Geography and Environmental Studies

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Browsing Geography and Environmental Studies by Author "Jury, M."

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    An evaluation of lake and river levels as indicators of climate variability in tropical Southern Africa
    (1999) Gwazintini, Maxwell Esau; Jury, M.
    Southern Africa has only a few.large lakes, one of which is Lake Malawi. It forms part of the lower Zambezi River catchment and is part of the Great Rift Valley. Lake Malawi plays a significant role in socio- economic development particularly in food production, health, energy, transport, recreation, and environment. Changes in its level are related to variations in rainfall and the surrounding atmospheric circulation. Runoff is greatest in late summer following an active-spell of the Inter Tropical Convergence Zone (ITCZ). This study looks at differences between years when the inflow to Lake Malawi is above and below normal using CEP reanalysis composites. An understanding offactors governing variations ofwater resources will be useful to hydrologists and resource managers who deal with seasonal and inter-annual fluctuations. To this end monthly lake level and rainfall data were analysed for variability in the period 1915 to 1995. From the lake level time series, differences in the minimum and maximum level from beginning to and of summer were calculated, then years with high inflow and low inflow were identified in the period overlapping with the NCEP reanalysis data 1958 - 1998. The Zambezi annual flows have been included to study regional coherence ofclimatic variability. Composites were generated during and prior to the season for sea surface temperature, outgoing long-wave radiation, wind, pressure, and circulation derivatives in the area 50S to 10N and 30W to 100E. The mean annual cycle oflake levels contains a peak in April-May and low point in November-December. The inflow index ( Maximum - Minimum) is well correlated with adjacent station rainfall (r =0.51 to 0.66). Spectral analysis on the inflow index indicates a cycle of 9.58 yrs in the period 1938-1976; and a lower frequency oscillation of 19.16 yrs in recent decades 1976-1995. Secondary oscillations occur at periods of 14.5 yrs, 8.3 yrs, 5.8 yrs (ENSO) and 2.05 yrs (QBO). The 9.58 yrs cycle is consistent with that of Abu-Zeid et al (1992) for the Nile River. Composite years include: Wet 1962,1963,1974,1978,1979,1989; and Dry 1959,1967,1972, 1977,1992,1994. Surface press.ure and wind show significant anomaly patterns for wet minus mean composites. The sea surface temperature patterns alternate over the Atlantic and Indian Oceans particularly in the 30S - 40S belt south east of Africa. Patterns in the precursor seasons: Sept - Nov and lun - Aug enable development of predictive models for Lake Malawi inflow at 3 to 6 months lead time.
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    The evolution of coastal lows along the south coast of South Africa
    (2005) Carter, T.J.; Jury, M.
    Coastal lows occur frequently along the South African coast. Coastal low passage along the coast of southernmost Africa is signaled by a marked change in both wind direction and speed. Occasionally, a dramatic and rapid increase in wind speed takes place, which may cause damage to property and is potentially hazardous to aviation and shipping. Current global scale numerical weather prediction models do not forecast the intensity of these mesoscale systems well. A study of the coastal low is made with the aim of improving knowledge about the coastal low in general, and of strong coastal lows in particular, in order to improve the ability to predict the strong to gale force southwesterly winds associated with the more intense coastal lows. An average climatic profile of the coastal low is established in order to study the basic nature and evolution of the coastal low. This profile is compared against a similar profile of strong cases in order to locate mesoscale and synoptic scale elements that distinguish between normal and strong coastal lows. Links between intense coastal lows, and higher than normal pre-coastal low temperatures at surface and in the lower levels of the atmosphere over the southern parts of the country, are examined. The upper level northwesterly winds, associated with the upper troughs that follow closely behind the coastal lows, are a contributing cause in the development of stronger coastal lows on the south coast.
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    Mechanisms and prediction of climate variability in tropical North Africa
    (University of Zululand, 2003) Yeshanew, Adebe; Jury, M.
    One of the monsoon regions of the world is the tropical North Africa. The Sahara Desert lies in contrast with the cool South Atlantic. The monsoon systems control the mean circulation of this region. Superimposed on this basic state, large-scale variability dictates the life of the people and their socio-economic activities. The tropical North Africa climate exhibits a spectra! energy mainly in the ENSO and decadal temporal band as revealed by wavelet transform. The lowland Sahelian climate swing reveals low frequency signals. The mountainous regions of tropical Northeast Africa exhibit higher frequency variability. This variability has one common factor: a large-scale east-west overturning that connects the Pacific and Atlantic. An upper-level velocity dipole is established that induces convection polarity between tropical North Africa and South America. The strength and the sign of opposing poles are determined by the Atlantic and Walker Circulations. This is verified using correlation based on longer timeseries (1950-2000). ENSO signal modulates tropical North Africa climate by surpassing other tropical SST through these Circulations. Tropical Atlantic SST modes and Indian Ocean SST dipole influence tropical North Africa climate variability through the connection of Atlantic Circulation. The impacts of these SSTs are more pronounced during non-ENSO years as their influences are masked by global ENSO mode of variability. The modulation of transverse Monsoon Circulation (in Indian Ocean where Tropical Easterly Jet is the upper limb) on this part of Africa climate operates in phase with Atlantic Circulation. More than 80% of the variance of the Sahelian climate variability is associated to this circulation. It as well imparts equally the Brazilian rainfall following the sign of the Atlantic Circulation velocity potential. One of the aspects that the Indian Ocean differs from other east-west Circulations is that the Monsoon Circulation leads the global ENSO in coherent mode and it explains 60% of the indo-Pacific SST variation. Locally, the African Easteriy Jet determines the north-south moisture and convection between Sahel and Guinea through Hadley Circulation. To understand the ocean's role in the tropical North Africa and South America convection polarity, subsurface thermocline temperature and heat content are analysed using singular value decomposition, correlation and composite analyses. One of the main results that come from these analyses is that the convection over tropical North Africa and South America are closely tied to subsurface properties of the tropical oceans. The nnost important ocean signal that is sensitive to Atlantic Zonal Circulation convection is the east-west sea-saw of the equatorial thermocline. The east-west upper-ocean dipole is manifested in the leading EOF modes in thermocline temperature and in heat content anomaly (HCA) in the Pacific Ocean and Indian Ocean. In the Atlantic however, the main climate signal is in the kinematic fields. In developing predictive equations for tropical North Africa climate variability, stable predictors were found: lower-level Atlantic and Pacific zonal wind. The key factor that leads to high hit rates in the prediction models is the 'memory' and stability of the equatorial ocean winds. The kinematic predictors outperfonn SST in hindcast fit by 33% with respect to Sahelian climate and river flow. The multi- decadal oscillation of angular momentum is shown to play a role in the predictability. The study therefore contributes to understanding of the climate variability and prediction of tropical North Africa climate by inclusion of the kinematic component of the climate system that is the means of ENSO transmission to Africa.
University of Zululand