Browsing by Subject "Climatology"

Now showing 1 - 5 of 5
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    Open Access
    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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    Open Access
    A coastal seawater temperature dataset for biogeographical studies: large biases between in situ and remotely-sensed data sets around the Coast of South Africa
    (Public Library of Science, 2013) Smit, Albertus J; Roberts, Michael; Anderson, Robert J; Dufois, Francois; Dudley, Sheldon F J; Bornman, Thomas G; Olbers, Jennifer; Bolton, John J
    Gridded SST products developed particularly for offshore regions are increasingly being applied close to the coast for biogeographical applications. The purpose of this paper is to demonstrate the dangers of doing so through a comparison of reprocessed MODIS Terra and Pathfinder v5.2 SSTs, both at 4 km resolution, with instrumental in situ temperatures taken within 400 m from the coast. We report large biases of up to +6°C in places between satellite-derived and in situ climatological temperatures for 87 sites spanning the entire ca . 2 700 km of the South African coastline. Although biases are predominantly warm (i.e. the satellite SSTs being higher), smaller or even cold biases also appear in places, especially along the southern and western coasts of the country. We also demonstrate the presence of gradients in temperature biases along shore-normal transects -- generally SSTs extracted close to the shore demonstrate a smaller bias with respect to the in situ temperatures. Contributing towards the magnitude of the biases are factors such as SST data source, proximity to the shore, the presence/absence of upwelling cells or coastal embayments. Despite the generally large biases, from a biogeographical perspective, species distribution retains a correlative relationship with underlying spatial patterns in SST, but in order to arrive at a causal understanding of the determinants of biogeographical patterns we suggest that in shallow, inshore marine habitats, temperature is best measured directly.
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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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    Understanding the variability and predictability of seasonal climates over West and Southern Africa using climate models
    (2015) Lawal, Kamoru Abiodun; Abiodun, Babatunde Joseph; Stone, Dáithí A
    A good understanding of seasonal climate and the limit to which it can be predicted is crucial in addressing various socio-economic challenges in Africa. However, how to improve the capability of the dynamical models of the climate system in reproducing the regional seasonal climate variability and in replicating the role of various atmospheric circulation anomalies on the regional variability remains a major challenge. Thus far, understanding of seasonal climate over these regions, as well as the ability of climate models to predict them, has focused on the agreement of simulations of dynamical models of the climate system, rather than considering outliers as potentially vital contributors to understanding and predictability. This thesis uses discrepancy in a large ensemble of climate simulations as a tool to investigate variability in dominant seasonal rainfall and temperature patterns (i.e. classes) over West and Southern Africa, to examine the capability of climate models in reproducing the variability, and to study the predictability of the seasonal climates over South Africa. The dominant classes of variability (of rainfall and maximum temperature fields) in both regions are examined based on the Self-Organizing Map (SOM) classifications. The sequences in which each class occurs cannot be linked simply to a single common index of global scale atmospheric circulation anomalies, implying that the chaotic regional atmospheric circulations that modulate the global scale modes of variability are indispensable. The climate model examined adequately reproduces the dominant classes of seasonal climate over West and Southern Africa.