Downstream development associated with two types of ridging South Atlantic Ocean anticyclones over South Africa

Abstract

Abstract. There are at least two types of ridging South Atlantic Ocean high pressure systems in the South African domain. Type-N events occur north of 40° S and Type-S occur south of this latitude line. This study shows that there is no evidence of surface downstream development in terms of the evolution of eddy kinetic energy and associated ageostrophic geopotential fluxes for both types of ridging high events. Rather, for these systems downstream development is an upper level process. The baroclinic waves associated with the ridging develop from baroclinic instability, by converting eddy available potential energy to eddy kinetic energy. The bulk of the conversion is located at the upstream end of the waves. The downstream trough, which is the part of the wave that influences upward motion over South Africa, develops from the transport of eddy kinetic energy across the trough axis by means of ageostrophic geopotential fluxes. These fluxes are stronger for Type-S events. The absence of downstream development at the surface and the presence of it aloft demonstrates that there are differences in the underlying dynamics in the evolutions of these systems in the vertical. The evolution of eddy kinetic energy associated with baroclinic waves that occur during the ridging events is different from what has been observed for cut-off low pressure systems in the South African domain.