Causes, structure and impacts of the 1992/93 drought in KwaZulu/Natal
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1998
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Abstract
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.
Description
Thesis submitted in fulfillment of the requirements for the degree of Doctor of Science in the Department of Geography and Environmental Sciences at the University of Zululand, South Africa, 1998.
Keywords
Drought forecasting--South Africa, Droughts--Kwazulu/natal (South Africa)