R E Okoola

Dr Pierre Camberlin

J M Ininda

Wet periods along the East Africa Coast and the extreme wet spell event of October 1997

Extreme wet spells affect the East Africa Coast (EAC) during March to June (long rains) and October to December (short rains). While these spells are less frequent during the short rains, some of the most extreme wet spells occur at this time of the year. The present study examined the general characteristics of the wet spells during the short rains. A detailed study of the anomalous wet spell event of October 1997, with record rainfall around Mombasa (4.0°S, 39.6°E), was also carried out.

Daily rainfall for 1962 -1997 and NCEP2 reanalysis data for 1979 -1997 were used to study the characteristics of the wet events. A high spatial coherence is found in the rainfall over the EAC. The circulation features that were common during most of the wet events were: weakening or reversal of the east-west (Walker type) circulation over the Indian Ocean, enhanced convergence between the northern and southern hemisphere trade winds and westward-moving disturbances in the low-level equatorial wind field.

During the 1997 wet event, it is shown that prior to the heavy rainfall event a ridge of high pressure, on the eastern coast of southern Africa, intensified and propagated eastwards leading to the strengthening of moist easterlies reaching the EAC. The zonal wind component along longitude 40°E showed shears in the flows that were associated with the development of the Mozambique Channel low/trough in the lower troposphere round which southerlies surged northwards. These southerlies converged with the easterlies near the EAC. Thus, the warm and wet air from the east interacted with the relatively cold and mainly continental air from the south generating instability at the EAC.

Introduction

The East Africa Coast (EAC, fig. 1) is regularly affected by heavy rainfall events. These events tend to occur within wet spells which simultaneously affect most of the coastal stations. A spectacular example is the wet spell that occurred from 17 th to 22 nd October 1997.

For example, Mombasa-Airport rainfall station received a record 617 mm in this 6 days interval, with over 200 mm on each of the two days, 18 th and 19 th . During this event, the whole of EAC, from Dar-es-Salaam in Tanzania (6°52'S) to the south, to Lamu in Kenya (2°16'S) to the north, was affected by very heavy rains. These rains had enormous socioeconomic impacts in the coastal region: 18 people died and many thousands were rendered homeless, more than 20 bridges were washed away, and sections of the Nairobi-Mombasa railway in Kenya and the main water pipeline serving the Mombasa area collapsed. These impacts prompted the Government of Kenya, on the 25 October 1997, to declare the coast region of Kenya a national disaster area due to the floods.

The wet spells that occur along the EAC have distinct characteristics from those, which affect the interior. These wet spells are more common during the long rains (March to June along the coast), the main rainy season in this region compared to other seasons [START_REF] Camberlin | Intraseasonal wind anomalies related to wet and dry spells during the "long" and "short" rainy seaons in Kenya[END_REF]. However, they are not uncommon during the short rains (October to December) especially during the El-Nino events. Some case studies of synoptic patterns, which induced a wet spell along the EAC during the long rains and the following austral winter, are available, suggesting the intervention of easterly waves and remnants of cold fronts from the Southern Hemisphere [START_REF] Lumb | Synoptic disturbances causing rainy period along the East African Coast[END_REF][START_REF] Fremming | Notes on an easterly disturbance affecting East Africa[END_REF][START_REF] Gichuiya | Easterly disturbances in the South-East monsoon[END_REF][START_REF] Okoola | Westwards moving disturbances in the Southwest Indian Ocean[END_REF]. However, few studies are available on the wet spells that occur during the short rains season. Moreover some wet spell events such as the one that occurred during the short rains of 1997 are so unique as to be attributed to the general mechanisms that cause rainfall during this season. October-December 1997 is recorded as one of the wettest period of the last 100 years over most of East Africa [START_REF] Wmo | World Meteorological Organisation statement on the status of the global climate in 1997[END_REF]. Although the anomalous rainfall recorded in this year attracted many studies (e.g., [START_REF] Birkett | Indian Ocean climate events bring floods to East Africa's lakes and the Sudd Marsh[END_REF][START_REF] Goddard | Importance of the Indian Ocean for simulating rainfall anomalies over eastern and southern Africa[END_REF][START_REF] Webster | Coupled oceanatmosphere dynamics in hte Indian Ocean during 1997-98[END_REF][START_REF] Black | An observational study of the relationship between excessively strong short rains in Coastal East Africa and Indian Ocean SST[END_REF][START_REF] Hastenrath | Circulation mechanisms of climate anomalies in the equatorial Indian Ocean[END_REF], showing a relationship with sea-surface temperature patterns in the Indian Ocean, little is known about the within-season organisation of the rains, especially the conditions which led to the extreme wet spells.

