The climatological link between cut-off low (COL) pressure systems that occur from 20 to 50 • S and Rossby wave breaking (RWB) in the Southern Hemisphere (SH) is examined for 1979-2008. It is shown that COLs are associated with either RWB events (89%) or with potential vorticity (PV) intrusions where there is north-south advection of high-PV air (11%). In the vast majority of COLs, the RWB events occur upstream, on or before the day of the COL formation. The evolution of the PV, geopotential heights, static stability, absolute vorticity and temperature advection fields during the COLs are consistent with the formation of high-PV anomalies that induce cyclonic circulations as predicted by theory. RWB plays a key role in producing the split flow associated with COLs in the SH, which in turn produces absolute vorticity anomalies by shear-curvature vorticity conversion, and creates static stability anomalies. The COLs associated with RWB at 330 K are deeper and more persistent than those associated with 350 K RWB and surface processes differ depending on the isentropic surface on which the associated RWB occurs. The seasonality of the RWB and COLs are similar, and is linked to the seasonal march of the westerly jets.

Austral summer rainfall over the period 1991/1992 to 2010/2011 was dynamically downscaled by the weather research and forecasting (WRF) model at 9 km resolution for South Africa. Lateral boundary conditions for WRF were provided from the European Centre for medium-range weather (ECMWF) reanalysis (ERA) interim data. The model biases for the rainfall were eval-uated over the South Africa as a whole and its nine prov-inces separately by employing three different convective parameterization schemes, namely the (1) Kain-Fritsch (KF), (2) Betts-Miller-Janjic (BMJ) and (3) Grell-Devenyi ensemble (GDE) schemes. All three schemes have gener-ated positive rainfall biases over South Africa, with the KF scheme producing the largest biases and mean absolute errors. Only the BMJ scheme could reproduce the intensity of rainfall anomalies, and also exhibited the highest cor-relation with observed interannual summer rainfall variability. In the KF scheme, a significantly high amount of moisture was transported from the tropics into South Africa. The vertical thermodynamic profiles show that the KF scheme has caused low level moisture convergence, due to the highly unstable atmosphere, and hence con-tributed to the widespread positive biases of rainfall. The negative bias in moisture, along with a stable atmosphere and negative biases of vertical velocity simulated by the GDE scheme resulted in negative rainfall biases, especially over the Limpopo Province. In terms of rain rate, the KF scheme generated the lowest number of low rain rates and the maximum number of moderate to high rain rates associated with more convective unstable environment. KF and GDE schemes overestimated the convective rain and underestimated the stratiform rain. However, the simulated convective and stratiform rain with BMJ scheme is in more agreement with the observations. This study also docu-ments the performance of regional model in downscaling the large scale climate mode such as El Nin˜o Southern Oscillation (ENSO) and subtropical dipole modes. The correlations between the simulated area averaged rainfalls over South Africa and Nino3.4 index were -0.66, -0.69 and -0.49 with KF, BMJ and GDE scheme respectively as compared to the observed correlation of -0.57. The model could reproduce the observed ENSO-South Africa rainfall relationship and could successfully simulate three wet (dry) years that are associated with La Nin˜a (El Ni˜no) and the BMJ scheme is closest to the observed variability. Also, the model showed good skill in simulating the excess rainfall over South Africa that is associated with positive sub-tropical Indian Ocean Dipole for the DJF season 2005/2006.

A 30-yr climatology of Rossby wave breaking (RWB) on the Southern Hemisphere (SH) tropopause is formed using 30 yr of reanalyses. Composite analysis of potential vorticity and meridional fluxes of wave activity show that RWB in the SH can be divided into two broad categories: anticyclonic and cyclonic events. While there is only weak asymmetry in the meridional direction and most events cannot be classified as equatorward or poleward in terms of the potential vorticity structure, the position and structure of the fluxes associated with equatorward breaking differs from those of poleward breaking. Anticyclonic breaking is more common than cyclonic breaking, except on the lower isentrope examined (320 K). There are marked differences in the seasonal variations of RWB on the two surfaces, with a winter minimum for RWB around 350 K but a summer minimum for RWB around 330 K. These seasonal variations are due to changes in the location of the tropospheric jets and dynamical tropopause. During winter the subtropical jet and tropopause at 350 K are collocated in the Australian-South Pacific Ocean region, resulting in a seasonal minimum in the 350-K RWB. During summer the polar front jet and 330-K tropopause are collocated over the Southern Atlantic and Indian Oceans, inhibiting RWB in this region.

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Using 38 years of ERA-Interim reanalyses, this study examines the flow and associated moisture fluxes induced by ridging South Atlantic Ocean anticyclones over South Africa. The flow and moisture fluxes are divided into their geostrophic and ageostrophic components. Composite analysis reveals that ridging high pressure systems are modulated by Rossby wave trains that develop upstream in the middle latitudes near South America and the South Atlantic Ocean near the Greenwich Meridian. The wave trains are similar to those that are associated with tropical temperate troughs (TTTs) and nearly all TTTs are followed by ridging, whilst 20% of the ridging events are linked to TTTs. Composite analysis also shows that ridging-induced moisture fluxes affecting South Africa originate from different areas of the surrounding oceans at different times during the evolution of ridging highs. In the early stages, moisture enters the country along the southeastern coast through ageostrophic processes from a moisture divergence region located adjacent to the coast and remains largely south of 30 S latitude line. The moisture fluxes have a northeastern orientation on land, following the geometry of the eastern coast. These ageostrophic fluxes contribute to the occurrence of rainfall during the ridging process in the region located between Lesotho and Swaziland. Ridging events are associated with about 60% of rainfall days in summer over southern Africa whilst TTTs contribute about 21% summer rainfall days. During the later stages of the evolution of ridging, geostrophic fluxes enter the southern parts of Mozambique and contribute the accumulation of moisture in that region. The associated moisture divergence region is always located ahead of its ageostrophic counterpart, with a local maximum eventually forming in the Mozambique Channel.

The link between Rossby wave breaking and ridging Atlantic Ocean anticyclones in the South African domain is examined using NCEP–DOE AMIP-II reanalysis data. A simple composite analysis, which used the duration of ridging events as a basis of averaging, reveals that ridging anticyclones are coupled with Rossby wave breaking at levels higher than the dynamical tropopause. Lower-stratospheric PV anomalies extend to the surface, thus coupling the ridging highs with the lower stratosphere. The anomaly extending from the 70-hPa level to the surface contributes to a southward extension of the surface negative anomaly over the Namibian coast, which induces a cyclonic flow, causing the ridging anticyclone to take a bean-like shape. The surface positive anomaly induces the internal anticyclonic flow within the large-scale pressure system, causing the ridging end to break off and amalgamate with the Indian Ocean high pressure system. Lower-stratospheric Rossby wave breaking lasts for as long as the ridging process, suggesting that the former is critical to the longevity of the latter by maintaining and keeping the vertical coupling intact.

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