow associated with cut-off lows over South Africa and surrounding oceans

22 Sep 2020

The zonal flow associated with cut-off lows (COLs) comprises two jet streaks of different spatial extents. The smaller scale jet streak, located north of the COLs, forms as a result of meridional divergence of vorticity advection and it is quasi-stationary, relative to the COLs. It dissipates as the COLs do the same. The larger scale jet streak gives rise to anticyclonic and equatorward Rossby wave breaking (RWB) as it propagates southeasterly to the base of the ridge, south of the COL and then northeasterly beyond that point. As the jet streak propagates it brings with it the anticyclonic barotropic shear that causes the Rossby waves to break. Its propagation is caused by zonal momentum advection by the zonal flow from jet streak entrance to its exit. As it propagates, its northwesterly/southeasterly orientation changes to one that is more zonal to become south-westerly/northeasterly at the end of the COL life cycle. This change in orientation is due to meridional advection of zonal momentum,where the meridional flow advects momentum southward (northward) at the jet streak entrance (exit). The jet streaks form a split jet structure and the winds between the streaks is decelerated by vorticity advection convergence. Because the flow and COL (and RWB) life cycle are coupled, understanding the dynamics that underlie the changes in the COL ambient flow contributes to resolving the outstanding RWB/COL causality problem.