19 Jan 2022
19 Jan 2022
Status: this preprint is currently under review for the journal WCD.

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

Thando Ndarana1, Lesetja E. Lekoloane2, Tsholanang S. Rammopo1, Chris J. C. Reason3, Mary-Jane M. Bopape2, Hector Chikoore4, and Francois A. Engelbrecht5 Thando Ndarana et al.
  • 1Department Geography, Geoinformatics and Meteorology, University of Pretoria, Hatfield, South Africa
  • 2Department of Research and Innovation, South African Weather Service, Centurion, South Africa
  • 3Department of Oceanography, University of Cape Town, Cape Town, South Africa
  • 4Unit for Environmental Sciences and Management, North-West University, Vanderbijlpark, South Africa
  • 5Global Change Institute, University of the Witwatersrand, Johannesburg, South Africa

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.

Thando Ndarana et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on wcd-2022-2', Anonymous Referee #1, 17 Feb 2022
  • RC2: 'Comment on wcd-2022-2', Anonymous Referee #2, 23 Feb 2022
  • RC3: 'Comment on wcd-2022-2', Anonymous Referee #3, 25 Feb 2022

Thando Ndarana et al.

Thando Ndarana et al.


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Short summary
This study considered the upper dynamics associated with the South Atlantic Ocean high pressure system that extends across the South African domain. This extension of this system occurs in two ways. The dynamics are studied by considering the evolution of energy and it is shown that the upper level processes are different between the two types of the extension. The weather systems that occur in the upper develop very differently and have different implications for rainfall over South Africa.