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Browsing Geography and Environmental Studies by Author "Jury, M.R."
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- ItemCauses, structure and impacts of the 1992/93 drought in KwaZulu/Natal(1998) Dube, Lawrence Thembokwakhe; Jury, M.R.A majority of the population in southern Africa rely on agriculture at a subsistence level for food, or at a more advanced level to generate income, and are therefore susceptible to changes in rainfall and climate. The high incidence of drought since the 1970's (Jury, 1996), and particularly the devastating droughts of 1982/83 (Jury and Levey, 1997) have laid a ground of motivation for a study on circulation changes associated with the 1992/93 drought over KwaZulu/Natal and its impacts. This research analyses the historical context of the 1992/93 drought in KwaZulu/Natal using ECMWF and NCEP/NOAA data at a resolution of 2.5° x 2.5°. To outline the causes and structure of the drought surface and upper-level meteorological data are utilised and impacts are assessed using agricultural production and water resource data. Pentad synoptic weather data for the drought period are composited to establish patterns of circulation and convection over the region, and departures from the historical mean computed. Satellite and conventional data sources are used and time series analysis is undertaken. Outgoing longwave radiation (OLR) and normalised difference vegetation index (NDVI) are used as a proxy for convective intensity and for the identification of impacted areas over South Africa. Wind data and derived parameters are employed to explore large-scale dynamical structures. Pongolapoort and Midmar Dams inflow levels, and agricultural production output data from sugar cane and maize industries are used to gauge the severity of the drought. Data analysis indicates that increased westerly winds with surface marine lows and continental highs prevailed over southern Africa. Anomalous divergence and subsidence occurred over the eastern subcontinent. This was coupled with reduced tropical moist inflows. Anticyclonic vorticity and subsidence via upper level convergence suppressed convection over KwaZulu/Natal. Mid-iatitude winds played a significant role in producing the drought over KwaZulu/Natal through the northward (southwards) movement of the subpolar (subtropical) jet streams which limited the supply of moisture into this region. The area of reduced water vapour flux extended from 15-33°Sandl5-35°E. The 1992/93 DJF analysis of OLR reflects a SE-NW oriented wave-train pattern over southern Africa with KwaZulu/Natal and Malawi anti-phase with the wet Zambezi. Negative anomalies of the streamfunction are obtained between 35°S and 15°S associated with anticyclonic circulation at the 50°E longitude. These are areas where negative SSTs are observed to the east. It is thus apparent that a Hadley cell is a driving mechanism behind the 1992/93 drought over parts of southern Africa south of 20°S. The atmospheric wave train pattern during SON is aligned in the same axis as in the DJF season but the anomaly values are higher in the latter season. This is an indication that even during the pre-summer season convection is suppressed over KwaZulu/Natal and parts of southern Africa south of 20°S. No propagation of these wave trains is observed. The values of below normal precipitable water within these axes increase in the peak summer season. During the DJF (summer) season, the Indian and Atlantic Oceans were anti-phase at the surface and upper levels, but show in-phase tendencies in the pre-summer velocity potential anomaly field. Surface temperatures over southern Africa led to evaporative losses which contributed to a decline in vegetation cover, dam and streamflow levels. The 1992/93 agricultural season was characterised by crop failure and inadequate food resources in some areas. Sugar cane yields in particular were the worst on record during the 1992/93 drought period compared to those of the past three decades. Midmar dam level inflows plunged from 100 mil m3 at the end of 1990 to 0.5 mil m5 during the 1992/93 summer season. The analysis suggests that the 1992/93 drought was not a strong El Nino-induced climatic event. There are signs observed in the velocity potential and divergence fields showing resemblance to an El Nino type of influence but most parameters analysed do not suggest patterns typical of ENSO. The westerly mid-latitude winds coupled with a prominent Hadley cell overturning at 15°S had a profound influence on the occurrence of drought during the 1992/93 summer season than SSTs. Furthermore, budget calculations indicate that kinematic (rotational) properties of the circulation structure had more contribution to the occurrence of the drought than thermodynamic properties. The north-south Hadley overturning between South Africa and the Zambezi implied an anti-phase circulation regime. This together with mesoscale internal dynamics in the meteorological structure of KwaZulu/Natal, sustained the drought for at least three years. As a result, substantial reduction in crop yield and streamlevel inflows had a deleterious repercussions on the community in KwaZulu/Natal.
