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C. J. C. Reason

Abstract

The Agulhas Current is much the strongest western boundary current in the Southern Hemisphere transporting about 65 Sv of warm water along the southeast coast of Africa. South of Africa, it retroflects back into the South Indian Ocean and flows east as the Agulhas Return Current. Large amounts of heat are lost to the atmosphere from the warm waters of the southern Agulhas Current, its retroflection, and the Return Current particularly during winter and the transition seasons. It is thought that this large exchange of heat may have a significant influence on the regional atmospheric circulation, the tracks and intensity of transient weather disturbances such as extratropical cyclones, and on rainfall received over large parts of neighboring southern Africa.

In an attempt to investigate the potential influence of the Agulhas Current on the regional atmosphere, two ensembles of year-long integrations of an atmospheric general circulation model are analyzed. In the first ensemble, the sea surface temperature (SST) forcing is a monthly climatology and therefore contains the signature of the greater Agulhas Current system at the model resolution. In the second ensemble, the pronounced signature of this current is smoothed out so that the waters of the region only show a typical latitudinal variation in SST similar to that found in the central ocean away from boundary currents. An objective vortex tracking scheme is used to assess the differences in the number, track, and intensity of extratropical cyclones in the Southern Hemisphere between the two integrations.

It is found that a near-surface cold anticyclonic anomaly is generated over the greater Agulhas Current region in the ensemble with the smoothed SST. Cyclonic systems are weaker and shifted farther south in this ensemble compared to climatology with significant reductions in rainfall over neighboring South Africa. Downstream, over the southeastern Australian–Tasman Sea region, there is an increase in cyclonic activity and rainfall. In general, the differences between the two ensembles are more coherent and pronounced during winter than in summer consistent with the surface temperature contrast between the Agulhas Current waters and the overlying atmosphere being larger in winter. With the caveats that the atmosphere–ocean system is nonlinear and that the model resolution does not represent the turbulent processes of air–sea heat/moisture exchange as well as one would like, the results provide some evidence for the potential importance of the greater Agulhas Current system for regional climate and weather patterns.

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J. C. Hermes and C. J. C. Reason

Abstract

A global ocean model (ORCA2) forced with 50 yr of NCEP–NCAR reanalysis winds and heat fluxes has been used to investigate the evolution and forcing of interannual dipolelike sea surface temperature (SST) variability in the South Indian and South Atlantic Oceans. Although such patterns may also exist at times in only one of these basins and not the other, only events where there are coherent signals in both basins during the austral summer have been chosen for study in this paper. A positive (negative) event occurs when there is a significant warm (cool) SST anomaly evident in the southwest of both the South Indian and South Atlantic Oceans and a cool (warm) anomaly in the eastern subtropics.

The large-scale forcing of these events appears to consist of a coherent modulation of the wavenumber-3 or -4 pattern in the Southern Hemisphere atmospheric circulation such that the semipermanent subtropical anticyclone in each basin is shifted from its summer mean position and its strength is modulated. A relationship to the Antarctic Oscillation is also apparent, and seems to strengthen after the mid-1970s. The modulated subtropical anticyclones lead to changes in the tropical easterlies and midlatitude westerlies in the South Atlantic and South Indian Oceans that result in anomalies in latent heat fluxes, upwelling, and Ekman heat transports, all of which contribute to the SST variability. In addition, there are significant modulations to the strong Rossby wave signals in the South Indian Ocean. The results of this study confirm the ability of the ORCA2 model to represent these dipole patterns and indicate connections between large-scale modulations of the Southern Hemisphere midlatitude atmospheric circulation and coevolving SST variability in the South Atlantic and South Indian Oceans.

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R. C. Blamey and C. J. C. Reason

Abstract

A combination of numerous factors, including geographic position, regional orography, and local sea surface temperatures, means that subtropical southern Africa experiences considerable spatial and temporal variability in rainfall and is prone to both frequent flooding and drought events. One system that may contribute to rainfall variability in the region is the mesoscale convective complex (MCC). In this study, Tropical Rainfall Measuring Mission (TRMM) Multisatellite Precipitation Analysis (TMPA) data is used to document the precipitation produced by MCCs over southern Africa for the 1998–2006 period.

Most of the rainfall associated with MCCs is found to occur over central Mozambique, extending southward to eastern South Africa. High precipitation totals associated with these systems also occur over the neighboring southwest Indian Ocean, particularly off the northeast coast of South Africa. MCCs are found to contribute up to 20% of the total summer rainfall (November–March) in parts of the eastern region of southern Africa. If the month of March is excluded from the analysis, then the contribution increases up to 24%. In general, the MCC summer rainfall contribution for most of the eastern region is approximately between 8% and 16%. Over the western interior and Botswana and Namibia, the MCC contribution is much less (<6%). It is also evident that there is considerable interannual variability associated with the contribution that these systems make to the total warm season rainfall.

