| dc.description.abstract | The period between September 1961 and January 1962 saw
the most severe floods experienced in East Africa (Kenya,
Uganda, Tanzania) this century. Flooded rivers damaged both
road and rail communications, agricultural activities were
disrupted and inhabitants of the worst-hit areas were rendered
homeless. Lake levels rose drastically, lake Victoria having
a record rise of about 1.8 metres by June 1962. An estimate
of the total loss and damage in Kenya alone was quoted at
about 40 million Kenya shillings.
This study attempts to establish the characteristic
features of the weather systems that gave rise to the extremely
heavy rains which culminated into the widespread floods of
1961/62 in East Africa. To"achieve this objective we first
establish the monthly mean wind flow patterns in the lowest 7 kID
AMSL over East Africa. The main weather disturbances during
the 1961/62 flood episode are then identified.
Daily wind vectors for 31 pilot balloon stations in
East Africa have been used to establish the mean wind flow
patterns. Data for 24 stations has been utilised in determining
the 1961/62 wind field for the weather disturbances.
The monthly mean wind fields are presented and discussed
under the four well-known seasons in East Africa
the northeast
monsoon season (December-February), the long rains season
xvi
(March-May), the southeast-southwest monsoon season (June-
August) and the short rains season (September-November).
The northeast monsoon air current is evident in the
lowest 3 km AMSL in December.
It reaches maximum development
in January-February. During this period, it extends to the
4 km level. The current bifurcates over northeastern Kenya
as part of it flows westwards into the interior of Africa
while the other branch flows southwards almost parallel to
the East African coast. The easterly current across northern
Kenya is generally the stronger of the two branches. The
northeast monsoon current begins to decay in March, beginning
first in the lowest levels and working upwards.
The long rains season is characterised by the retreat
of the northeast monsoon current and the establishment of the
southeast-southwest monsoon current. The flow between 700 mb
and 500 mb in April and May.depicts a well-defined convergence
zone (ITCZ) between northerly and southerly currents.
The southeast-southwest monsoon current is evident at
the lowest levels as early as April and May. The current
intensifies progressively and reaches maximum intensity in
July/August. One major feature of the low level flow during
this season is the well-known East African low level jet
-1
which has mean peak speeds of about 13.5 ms in July/August
at the 850 mb level. The southeast-southwest monsoon current
is, like the northeast monsoon current, highly effluent
between 3SoE and 40oE, and confluent in the lake Victoria
region.
Over coastal Tanzania, one branch of the monsoon air
flows westwards to form a speed maximum of about 8 ms over
central Tanzania. Similar branching is observed over northern
Kenya. The SE-SW monsoon current influences East Africa for
a longer period (April-October) and is more intense than the
northeast monsoon current (December-March). However, the
former is shallower (surface-3 km AMSL) than the latter which
extends to the 4 km level during its period of maximum
intensity.
There are two weather phenomena which are persistently
evident in the lower tropospheric flow over East Africa during
the SE-SW monsoon season. These are the cyclonic disturbance
over the lake victoria region and western Kenya/eastern Uganda
and the 600-500 mb level anticyclone in the vicinity of Nairobi.
The observed active weather conditions in the lake Victoria/
western Kenya region durin~July/August, an otherwise dry
period for the rest of East Aptica,is due to interactions
between land-lake breezes and this cyclonic disturbance. The
dry westerly winds which have been referred to in previous
studies are found to be pa~t of the outflow from the mid-
tropospheric anticyclone that is centred near Nairobi.
The short rains season is characterised by the retreat
of the SE-SW monsoon current in September-October and the
establishment of the ITCZ.
The divergence patterns associated with the mean flow
show that the monsoon currents are not only diffluent but
also highly divergent in the eastern low-lying parts of Kenya
-5 -1
Maximum divergence is about 3xlO s and isand Tanzania.
observed at the 850 mb level over eastern Tanzania and eastern
Kenya. This area of maximum divergence at the 850 mb level,
roughly coincides with the well-known zone of rainfall deficiency
in Kenya and Tanzania. During the two wet seasons, the ITCZ
-5 -1
is depicted as a zone of maximum convergence (_1.5xlO s ) with
-2 -1 .
ascending motion of upto 4xlO ms at the 500 mb level. The
ITCZ has a slight equatorward tilt with height.
The onset of the 1961/62 flood episode was marked by
dramatic 2-3 day intense falls at the Kenyan coast towards the
end of September 1961.
The major weather disturbances in September and October
1961 included anticyclones southern Tanzania and off
the northeastern part of East Africa. Outflow from the anti-
cyclone over southern Tanzania was well-organised and converged
into~he central parts of East Africa. The low level air
flow in this region was characterised by well-marked air inflows
in the entire lower troposphere. This was capped aloft by a
well-organised air outflow. This configuration gave rise to
the heavy rainfalls during October 1961.
The wettest month during the flood period was November
1961. During this month, the whole of East Africa was engulfed
by an anomalous westerly air current which converged into the
vertices that are found in the ITCZ over the Kenya highlands
and off the Kenyan coast. The air current was convergent in
a deep layer extending to the 500-400 mb levels. The magnitude
of convergence associated with the westerly perturbation was
-5 -1
about 1.5xlO s while the maximum upward motions were about
-2 -1
5xlO ms .
In the next two months, that is, December 1961 and
January 1962, the westerly disturbance intensified and the air
current converged into the ITCZ vortices now centred over
southeast Tanzania/northern Mozambique. The westerly
disturbance that prevailed during this period gave rise to a
westerly flow which completely replaced the normally diffluent
northeast monsoon current in December and January. The origin
of the westerly air flow, which was dominant in December 1961
and January 1962, seems to have consisted of two sources
the Congo/SE Atlantic westerlies and a recurved northeast
monsoon air current from the southwest Indian Ocean . | en |