There is therefore need to understand the mechanisms that were associated with the 17-22 October 1997 extreme rainfall event in particular, and the coastal wet spells in general. [START_REF] Mutai | East African Rainfall and the tropical circulation/convection on intraseasonal to interannual timescales[END_REF] found evidence of eastward propagating circulation anomalies from the equatorial Atlantic, in association with intraseasonal rainfall variations during the 1979-1996 short rains, but these results applied to a much larger East Africa region. [START_REF] Kijazi | Relationships between intraseasonal rainfall variability of coastal Tanzania and ENSO[END_REF] analysed intraseasonal rainfall variability in coastal Tanzania; however, like [START_REF] Mutai | East African Rainfall and the tropical circulation/convection on intraseasonal to interannual timescales[END_REF], they considered time-scales longer than those relevant to the present study.

While there is a strong coherence in the interannual variability of the short rains throughout East Africa [START_REF] Ogallo | The spatial and temporal patterns of the East African seasonal rainfall derived from principal component analysis[END_REF][START_REF] Beltrando | Space-time variability of rainfall in April and October-November over East Africa during the period 1932-83[END_REF], the intraseasonal timescale shows more spatial variability during this season, as was the case of 1997. The coast is actually known to be strongly affected by the large-scale changes which take place over the equatorial Indian Ocean in some years, resulting into very wet conditions [START_REF] Hastenrath | Atmospheric-hydrospheric mechanisms of climate anomalies in the western equatorial Indian Ocean[END_REF][START_REF] Black | An observational study of the relationship between excessively strong short rains in Coastal East Africa and Indian Ocean SST[END_REF][START_REF] Clark | Interdecadal Variability of the Relationship Between the Indian Ocean Zonal Mode and East African Coastal Rainfall Anomalies[END_REF][START_REF] Hastenrath | Circulation mechanisms of climate anomalies in the equatorial Indian Ocean[END_REF]. For 1997, a preliminary investigation of daily rainfall distribution suggests that in the interior the wet conditions were much more spread over the season than along the coast, where distinct, organised wet spells seem to be found. It is therefore questioned what synoptic conditions caused the extreme coastal rains of October 1997, and how they fit into the interannual anomaly which affected the whole of East Africa and Western Indian Ocean. More generally, the following two questions arise: (i) how spatially consistent are any wet spells along the coast during the short rains, and how do they relate to rainfall in the interior? (ii) what is the circulation anomaly patterns which generally accompany these wet events? Answering these questions is important in that it may increase our capacity to anticipate such heavy rains and the possible resulting flooding.

After a presentation of the data and methods (section 2), section 3 will show the intraseasonal distribution of the rains along the EAC during the short rains, and a comparison with inland East Africa. The case of 1997 will be presented, and then a more general analysis will be carried out based on 36-yr (1962-1997) daily rainfall data. In section 4 the circulation patterns associated with coastal wet spells will be shown. A separate assessment of the 1997 extreme rainfall will be provided in section 5. Finally, section 6 will discuss the results through comparisons with earlier studies on East Africa rainfall variability.

Data and methods

The data used in this study include daily rainfall totals, daily and twice-daily reanalysis data for a range of atmospheric variables.

Rainfall Data

The rainfall data consists of daily records for the period 1962-1997 at five stations in Kenya (three along the coast: from north to south, Lamu, Malindi-Airport, Mombasa-Airport, and two inland: Voi and Makindu), and two in coastal Tanzania (Tanga and Dar-es-Salaam)(fig. 1). These daily rainfall data were obtained from the Kenya and Tanzania Meteorological Services. The short rains season, October to December (OND), was retained.

2.2

The NCEP-DOE AMIP-II Global Reanalysis Dataset

This study also utilised the National Centre for Environmental Prediction -Department of Energy (NCEP-DOE) Atmospheric Model Intercomparison Project (AMIP-II) reanalysis fields, described in detail by [START_REF] Kalnay | The NCEP/NCAR 40-Year Reanalysis Project[END_REF] and [START_REF] Kanamitsu | NCEP-DOE AMIP-II Reanalysis (R-2)[END_REF]. The spatial resolution is 2.5° latitude by 2.5° longitude. Data were extracted for the low and mid-latitude sector 50°N to 50°S, and between longitudes 30°E to 110°E. They cover the period 1979-1997. This dataset is an updated NCEP-NCAR reanalysis, featuring newer physics and observed soil moisture forcing, and eliminating several previous errors.