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R. C. Blamey and C. J. C. Reason

Abstract

The maximum spatial correlation technique (MASCOTTE) is an objective and automated method developed to simultaneously determine both the structural properties and evolution (tracking) of cloud shields of convective systems. Originally designed to monitor systems over the Amazon region, this method has now been adapted for subtropical southern Africa. In this paper, a detailed climatology of 70 mesoscale convective complexes (MCCs) that occurred during the austral summer months over southern Africa during the 1998–2006 period are presented. Most MCCs are clustered along the eastern regions of southern Africa, adjacent to the warm waters of the Mozambique Channel and Agulhas Current. A few infrequent systems are found to be developing in Namibia and Botswana. The systems are found to predominantly occur during the months of November–February, with maximum activity occurring in November and December. The transition from a more midlatitude-dominated circulation to a tropical circulation over the region during the late summer leads to an uncharacteristic equatorward migration of the MCC distribution then. The analysis also suggests that there is variability in MCC frequency on monthly and seasonal time scales. Although fewer in number (about nine per season) compared to MCC populations in other regions, the systems do tend to follow the nocturnal life cycle as documented elsewhere.

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Ross J. Murray and C. J. C. Reason

Abstract

Connections between basins below 4000 m, which may or may not be resolved in computer-generated model topography, have been found to have a considerable impact on the movement of Antarctic Bottom Water and on deep ocean temperatures. Blocked ridges produce sharply stepped tracer distributions and a complete absence of Antarctic influence in the North Atlantic. With channels opened, mass transports through them are of the right order, but interbasin gradients are too small and the water is too dense. Because mixing processes in the channels are poorly represented, some modification of channel properties may be necessary in order to achieve realistic water property distributions. This may be done by making local adjustments of diffusive and viscous coefficients, by the use of single width channels (which may be included inadvertently or by design in topography generation and which allow diffusive and limited advective exchanges between basins), and, when vertical resolution allows it, by an appropriate choice of sill depth at the interbasin channels. The various methods work by controlling the mixing of bottom water with deep water by advection and diapycnal diffusion.

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Ross J. Murray and C. J. C. Reason

Abstract

Unphysical extrema are found in ocean simulations employing space-centered advection even when horizontal mixing coefficients are sufficient to suppress checkerboard behavior. They are mostly found as isolated spurious temperature minima (density maxima) occurring in close association with topography and in places of concentrated upwelling or overflowing adjacent to the margins of deep basins, where velocities considerably exceed the vertical, or sometimes horizontal, grid Péclet condition. Temperature depressions may be as large as the expected temperature difference between levels and are capable of contaminating upstream water mass properties through horizontal diffusion and convective adjustment. Examples are discussed with reference to the finite difference solution for a stream of variable speed and diffusivity. Increased vertical resolution disproportionately reduces the magnitude but not necessarily the incidence of depressions. Flux correction eliminates the extrema and the contamination, but it can be very diffusive in the deeper layers and cannot overcome problems due to failure to resolve circulations.

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Ross J. Murray and C. J. C. Reason

Abstract

It is shown that a global curvilinear grid with variable resolution is an efficient way of providing a high density of grid points in a particular region. In equilibrium experiments using asynchronous time steps, this type of grid has been found to allow a better representation of smaller-scale features in the high-resolution region while maintaining contact with the rest of the World Ocean, provided that lateral mixing coefficients be scaled with grid size so as to maintain marginal numerical stability. In this study, the region of interest is the southern Indian Ocean and, in particular, that of the South Indian Ocean Current. In all experiments, decreased viscosities and diffusivities generally led to increased currents and tracer gradients. In horizontal mixing simulations, maximum current speeds in the frontal region were mainly determined by local (i.e., high-resolution region) viscosities, while maximum temperature gradients were determined by local values of both lateral viscosity and diffusivity. With eddy-induced transport experiments, maximum values were analyzed on isopycnal surfaces. Isopycnal diffusivities were found to control tracer gradients on isopycnals but not isopycnal slopes, while thickness diffusivities controlled isopycnal slopes but only to a small degree tracer gradients. Changes to mixing coefficients in the coarse part of the grid had hardly any influence on the frontal properties examined, although they did affect currents in the Indian Ocean to some extent via their control on size of the Antarctic Circumpolar Current and the Pacific–Indian Throughflow.

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C. J. C. Reason and D. G. Steyn

Abstract

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C. J. C. Reason and A. Keibel

Abstract

February–March 2000 saw devastating floods in Mozambique, Zimbabwe, and South Africa. Due to the huge damage and loss of life, global media attention was extensive. Less well known is that one of the weather systems that contributed to these floods (ex-Tropical Cyclone Eline) tracked almost 2000 km across southern Africa toward the cool southeast Atlantic and led to substantial rainfall over arid to semiarid southern Namibia (over two standard deviations above average for these two months and the wettest summer since 1976). Less than 5% of southwest Indian Ocean tropical cyclones actually make landfall on the east coast of southern Africa and even fewer significantly penetrate into the interior, because of the relatively dry 1–1.5-km-high interior plateau that covers most of the region. It is argued that the precursor synoptic conditions together with large-scale circulation and SST anomalies over the Indian Ocean associated with a strengthening La Niña were highly favorable for this unusual evolution and track of Eline.

A summary of the accuracy of La Réunion and Met Office forecasts of Tropical Cyclone Eline over the Indian Ocean is given. Over the mainland, almost all countries do not have any NWP capacity, and the challenges and potential solutions for improved forecasting for the region are discussed. It is argued that by keeping informed of current rainfall, vegetation, and soil moisture conditions over southern Africa, as well as evolving climate signals in the tropical oceans, local forecasters could at least be in a state of heightened alert in advance, since these factors significantly influence extreme weather event characteristics in the region.

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C. J. C. Reason and D. G. Steyn

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No abstract available

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