A quasi-geostrophic diagnosis of the zonal flow associated with cut-off lows over South Africa and surrounding oceans
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A quasi‑geostrophic diagnosis of the zonal flow associated with cut‑off lows over South Africa and surrounding oceans Thando Ndarana1 · Tsholanang S. Rammopo1 · Hector Chikoore2 · Michael A. Barnes3 · Mary‑Jane Bopape4 Received: 16 March 2020 / Accepted: 29 July 2020 © Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract 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 quasistationary, 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. Keywords Cut-off low · Jet streak · Rossby wave breaking · Geostrophic flow
1 Introduction Cut-of-low (COL) pressure systems are one of the most important all-seasons (Singleton and Reason 2007) precipitation producing systems over South Africa, with an annual frequency maximum occurring in autumn and a secondary maximum in spring (Singleton and Reason 2007; Pinheiro et al. 2017). During the spring, the rainfall associated with these systems is more intense and widespread (Favre et al. 2013), thus contributing the most to annual COL precipitation. During the winter months COLs can also bring about * Thando Ndarana [email protected] 1
Geography, Geoinformatics and Meteorology, University of Pretoria, Hatfield, South Africa
2
Unit for Environmental Sciences and Management, North-West University, Vanderbijlpark, South Africa
3
Marine Research Unit, South African Weather Service, Cape Town, South Africa
4
Department of Research and Innovation, South African Weather Service, Centurion, South Africa
heavy snowfall, when coupled with low level systems that facilitate cold temperature advection at the surface (Stander et al. 2016). COLs may also occur together with ridging high pressure systems. The latter facilitate the influx of moisture from the Indi
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