The general circulation parameters derived from these records include wind fields, mean-sealevel pressure (MSLP) and geopotential heights and the associated wind derived parameters at all mandatory levels from 1000 to 100 hPa (300 hPa for relative humidity). These fields are influenced by the observed data. Some use was also made of vertical velocity, but it should be noted that this variable is not as reliable since it is dependent on the model's parameterization [START_REF] Kalnay | The NCEP/NCAR 40-Year Reanalysis Project[END_REF]. Both daily and twice-daily means were used. The latter were for morning (average for 00 and 06 UTC) and afternoon (average for 12 and 18 hours UTC) mean values. Many researchers including [START_REF] Mutai | East African Rainfall and the tropical circulation/convection on intraseasonal to interannual timescales[END_REF], [START_REF] Camberlin | The East African March -May Rainy Season: Associated atmospheric dynamics and predictability over the 1968-97 period[END_REF], [START_REF] Zorita | Rainfall variability in northern Tanzania in the March to May season (long rains) and its links to large-scale climate forcing[END_REF] among others have used the NCEP datasets for various studies over the region. These researchers have shown that the NCEP analyses were good approximations of the real flows over Africa and the adjacent ocean areas.

Methods

The relationship between rainfall variations in the diverse stations of the region under study, and between rainfall and atmospheric circulation, was studied based on simple correlation analysis and composite analysis, using data for 1962-1997 (rainfall) and 1979-1997 (atmospheric circulation). The composites were made of a selection of wet days along the coast, as defined below. Maps showing the difference between the mean circulation found during those days and the long-term mean were plotted, with significance testing using the Student's t-test.

For the 1997 case study, cross-sectional analyses of Mean Sea Level Pressure (MSLP) and wind data were used in order to study the propagation of disturbances across the western Indian Ocean. These were time-longitude, and latitude-time sections. Also, moisture flux and the horizontal wind at various levels averaged between latitudes 2°S and 5°S and longitudes 39°E to 43°E were plotted as height-time sections to reveal the variation of horizontal wind and moisture flux with height in the EAC area.

Rainfall patterns along the East African Coast during October to December season

Equatorial East Africa experiences two distinct rainy seasons. These are the long rains season, which occurs during March to May, and the short rains during October to December.

At the coast however (fig. 2), the long rains generally extend into June and the short rains are much lower in amount compared to both the long rains and the stations further inland. The lower amount during the short rains may be largely due to the direction of the prevailing wind flow, which is orthogonal to the coastline in the long rains but more parallel to it in the short rains. An additional factor is the sea-surface temperature, which is much greater than the land temperature in the long rains [START_REF] Camberlin | Coastal precipitation regimes in Kenya[END_REF]. The short rains are also characterised by a very large interannual variability.

The monthly rainfall totals for 1997 are superimposed on the mean monthly rainfall in Figure 2 to show the extent of rainfall differences during a very wet year (1997) and the normal year, especially during the short rains months. Heavy seasonal rains were also observed during 1982during , 1967during , 1972during and 1977during . It happens that the years 1997during , 1982during , 1972during and 1977 were were El Nino years confirming the strong relationship between El Nino and rainfall at the EAC during this season [START_REF] Farmer | Seasonal forecasting of the Kenya Coast short rains[END_REF][START_REF] Ogallo | Relationships between seasonal rainfall in East Africa and the southern oscillation[END_REF][START_REF] Indeje | ENSO Signals in East African Rainfall Seasons[END_REF]. However, a few heavy rainfall years (1967 and, outside the period under investigation, 1961) do not coincide with El Nino events.

The distribution of daily rainfall at the three Kenyan coastal stations of Mombasa, Malindi and Lamu, for OND 1997, is presented in fig. 3. There is a clear grouping of the wet events at the three stations. The 17 -22 October stands out as a very wet sequence. Rainfall even reached 755 mm in those 6 days at Mtwapa, between Mombasa and Malindi, with a record 337 mm on 18 th October. Further south in northern Tanzania heavy rainfall was also recorded: Tanga (5°05'S) received 145mm on 20 th October, Dar-es-Salaam 150mm on 19 th .

Voi, in Kenya at 110 km from the coast recorded much less rainfall (45 mm in the same 6day period). Further inland at Makindu and Garissa less than 10 mm were recorded. More precipitation occurred in the Kenya Highlands and in northern Kenya, but the amounts remained much lower than those recorded along the coast. Later in the season, abundant rainfall continued. Definite spells are harder to track down; some events appear to be synchronous inland and on the coast, others not. In view of the specificity of the 17-22

October 1997 heavy rainfall event along the coast, this sequence will be selected for intensive study in section 5.

