Browsing by Author "Reason, C J C"

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    A model investigation of interannual winter rainfall variability over southwestern South Africa and associated ocean-atmosphere interaction
    (2003) Reason, C J C; Jagadheesha, D; Tadross, M
    We have investigated the variability of inter-annual winter rainfall over the southwestern Cape region of South Africa and associated large-scale atmosphere-ocean interaction upstream over the South Atlantic using the HadAM3 atmospheric general circulation model. This model was run for the period from 1990 to 1999 using mean monthly global sea-surface temperature (SST) as surface boundary condition over the global ocean. Diagnostics of winter (May to September) model output averaged over 1990-99 suggest that the HadAM3 model represents the general circulation in the South Atlantic / African sector reasonably well for this season at least. In addition, model years with wet and dry winters over the study area tended also to be those that were observed to be anomalously wet or dry. Wet minus dry season composite fields were used to investigate the model's inter-annual variability. The composite difference fields for low- and mid-level winds, sea-level pressure, and moisture flux all indicated wet winters being associated with increased inflow from tropical South America (originating in the equatorial western Atlantic at low levels) contributing relatively moist air to the westerly flow heading towards the southwestern Cape. A stronger jet over the South Atlantic promoted the passage of storms towards the Cape. Large areas of cyclonic vorticity anomalies, enhanced eddy activity, increased thickness in the lower atmosphere and low-level convergence near and upstream of the southwestern Cape in the model composite differences all favoured increased storm systems as well as their local intensification, implying enhanced rainfall. The results presented here suggest that the model can represent the interannual variability of winter rainfall over the study region and shed light on the mechanisms potentially associated with anomalously wet winters there.
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    A review of South African research in atmospheric science and physical oceanography during 2000-2005
    (2006) Reason, C J C; Engelbrecht, F; Landman, W; Lutjeharms, J R E; Piketh, S; Rautenbach, C J W; Hewitson, B C
    The purpose of this article is to review progress in the fields of atmospheric science and physical oceanography made by workers based at South African institutions over approximately the last 5 years. Research published by South African scientists working abroad is not included. Most published research in these fields falls within the broad areas of climate variability, climate change, aerosols and atmospheric pollution, seasonal forecasting, numerical modelling (both atmospheric and oceanic), and the physical oceanography of the Agulhas and Benguela current systems. Most but not all of the atmospheric science papers relate to South Africa or southern Africa; however, some work pertaining to the southern hemisphere as a whole or to other regions has been done. We note that funding and institutional support for atmospheric science and physical oceanography research in South Africa remains poor and this situation significantly hampers local efforts.
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    Sensitivity of the atmospheric response to sea-surface temperature forcing in the South West Indian Ocean: A regional climate modelling study
    (2006) Hansingo, K; Reason, C J C
    The MM5 regional climate model has been used to investigate the sensitivity of the atmospheric response to sea-surface temperature (SST) forcing in the South West Indian Ocean. Two model runs were analysed and compared against each other; namely, one in which the model was forced by an observed warm SST anomaly during a summer season with above-average rainfall over southern Africa, and the other in which the model was forced with a smoothed representation of this anomaly but with the centre shifted closer to the east coast of South Africa. The latter experiment was motivated by correlation analyses between rainfall and SST and by previous experiments with coarser-resolution global circulation models, which suggest that the model response over the land is larger if the SST forcing is shifted closer to it. Analysis of the differences in the model response between the two runs suggests that, consistent with the global models, the MM5 response is indeed larger over southern Africa and more conducive to above-average rainfall in the experiment with the smoothed and westward shifted SST forcing. Increased evaporation over the South West Indian Ocean, local uplift and enhanced moisture flux westwards into southern Africa (as well as southwards over the land from the equatorial region) all play a role in enhancing the regional atmospheric conditions favourable for rainfall over a large area of southern Africa during the season simulated
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    South Atlantic response to El Niño–Southern Oscillation induced climate variability in an ocean general circulation model
    (American Geophysical Union, 2004-12-14) Colberg, F; Reason, C J C; Rodgers, K
    [1] The response of the South Atlantic Ocean to El Niño-Southern Oscillation (ENSO) is investigated by means of an ocean general circulation model (ORCA2) forced with National Centers for Environmental Prediction (NCEP) reanalyses for the 1948–1999 period. Seasonal ENSO composites suggest that the ENSO-induced wind anomalies play a major role in driving upper ocean temperatures by altering the net surface heat fluxes, the meridional Ekman heat transport, and Ekman pumping. Model diagnostics indicate that the Ekman heat transport changes are in better agreement with the upper ocean temperature anomalies during the first half of the ENSO event whereas, in the latter half, the surface heat flux anomalies agree better. In general, the atmospheric forcing tends to lead to a coherent ocean response with a time lag of about one season. Subsurface temperatures evolve more slowly in response to ENSO forcing than the upper ocean. They receive time-filtered ENSO signals from mainly Ekman pumping (suction) and variations in thermocline depth that result in the poleward and equatorward margins of the subtropical gyre exhibiting temperature anomalies of the same sign but opposite to those in the central regions of the gyre.
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    Tropical Cyclone Eline and Its Unusual Penetration and Impacts over the Southern African Mainland
    (2004) Reason, C J C; Keibel, A
    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.