On The Monthly Mean Lower Tropospheric Circulation And The An0i1alous Circulation During The 1961/62 Floods In East Africa
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 .