The 1997 case suggests that there is a preference for the wet events to occur relatively synchronously along the coast. In order to assess whether this is a recurrent feature, correlations between daily rainfall time-series from both coastal and inland stations have been computed for OND 1962-1997. All rainfall amounts are first square-root transformed to reduce skewness [START_REF] Bärring | Regionalization of daily rainfall in Kenya by means of common factor analysis[END_REF][START_REF] Stephenson | Extreme daily rainfall events and their impact on ensemble forecasts of the Indian Monsoon[END_REF]. Only the results obtained using the coastal station of Mombasa (at a central location along the EAC) are shown (table 1), but computations using other coastal stations led to a similar interpretation. The correlations (for both daily and 3-day averaged time-series) with the four other Kenyan stations are stronger in the north-south direction (that of the coast) than in the east-west direction. The correlations markedly decline with distance inland. Although some events may co-occur along the coast and inland, there is a tendency for more in-phase rainfall variations in the coastal belt.

A further comparison is done by selecting heavy daily rainfall events (over 20 mm) at the same five Kenyan stations. Such events represent between 11 and 17% of all wet days in any station. When the co-occurrence of heavy rainfall at Mombasa and at the other stations is analysed (table 2), it is found that there is again a strong preference for events to develop simultaneously along the coast, but much less so inland. For instance, whereas only 13% of the days with heavy rains at Voi, inland, are also very wet at Mombasa, the percentage is 36% for Malindi vs Mombasa, 100 km north of Mombasa along the coast, and still 24% a further 140km northward at Lamu. These percentages markedly increase, along the coast, if we consider a three-day period spanning the wet day (not shown). However, on average, the greatest consistency between heavy rains along the coast is obtained at a zero-day time-lag.

Similarly, the consideration of a time-lag between the coastal and the inland stations does not improve the consistency. Similar results were obtained using slightly lower or higher daily rainfall thresholds.

These observations point to partly distinct rainfall mechanisms between the coast and the interior, and to the episodic nature of the rains at the coast, a likely reflection of the occurrence of organised disturbances. Thus, the rest of the study will concentrate on wet spells affecting the coastal area, though it should be remembered that some of these events are also coincident with rain occurrences further inland.

Atmospheric Dynamics associated with wet events

This section presents the mean circulation patterns associated with coastal wet events, in order to provide a background for the case study of the 1997 exceptional rains.

Based on the above results, wet events along the East Africa Coast (EAC) were selected as days recording precipitation in at least 50% of the coastal stations, or rainfall above 5 mm in at least one station. Between 1979 and 1997, 567 such events (a little less than one third of all days) are found. Using NCEP-NCAR DOE-2 reanalysis daily data, a composite was formed of these wet events. Prior to compositing, all reanalysed data were deseasonalised by removing a smoothed (21-day moving average) mean seasonal cycle, computed on the period 1979-1997. Composite fields are therefore expressed as anomalies from the mean seasonal cycle. On the maps (figures 4-8) only significant anomalies are displayed, according to Student's t-test (arrows are proportional to the t value of the test for the zonal and meridional components of wind, geopotential height and temperature are shown as contours, and vertical velocity as shading). The most prominent features associated with the wet events are found at 850 hPa, with relatively similar patterns at 700 hPa. Both 0-lead and 3-day-lead anomalies are retained for display, the latter in order to detect possible precursors. Unfiltered deseasonalised data are first considered, and then a high-pass filter is applied.

On unfiltered data (fig. 4), it is noticeable that wet events are accompanied by large-scale circulation anomalies across the whole Indian Ocean. At zero-lead (d0), a belt of anomalous easterlies covers the whole equatorial Indian Ocean. They are fuelled by two broad highpressure centres on both sides of the equator. Enhanced upward motion is found from the central Indian Ocean to the EAC, with very significant values at the latter location. It does not extend much inland. The rising motion area clearly results from a strong convergence between the northeasterly and the southeasterly trades. Surface fields (not shown) display in the west (east) of the basin an excess (deficit) of moist static energy (which combines latent, potential and sensible energy). This translates into surface easterly anomalies (actual easterlies or weakened westerlies, since surface westerlies tend to prevail at this time of the year over a narrow belt in the equatorial Indian Ocean in association with the ITCZ, [START_REF] Sadler | Vol. I: Indian and Atlantic Oceans[END_REF]. It is noticeable that three days before d0, most features described above are already present, though generally with a smaller magnitude (fig. 5). In particular, the lowlevel easterly anomalies are spread out all along the equatorial Indian Ocean. Anomalous rising motion is already there over the Western Indian Ocean, but it does not reach the EAC.

