A quasi-geostrophic analysis of summertime southern African linear-regime westerly waves

Linear-regime westerly waves that propagate across the South African domain are often linked to well-known rainfall producing systems such as tropical temperate troughs and synoptic scale tropical low-pressure systems, and ridging South Atlantic Ocean anticyclones at the surface. It is accepted that the baroclinic waves that propagate across the domain provide the lifting mechanism that causes the required vertical motion for rainfall to occur. This study shows that there exists a jet streak embedded in these waves that is located downstream of the trough axis, to the east of which vertically upward motion is expected to occur. The entrance of the jet streak passes just south of the country, as the waves propagate past the domain. The study further shows that for this class of waves, the vertical motion that causes rainfall to occur is induced by the thermally direct transverse ageostrophic circulation that is located at this jet entrance. This is instead of the conventional upper air divergence that is located at the infection point east of the trough axis. Using a method of decomposing the Q-vector into its transverse (Qn) and shear (Qs) components, the divergence felds of which are used to decompose the vertical motion into the corresponding components, i.e 휔n and 휔s, respectively; it was shown that the vertical motion over South Africa is explained more by the former than the latter. Therefore, the uplift over the country and that located at the infection point east of the trough are dynamically distinct processes. Taking the limitations of the quasi-geostrophic framework into consideration, the study concludes that during the passage of linear-regime waves vertical motion that might lead to rainfall is caused by the circulation at the jet entrance and not the divergence in the baroclinic wav

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

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.Water Research Commissionhttp://link.springer.com/journal/3822021-08-17hj2020Geography, Geoinformatics and Meteorolog

Downstream development during South African cut-off low pressure systems

Using 39 years of ECMWF renalysis data, an established energetics framework and simple composite analysis this study has shown that South African cut-off low (COL) pressure systems are preceded by downstream development of a baroclinic wave. Downstream development is characterised by the evolution of two energy centres, one located upstream in the midlatitudes and another downstream in the subtropics. The upstream eddy kinetic energy, which is associated with a midlatitude jet streak, develops and reaches its maximum before the formation of the closed COL cyclonic circulation. The downstream eddy kinetic energy centre maximises at the point where the closed circulation forms. The upstream eddy kinetic energy centre grows from baroclinic conversion from eddy available potential energy to eddy kinetic energy, whilst the downstream centre grows by receiving energy by means of ageostrophic geopotential fluxes that transport eddy kinetic energy in a north-eastward direction from the upstream centre. These ageostrophic geopotential fluxes are induced, increased in magnitude and directed by processes associated with Rossby wave breaking (RWB) on the midlatitude dynamical tropopause and so the downstream energy transfer connects South African COLs to midlatitude processes. The study has further shown that the baroclinic kinetic energy configuration previously associated with wet seasons over South Africa is consistent with times when COLs forms over the country.The Water Research Commission, South Africahttps://www.elsevier.com/locate/atmosreshj2022Geography, Geoinformatics and Meteorolog

Two types of ridging South Atlantic Ocean anticyclones over South Africa and the associated dynamical processes

Using 41 years of ERA5 reanalysis, two types of ridging South Atlantic Ocean high pressure systems were identified in the South African domain. Type-N events have a zonal structure and the ridging component breaks off from the parent South Atlantic Ocean anticyclone, after extending across the South African mainland. Type-S events extend south of the mainland and then break off. The Type-N (Type-S) ridging component is weaker (stronger) leaving behind a stronger (weaker) South Atlantic Ocean high. The two types of ridging events are associated with different configurations of Rossby wave packets that propagate across the South Atlantic Ocean. Surface and upper tropospheric anomalies associated with Type-S wave packets are stronger than those associated with Type-N events and the vertical coupling of the anomalies is much stronger during Type-S events. Type-N events are associated with a double jet streak structure, with the downstream jet contributing to upward motion over the landmass by means of its thermally direct circulation at its jet entrance. The upstream jet during Type-N events induces downward motion over the southern half of South Africa as it propagates eastward. The Type-S upstream jet streak, which only appears during winter, has limited zonal extent and does not induce downward motion over the country. Type-N ridging is associated with stronger ageostrophic moisture fluxes along the southern coast leading to higher moisture content and precipitation along the south eastern and eastern coasts of South Africa.The Water Research Commission, South Africahttp://www.elsevier.com/locate/atmoshj2021Geography, Geoinformatics and Meteorolog