- ItemClimate variability and predictability in tropical Southern Africa with a focus on dry spells over Malawi(1999) Mwafulirwa, Nicholas Dennis.; Jury, M.R.The climate variability and predictability in tropical southern Africa at inter-annual time scale is studied. Dry spells are identified using Malawi daily rainfall and circulation patterns are investigated at intraseasonal time scale. The study first examines the temporal and spatial variations of Malawi summer rainfall and relationships with global/regional environmental variables are established. Malawi summerrainfall shows that extreme weather has become more frequent in the last three decades. Three oscillations are identified in Malawi summer rainfall; these are the 2.4 and 3.8 year cycles followed by the 11.1 year cycle. These cycles may be associated with the Quasi-Biennial Oscillation (QBO), El Nino/Southern Oscillation (ENSO) and solar cycle respectively. The summer rainfall indicates strong correlation with Nino3 sea surface temperature and Indian Ocean outgoing longwave radiation (OLR), three to six months prior to the rainy season. Malawi pentad summer rainfall at intraseasonal time scale shows similarity in distribution with other regional pentad summer rainfall, (e.g., onset and cessation dates). Results from spectral analysis of daily rainfall indicate two major cycles, namely the 10-25 and the 30- 60 day cycles. Results from composite analysis, using the NCEP reanalysis data set, reveal distinct circulation patterns prior to occurrence of dry summers over Malawi. The circulation patterns prior to dry summer are dominated by westerly flow which changes latitude, causing subsidence. Below (above) normal sea surface temperatures are observed across the east Atlantic(central Indian Ocean), a pattern typical of El Nino conditions. Pronounced below normal geopotential heights occur during dry summers to the south of Africa and east of Madagascar with corresponding south westerly wind anomalies. Results from case studies of dry spells at intraseasonal time scale indicate the prominence of deep low pressure cell east of Madagascar and the absence of a well defined Inter-tropical Convergence Zone (ITCZ) over southern Africa. Dry spells are dominated by high pressure cell over Botswana in conjunction with divergent southerlies and subsidence over southern Africa. Results from composites reveal possible predictors for Malawi summer rainfall: zonal winds, geopotential heights east of Madagascar, SST in the central Indian Ocean and in the south east Atlantic Ocean, and low-level velocity potential over northern Madagascar. Results from multivariate regression analysis show that lowest predictability is found in early Malawi summer rainfall (NDJ) while highest predictability is found with FMA rainfall. The highest predictability for November-April (NA) Malawi summer rainfall is associated with sea surface temperature in the southeast Atlantic Ocean and Indian Ocean, sea level pressure over Indian Ocean and QBO in JAS months. This study has contributed to the understanding of summer rainfall in tropical southern Africa. The knowledge gained can be used by decision makers, farming community, water resource managers for planning and operational purposes. Further statistical forecast models could be developed from precursors (predictors) identified in the study to assist in mitigating the negative effects of climate variability.