The persistence of these features, and their pattern, suggest that the signal is mainly related to interannual variability. The large interannual rainfall variations of the short rains have been shown to be associated with large-scale Walker (east-west) circulation changes over the Indian Ocean. Wet years, especially along the coast [START_REF] Hastenrath | Atmospheric-hydrospheric mechanisms of climate anomalies in the western equatorial Indian Ocean[END_REF][START_REF] Mutai | East African Rainfall and the tropical circulation/convection on intraseasonal to interannual timescales[END_REF][START_REF] Black | An observational study of the relationship between excessively strong short rains in Coastal East Africa and Indian Ocean SST[END_REF][START_REF] Clark | Interdecadal Variability of the Relationship Between the Indian Ocean Zonal Mode and East African Coastal Rainfall Anomalies[END_REF][START_REF] Hastenrath | Circulation mechanisms of climate anomalies in the equatorial Indian Ocean[END_REF], coincide with anomalous equatorial easterlies in the lower levels, anomalous rising motion in the west of the basin, anomalous descending motion in the east. These circulation features result from marked changes in the sea-level pressure and surface temperature zonal gradients, well shown in the present analysis. Since the interannual variations of the EAC short rains are very large, and consistent over one season, it is therefore not unexpected that the daily composites of wet events strongly replicate the patterns found at the interannual time-scale.

Filtered data are next analysed (fig. 6). The filter is designed to remove interannual and slow intra-seasonal variations (longer than 70 days) in the circulation variables. At d0, there is a noticeable change from unfiltered data. The low-level equatorial easterly anomalies are not found any more. The trade winds enhancement, present in the unfiltered data, is reinforced.

In the northern hemisphere, it is associated with an elongated ridge covering the whole of South Asia. High geopotential heights are also found in the southern hemisphere, over and around Madagascar. The convergence in the western Indian Ocean is striking. The upward motion anomalies along the East African coast are also conspicuous. They are consistently found at all levels between the surface and 400-300 hPa. In the upper troposphere (not shown), there is evidence of a "double trough" pattern (i.e., one trough on each side of the equator, at the longitude of East Africa). Back in time from d0 to d-3, it is found that the trade wind enhancement is a relatively robust feature throughout the 4-day period (fig. 7).

Rising motion anomalies gradually build up to the south of the equator, in the western Indian Ocean, as a result of the south-easterlies strengthening. The wind gust reaches the Kenya Coast at d0, while it was restricted to the south of it till then. The upper tropospheric double trough is also already installed at d-3. It promotes upper diffluence around 45-50°E.

The comparison between the unfiltered and the filtered maps suggests the following observations. Wet events along the EAC require favourable large-scale conditions on an interannual time-scale (i.e., low-level equatorial easterlies, which prevent moisture from being exported to the eastern Indian Ocean). Once these conditions are met, wet events will occur in conjunction with sustained northeasterly winds over the northern Indian Ocean, and a strengthening of the trade winds in the western Indian Ocean. The development of a high in the southwestern Indian Ocean appears instrumental in it.

In the above composites, all wet days were used. Since some of these days occur in sequence, a separate composite analysis was carried out by selecting those days which represent the onset of a wet spell on the EAC (i.e., all wet days preceded by a dry day, and followed by a wet day as defined from the above criteria, in order to ensure that we were not picking a very isolated rain event). The sample comprised of 125 events. The significant anomalies (fig. 8) are on a much smaller scale than in the previous analyses. However, between d-3 and d0, the 850 hPa maps show an easterly surge in the equatorial western Indian Ocean, which shifts westward from around 75°E (d-3) to reach East Africa (30-40°E, d0). This surge is in connection with a build-up of high geopotential heights to the south, in the Madagascar -Mozambique Channel area. It is found that the pressure rise and associated easterly surge is further fuelled by an influx of dry air from the southern Indian Ocean, east of the Mascarene Islands. The surge induces upward motion anomalies along the EAC, peaking at d0. The upward motion is suggested to result from both horizontal convergence and orographic lifting as the air mass is directed from sea to land. Though not as well defined, such surges are found to be quite similar to the easterly disturbances reported for the northern summer season in the south-western Indian Ocean [START_REF] Fremming | Notes on an easterly disturbance affecting East Africa[END_REF][START_REF] Gichuiya | Easterly disturbances in the South-East monsoon[END_REF][START_REF] Okoola | Westwards moving disturbances in the Southwest Indian Ocean[END_REF]. [START_REF] Okoola | Westwards moving disturbances in the Southwest Indian Ocean[END_REF] found the area just south of the equator and between 60-75°E to be the source of these easterly disturbances. [START_REF] Kijazi | Relationships between intraseasonal rainfall variability of coastal Tanzania and ENSO[END_REF] documented irregular westward propagating OLR anomalies between 70°E and the Tanzania coast in OND, with more stationary features in some El-Nino years like 1997, but the time-scale they used (30-50 day filtered data) and the absence of statistical testing of the relationship with coastal Tanzania rainfall prevents easy comparisons with the present results.

