| dc.description.abstract | Many socio-economic activities over the Equatorial Eastern Africa
(EEA) region are rain-dependent. Occurrences of rainfall anomalies
have therefore been associated with losses of life, property and many
other socio-economic lniseries. The ITCZ has been recognised as the
major large-scale system controlling the seasonal rainfall over the
region.
In this study, the objective was to investigate the space-time
evolutions of the ITCZ and the associated circulation patterns over
Equatorial Eastern Africa during normal and anomalously wet and dry
years. The understanding of space-time evolutions of ITCZ is an
important step towards the development of the medium and longer range
rainfall forecasting in the region. The study was concentrated within
the months of March-May, which constitute the major rainfall season
over the region normally referred to by local climatologists as the
Long Rainfall Season.
The major data sets used included daily and monthly station
rainfall, gridded Outgoing Longwave Radiation (OLR) , monthly Sea
Surface temperatures (SSTs), and the European Centre for Medium-range
Weather Forecasts (ECMWF) Global Analyses data sets for wind at
various levels of the atmosphere. The data used extended within the
period 1961-1990.
Statistical and diagnostic methods were used to examine the
space-time evolution of the ITCZ including determination of the
anomalous wet/dry composite years, onset, withdrawal, duration and
other space-time evolutions of the various anomaly spells. The
statistical methods used included Composite, Principal Component
Analysis (PCA) , and Time-Cross Sectional analyses. The general
circulation patterns associated with the dry/wet composites were also
investigated. These patterns were derived from kinematic parameters
and other basic wind fields including vorticity, and divergence.
The years which were identified as representive of the typical
wet/dry cases, based on both simple rainfall indices and rotated
principal component T-mode analysis, were 1981/1984. The wet/dry
pentads during each of the two years were used In the composite
analysis.
The seasonal evolutions of the centres of convective activity,
ITCZ and the rainfall belt during a normal Long rainfall season and
the anomalously wet/dry cases were obtained from the analysis of the
station rainfall and gridpoint OLR records. From these data, distinct
northward/eastward evolution patterns of the zonal/meridional arms of
the ITCZ and convective activity over the EEA region during the long
rainfall season were delineated.
The results from the vector wind analyses showed that during
normal years, maximum intensity of westerlies was generally observed
at 700 hPa level, while easterlies were dominant at the upper levels
during the Long-rains season. This alignment of mid-level westerlies
and upper level easterlies links active convection with negative zonal
wind shears in the layer 700-200 hPa. The anomalously wet/dry spells
were associated with low-to-mid-level westerlies/easterlies over most
parts of the region.
Composite results of vector wind analyses identified close
positive associations between low-to-mid-level westerlies/easterlies
over most of the EEA region and southerly/northerly winds through the
Mozambique Channel. The southerlies were associated with the formation
and persistence of cyclonic/anti8yclonic circulations over
Madagascar/South Africa regions while the northerlies were associated
with anticyclonic/cyclonic circulations over Madagascar/South Africa
respectively.
Composite results of relative vorticity and horizontal velocity
divergence showed that maximum positive relative vorticity in the
lower-to-mid-troposphere and maximum positive divergence in the upper
troposphere were associated with the belt of maximum convective
activity.
Inspection of SST anomaly maps showed that there were anomalously
warm/cool waters over the West Indian Ocean/Eastern Atlantic Ocean
during the wet/dry years.
The results from the vertical cross sections of zonal wind speeds
revealed the transition of westerly/easterly zonal winds .in the
lower/upper troposphere across the equatorial eastern Africa region.
This vertical alignment of zonal wind is typical of tropospheric zonal
winds in the monsoonal regions and therefore the observed middle
level westerlies over EEA region during the long rainfall season are
deduced to be indicative of the monsoonal transition in this region.
This study has delineated unique atmospheric and oceanic
circulation patterns associated with anomalous ITCZ/rainfall
evolutions in the region. These patterns included anomalous basic
flow, divergence, vorticity, and pools of cold and warm waters. These
patterns may be used to develop a seasonal prediction regional model
for monitoring and forecasting of above/below normal (anomalous)
rainfall development over the region. Such information is crucial to
the development, planning and mitigation of all rainfall dependent
activities, and will help the governments of the EEA region to
minimise the severe socio-economical implications of the abnormal
rainfall events which are rampant in the region. | en |