- ItemMarine weather variability, tropical cyclone prediction and impacts in the Southwest Indian Ocean(2005) Chang-Seng, Denis; Jury, M.R.The southwestern Indian Ocean is characterized by significant inter-annual climate variability and numerous tropical cyclones. Long-and short-term variability and associated oceanic and atmospheric fields are analyzed using model-assimilated data. The evolution of monsoon circulations associated with composite tropical cyclones distinguished by trade is studied. Case study tropical cyclones are investigated and impacts evaluated through comparison of Yea!' and 'model' datasets. The seasonal variability of intense TC is studied through composite, statistical correlation analysis, cross-modulus wavelet-filter, hovmoller analysis and multi-variate modeling. The intense TC index is characterized with biennial to decadal cycles that may be related with the QBO and the ocean thermohaline circulation respectively. The decade 1960-69 was the most active while 1980-89 was the least active in terms of intense tropical cyclone days in SWIO. New predictors are uncovered that significantly improve the seasonal prediction of intense tropical cyclones. Te new multivariate models are performing about 42 % better than the previous model of jury et al (1999) in the period 1960-2002. One predictor is the geo-potential height (explaining 31 % TC days variability) in the southeast Pacific. It appears to foretell of downstream oscillations in the sub-tropical jet stream which bifurcates and govern wind shear over the Indian Ocean. A duct pattern seems to promote negative vorticity anomalies of -2.0X1061 and convergence anomalies of +1.2X106s *over SWIO. An anti-phase association between South America monsoon convection and intense TCs is found through the Atlantic Zonal circulation especially in the ENSO years. A mild La Nina type of mode is favorable for more TCs. A westward propagating cyclonic circulation is coupled with a transient ocean Rossby wave in respect of the year-to-year variability in tropical cyclone days in the SWIO. The fish resources of the SWIO are negatively associated with intense TC days, e.g. more TC implies lower catch rates. The east South African fish catch also responds to the GPH wave three patterns. Tire daily variability is investigated through composite, hovmoller analysis, standard empirical tables and equations. Daily sequences of composite conditions for tropical cyclones moving west, southwest and southward are compared. A link is established with the northern sub-tropical jet stream that may influence the intensity and track of cyclones in the SWIO. The temporal and spatial variability of TC rainfall and radial winds is significant. Westward and southwestward-moving TCs have rain bands of rainfall intensity of 30 mm h1 that affect the equatorial region. Southwestward-moving TC maintains its intensity while southward-moving TC endures rapid kinematic and therrnodynamfc transformation. Westward-moving TC suffers from the blocking effect of Madagascar highlands. In our case study analysis, it is found that the NCEP model consistently underestimates wind speed by a factor of two within a 300 km radius of tropical cyclones, when compared to QuikSCAT satellite winds. As a consequence swell-driven storm surges are also underestimated. Infrared and microwave satellite rainfall comparisons are done and results are presented.
- ItemMechanisms of inter-annual rainfall variability over tropical highlands of Africa and its predictability potential(2002) Mpeta, Emmanuel Jonathan; Jury, M.R.Modes of climate variability over tropical Africa and adjacent Atlantic and Indian Oceans are investigated using continental monthly averages of gridded rainfall and temperature and marine environmental data, which consists of monthly averages of sea surface temperature, sea level pressure and zonal and meridional wind components. Rainfall and temperature data are gridded at a resolution of 2° x 2° latitude, longitude and based on conventional station data reports (University of East Anglia CRU data set) in the domain 10°N-35°S over Africa except Madagascar. The environmental data is at a resolution of 10° x 10° latitude, longitude and based on ship reports (COADS) ia the domain 30QN - 40°S, 70°W - 100°E. The data sets are subjected to Principal Component Analysis (PCA) using the correlation matrix technique, resulting in the gridded data being standardized. The data are analyzed for annual cycle and the departure therefrom (i.e. Inter-annual fluctuations). The time-varying spectral energy nature of time series is investigated using wavelet analysis technique. PCA (with annual cycle) yielded 12 and 7 homogeneous rainfall and temperature regions, over the African continent south of 10°N, which explained a total of 81 and 91 variance respectively. PCA on marine environmental parameters over the Atlantic and Indian Oceans revealed a number of homogeneous regions, some of which could be associated with synoptic systems. Similarly, PCA was performed on the same data after removing the annual cycle.