To summarize, the three composite analyses enable us to draw the following picture: OND wet spells along the coast generally develop as a result of a combination of 3 features representative of 3 different time-scales: (i) a weakening / reversal of the Indian Ocean Zonal Mode (Walker circulation) at the interannual time-scale; (ii) an enhanced convergence between the northern and southern hemisphere trade-winds in the western Indian Ocean, which is an intra-seasonal but persistent feature; (iii) a westward-moving disturbance in the low-level equatorial wind field at the daily time-scale.

Atmospheric Dynamics Associated with the Extreme Event of 17-22 October 1997

In this section the mean sea level pressure patterns and the wind fields associated with the above extreme rainfall event are discussed.

Mean Sea Level Pressure Analysis

Mean Sea Level Pressure (MSLP) patterns for October (1979 -2002 climatology) are presented in figure 9a. North-west of the Mascarene High, it shows a ridge over the eastern Africa coast extending from about 25°S to the equator. The Mozambique Channel (MC)

trough is also noticeable in this figure. The pressure pattern during the 1997 wet event is discussed and contrasted against the mean MSLP pattern.

On 15 October, before the wet event, the equatorial Indian Ocean displayed a weak pressure region (between latitudes 12°S and 12°N) stretching from the African Coast eastwards to 90°E and beyond (Fig. 9b). Low pressure centres formed in this generally weak pressure region (e.g. low LA in Fig. 9b) and propagated westwards in the easterly flow. Other important features in the pressure pattern included:

(i) Three ridges in the Southern Hemisphere (SH) may be identified using the pattern defined by the 1019 hPa isobar, though pressure in the southern subtropics remained lower than normal. Two ridges in the Northern Hemisphere (NH) are observed based on the patterns of 1013 hPa isobar. The better organized ridge in the NH is that lying in the Southwest/Northeast direction over India. These ridges in the SH and NH constrained the flow over the Indian Ocean to be generally easterly,

(ii)

The Mozambique Channel Low (LB) is evident in Fig. 9b and a day later it deepened to form a low that travelled southwards leaving large pressure rises behind it, (iii)

The eastern Africa continental ridge (climatology) was evident, and remained a persistent feature throughout the subsequent wet event (Figures 9b,c). It contributed to the observed low-level southerly winds in the region.

Figure 9c, for 19 October 1997, presents an example of the pressure field at the height of the EAC wet event. The pressure field is more intense, especially to the west of about 70°E, compared to the climatological mean for October (Figure 9a). An intense high pressure with centre of 1031 hPa was observed to the south of Madagascar and was associated with two ridges, one on continental eastern Africa and the other extending northeastwards. Ahead of the continental ridge southerly air was transported equatorwards into the EAC.

Time-longitude cross section (Fig. 9d) along 20°S (latitude of central Madagascar) was used to monitor the movement of the northeastwards ridge (a breakaway ridge) with time. Note that the continental ridge based on the 1019 hPa isobar remained to the west of 40°E from 16

to 20 October 1997 but the breakaway ridge is seen on the 17 October propagating eastwards.

This breakaway ridge intensified the pressure gradient force between 20°S and the low pressure region near the equator. This is expected to strengthen the easterlies in the region, and to enhance their convergence with the southerlies found ahead of the continental ridge (see below). The propagation speed of the breakaway high was estimated at 6 ms -1 .

It is concluded that the heavy rainfall event at the EAC was linked to the occurrence of eastwards propagation of a middle latitude pressure ridge (Fig. 9c-d) along latitude 20°S.

The breakaway high was associated with an increase in the meridional pressure gradient between 20°S and the equator, which was expected to modulate the intensity of the easterly winds near latitude 12°S. Also, prior to the occurrence of the heavy rainfall event the Mozambique Channel low intensified, and between this low and the continental ridge a surge of the southerly winds could be expected to reach the EAC region.

Wind Analysis

Map patterns and cross sectional analysis of daily wind fields are presented in order to determine the three dimensional structure of the atmosphere during the heavy rainfall event of 17 -22 October 1997 at the EAC.

Vector Wind Maps

Examples of the total wind maps for the 19 October 1997 at 925 hPa and 700 hPa levels are presented in Figures 10a and10b. Along the equator, westerlies are almost absent, an outstanding feature of the 1997 season, associated with the reversal of the Indian Ocean Zonal Mode as discussed above [START_REF] Black | An observational study of the relationship between excessively strong short rains in Coastal East Africa and Indian Ocean SST[END_REF][START_REF] Hastenrath | Circulation mechanisms of climate anomalies in the equatorial Indian Ocean[END_REF]. At both 925 hPa (Figure 10a) and 850 hPa (not shown) the easterly winds between latitude 5ºS and 12ºS were strong and this flow extended from 40ºE (EAC longitude) to 90ºE. The easterlies from due east, forced by the eastwards moving breakaway high as shown above, converged with the southerlies from the Mozambique Channel area (Figure 10a). It is suggested that this convergence of the drier mainly continental southerlies with the moister maritime air enhances the instability of the atmosphere at the EAC.

