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.

Highlights

  • The subtropical South Atlantic Ocean is characterised by a quasi-stationary anticyclonic circulation that exhibits pronounced 25 variability in intensity and position at intra-annual and inter-annual (Sun et al, 2017) and multi-decadal (Reason, 2000) time scales

  • 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

  • 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

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Summary

Introduction

The subtropical South Atlantic Ocean is characterised by a quasi-stationary anticyclonic circulation that exhibits pronounced 25 variability in intensity and position at intra-annual and inter-annual (Sun et al, 2017) and multi-decadal (Reason, 2000) time scales. Under certain stratospheric conditions (Ndarana et al, 2018), this anticyclone extends east and ridges around the South African landmass to induce an influx of moisture into the country (Cook et al, 2004; Dyson, 2015; Engelbrecht et al, 2015; Ndarana et al, 2021a). This eastward extension process 30 is referred to as ridging. Ridging highs contribute to the moisture budget of southern Africa, with the South Atlantic and Indian Oceans being important moisture source regions (Rapolaki et al, 2020).

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