-The time evolution of these PC modes or time scores were shown to oscillate with periods of 2-4, 4-6, 8-12 and 14 -16 years. Long period oscillating modes were revealed over tropics, particularly the Atlantic Ocean. Possible causes of rainfall and temperature variability over the tropical highlands of Africa were investigated by correlation analysis between rainfall and temperature, and environmental parameters and other indices at lag 0, using monthly data. Some modes of marine environmental parameters over the Atlantic and Indian Oceans have shown good association with the climate over the tropical highlands of Africa up to three months. Results also show that modes over tropical equatorial belt of the Atlantic and Indian Oceans explain rainfall variability up to about 25% for some areas over Africa. Temperature variability over the continent is associated with SST modes over the equatorial tropical oceans and some of these explain more than 64% of temperature variability. ENSO has also shown good association with temperatures over the continent while QBO is poorly associated with temperatures over the continent. Analysis results have shown further that rainfall variability over east and southern Africa regions is associated with the variability of SST, SLP and U wind component in the box 60°E~90oE,5°N-10oS, over the Indian ocean. Time delay analyses have shown that generally environmental parameter indices lead rainfall for between 2 to 20 months. Results indicate that there is no clear picture as to which parameter leads the other, as oscillations are not always in phase or exhibiting constant lag. The un-harmonious nature of these features could sometimes undermine predictability in statistical modelling. So the idea of reviewing predictor-predictant relationships in a time varying manner is well justified. Some parameters extracted from key areas show persistency and stability up to three months while the Nino3 index has shown persistency and stability of up to 6 months. Results have also revealed good associations between some environmental parameters over some key areas of the Atlantic and Indian Oceans and rainfall over some areas of tropical Africa, at 0 to 3 month lags. Analyses results have shown that the potential of forecasting rainfall over some areas of tropical highlands of Africa exists.
- ItemSatellite derived rainfall over tropical Africa : its pattern variability and calibration(2004) Tazalika, Lukiya; Jury, M.R.The major spatial and temporal structure of rainfall and zonal wind at seasonal and intra-seasonal time-scales over tropical Africa are identified by principal component analysis. Pentad CMAP rainfall and NCEP 700hPa zonal wind for the period 1980-2000 were used. Wavelet analysis is used to filter the data sets for cycles between 20-70 days for the intra-seasonal oscillations. The first mode of seasonal rainfall that explains 36% of the total variance is dominated by the annual cycle associated with the ITCZ and is loaded over equatorial Africa. The dominant mode for the zonal wind is loaded over eastern Africa with an extended axis in the Indian Ocean (25.8%). A strong relationship exists between the zonal wind and rainfall modes at the seasonal time scale. The dominant mode for the intra-seasonal rainfall is loaded over northern Congo (29%). The second and third modes are over northeast Angola and East Africa respectively. The intra-seasonal oscillations are phase locked to the seasonal cycle and reveal cycles of 30-50 days typical of Madden Julian oscillation. The 700hPa zonal wind intra-seasonal oscillation reveals a dominant mode over the equatorial east Atlantic Ocean. The second and third modes are over the west Indian Ocean and southeast Atlantic. A strong relationship between zonal wind over the equatorial Atlantic and rainfall over both northern Congo and east Africa is observed. Cross correlation reveals that the zonal wind leads rainfall by a few pentads. Hovmoller plots for OLR and velocity potential reveal that the intra-seasonal oscillations over tropical Africa are dominated by eastward propagation. However, standing and westward propagation are also observed. Propagation is weak over east African coast due to strong meridional flow of the Indian Monsoon. Results from the satellite rainfall calibration and validation are examined. While satellite radiance values generally underestimate rainfall over Uganda, they naturally provide better spatial distribution than the gauges. Following calibration algorithm development, validation tests indicate that satellite rainfall explains 81% of rainfall received in the year 2000. These results should be interpreted with caution since rain gauges give point measurements while satellites provide pixel values.