Geography and Environmental Studies

Permanent URI for this collection

https://hdl.handle.net/10530/361

Browse

Browsing Geography and Environmental Studies by Subject "Climate variability"

Now showing 1 - 2 of 2
  • Thumbnail Image
    Item
    Climate 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.
  • Thumbnail Image
    Item
    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