Abstract:
Ridging South Atlantic high pressure systems (ridging highs) are often accompanied by cut-off low (COL) pressure systems
aloft, but may also occur without them, in which case a linear baroclinic wave would be observed propagating across the
South African domain in the upper troposphere. Using 41 years of ERA-5 reanalysis data, this study documents differences
between the characteristics of the prevailing dynamical processes and associated local eddy kinetic energy generation, its
downstream transfer and dissipation during these two scenarios. The study shows that when COLs are present then baroclinic
conversion is strong and it is confined east of the Greenwich Meridian, whereas it is located downstream of South
Africa and it is much weaker, when ridging occurs without COLs. The differences in strength and locations of the baroclinic
conversion are associated with the differing jet streak configurations between the two scenarios; which lead to Rossby wave
breaking and the absence thereof when there are COLs and when ridging occurs without COLs, respectively. The presence
of breaking during COLs leads to trans-ridge downstream development that facilitates energy transfer from the midlatitudes
into the South African domain. When there are no COLs present, the trans-trough downstream development is stronger than
it is across the upstream ridge. Barotropic conversion from eddy kinetic energy to mean kinetic energy occurs in the South
African domain during COLs, but occurs much further downstream when there are no COLs during ridging highs. The difference
in the characteristics identified in this study can be traced back to the differences in the potential vorticity anomaly
structures, which are largely due to whether the waves break or do not during the evolution of ridging events in the South
African domain.