On the 700 hPa level (Figure 10b) the wind reaching the EAC is much weaker and the southerlies through the Mozambique Channel are lacking. Instead, there is a low pressure centre in the southern part of the Mozambique Channel that is attracting the wind flow southwards at this level. It is noteworthy that the circulations near the Horn of Africa and to the northwest of Madagascar depict ridge patterns (R on Figure 10b) that determine a convergence over the western Indian Ocean. The winds at 600 hPa are similar to those at 700hPa but weaker still (Figures not shown).

Time-latitude cross sections of the Zonal Wind component

Time-Latitude cross sections of zonal wind across longitude 40°E (longitude of the EAC) are presented for the lower (925 hPa) and middle (700 hPa) troposphere, in order to depict the sequence of events during the wet spell (Figures 11 a,b). Regions of maximum easterly wind components are found from the Equator to 10°S. A band of westerly zonal wind component is observed further south. These easterly/westerly components of wind create a shear zone around 10°S. It is noteworthy that the first pair of closed maximum westerly/easterly wind events (Figure 11a) occurred on 17 October and was associated with the start of the wet spell at the EAC. The zonal wind shear is consistently found from 925 to 700 hPa (Figures 11a,b), suggesting that the Mozambique Channel (MC) low/trough extends at least to the 700 hPa level. This trough is generated by the Madagascar mountains ranges when there is prevailing easterly wind.

Time-latitude cross-sections of the Meridional Wind component

Figures 12(a,b) present time-latitude cross sections for meridional wind component at 925 and 700 hPa levels along 40°E. A region of maximum southerly wind component (15 ms -1 ) is found near latitude 30°S on the 16 October. On 17 October the 5 ms -1 isotach crossed into the region including the Equator and 5°S (case study region) and was associated with the start of the heavy rains in the region. During the 18 and 19 October the intensity of the meridional wind component rose to 7.5ms -1 in the study region and this period was marked by very heavy rainfall over the whole of the EAC. There was a general relaxation in the intensity of the southerly winds to below 5ms -1 after the 21 October and these were followed by a decrease in the rains on the 22 October (Figure 3). The meridional winds intensified again on 23 October with a 5 ms -1 isotach observed but this time the flow was mainly equatorial without the higher latitude fetch. At the 700 hPa level (Figure 12b) there were strong southerlies on the 16 October which propagated equatorwards bringing the 2.5 ms -1 isotach in the region between the equator and 5°S on the 17 and 18 October. These strong southerlies, reaching the EAC during the wet spell, occurred as a combination of the climatological ridge on the eastern Africa coast and the persistent MC low/trough.

Time-height cross section of horizontal total wind over the EAC region

Figure 13 displays the height-time section of horizontal wind vectors and isotachs in the area including Mombasa (2°-5°S, 39°-43°E, see figure 1). The direction of the wind is indicated by the vector arrow and the speed is contoured at an uneven interval so as to bring out clearly regions of relative maxima in wind speed. It is observed (Figure 13) that the winds were strong (>9 ms -1 ) on various days between 18 to 23 October 1997 in various layers of the atmosphere. These strong winds occurred in form of jetlets (>9 ms -1 ) near 850hPa level during this major wet event at the EAC and the vertical shear associated with the jetlets was conducive to the lifting of moist air in the Mombasa area. It has been shown in sections 5.2.1 and 5.2.2 that the winds in the Mombasa area, in the lower troposphere, originated mainly in the east near longitude 90°E and by the time they arrived at the EAC they had had a long sea track. Also, it is noted that the sea surface temperature anomalies were strongly positive in 1997 [START_REF] Birkett | Indian Ocean climate events bring floods to East Africa's lakes and the Sudd Marsh[END_REF][START_REF] Webster | Coupled oceanatmosphere dynamics in hte Indian Ocean during 1997-98[END_REF][START_REF] Black | An observational study of the relationship between excessively strong short rains in Coastal East Africa and Indian Ocean SST[END_REF] with possibility of injection of moisture into the lower atmospheric layers.

Moisture Analysis

The height-time section of humidity flux, the product of specific humidity and the wind intensity across the Mombasa area (2°-5°S, 39°-43°E), for October 1997 depicts two high moisture flux events; one between 2-5 October (Figure not shown) and the other between 17-24 October. Figure 14 offers a detailed analysis of the second event with both morning and afternoon moisture flux data used. We observe a dry region, centred on the 16 October, between 925-700 hPa levels showing low humidity flux of < 25 kg m -2 s -1 . Similarly, low flux values were evident again on the afternoon of 25th October at the end of the wet event.

