Mechanisms and prediction of climate variability in tropical North Africa
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Date
2003
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Publisher
University of Zululand
Abstract
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.
Description
Submitted to the Faculty of Science in partial fulfillment of the degree of Doctor of Philosophy in the Department of Environmental Studies at the University of Zululand, South Africa, 2003.
Keywords
Climate variability