The heavy rainfall event was associated with moisture flux of 100 kg m -2 s -1 or higher in the lower troposphere. It is also noted that periods of high values are associated with the appearance of jetlets in the area.

Discussion and conclusion

Along the East African Coast, much of the seasonal totals of the short rains tend to be concentrated over short periods, as demonstrated by the analysis of 36 years of daily rainfall records. This often leads to flooding, as was the case for 1997. The occurrence of wet events tends to be synchronous along the EAC as evidenced from high correlation values observed between the coastal stations. These results were consistent with the previous studies that have shown homogeneity in rainfall variability along the EAC North of Dar-es-Salaam [START_REF] Bärring | Regionalization of daily rainfall in Kenya by means of common factor analysis[END_REF]Nyenzi, 1988;[START_REF] Ogallo | The spatial and temporal patterns of the East African seasonal rainfall derived from principal component analysis[END_REF][START_REF] Kijazi | Relationships between intraseasonal rainfall variability of coastal Tanzania and ENSO[END_REF]. The coherence of the rainfall over a larger area suggests the association with a large-scale system. The study showed a weakening or reversal of the east-west (Walker type) circulation over the Indian Ocean, as a pre-condition for coastal wet spells. [START_REF] Hastenrath | Atmospheric-hydrospheric mechanisms of climate anomalies in the western equatorial Indian Ocean[END_REF] and [START_REF] Ininda | Simulation of the impact of sea surface temeparature anomalies on the short rains Over East Africa[END_REF] have shown that there is a descending arm of the east-west circulation over the western equatorial Indian

Ocean and an ascending arm over the eastern equatorial Indian Ocean. During El-Nino events and / or cooling in the Eastern Indian Ocean, the pressure builds up over the eastern Indian Ocean, which weakens or even reverses the east-west circulation over the equatorial Indian Ocean. This condition is favourable for enhanced low-level convergence in the western Indian Ocean. The other feature that was common during the wet spells was the enhanced convergence between the northern and southern hemisphere trade winds in the western Indian Ocean. This feature has also been noted by [START_REF] Anyamba | On the monthly mean lower tropospheric circulation and the anomalous circulation during the 1961/62 floods in East Africa[END_REF], who observed that the floods which occurred during the short rains of 1961 were as a result of the intensification and zonal orientation of the Arabian ridge that resulted into maritime flow of the north easterlies. Whereas the favourable large-scale features were common to most years that were wet during the short rain season, the wet spells along the coast were accompanied by westward moving disturbances in the low-level wind field between Madagascar and the equator. The easterly disturbances over the Indian Ocean have been studied by various others including [START_REF] Fremming | Notes on an easterly disturbance affecting East Africa[END_REF], [START_REF] Gichuiya | Easterly disturbances in the South-East monsoon[END_REF] and [START_REF] Okoola | Westwards moving disturbances in the Southwest Indian Ocean[END_REF]. These disturbances provide the energy that triggers or enhances the weather activities over the East Africa coast.

The 1997 case study revealed the presence of the above features with additional of occurrence of some unique features. It is shown that prior to the heavy rainfall event a ridge of high pressure, on the eastern coast of southern Africa, intensified and propagated eastwards leading to the strengthening of easterlies reaching the EAC. The zonal wind component along longitude 40°E showed shears in the flows that were associated with the development of the Mozambique Channel low/trough in the lower troposphere round which southerlies surged northwards to the EAC. These southerlies converged with the easterlies near the EAC. Thus, the warm and wet air from the east interacted with the relatively cold and continental air from the south generating instability at the EAC. The availability of warm moist air and instability, combined to create an environment that gave sustained heavy daily rains at the East Africa Coast. 1979-1997, day d0). Reanalysis data are deseasonalised but not filtered. Vectors : grid-points with significant (Student's t-test, 95% c.l.) U or V wind anomalies ; vector size is proportionate to the t value and points in the direction of the wind anomaly. Contours : significant geopotential height anomalies ; bold solid line : positive anomalies significant at 95% c.l., heavy bold : at 99% c.l. ; dashed lines : same but for negative anomalies. Shading : significant vertical velocity anomalies ; dark grey : upward motion ; light grey : downward motion. (567 events, 1979-1997, day d0). Reanalysis data are deseasonalised but not filtered. Vectors : grid-points with significant (Student's t-test, 95% c.l.) U or V wind anomalies ; vector size is proportionate to the t value and points in the direction of the wind anomaly. Contours : significant geopotential height anomalies ; bold solid line : positive anomalies significant at 95% c.l., heavy bold : at 99% c.l. ; dashed lines : same but for negative anomalies. Shading : significant vertical velocity anomalies ; dark grey : upward motion ; light grey : downward motion.

Figure Captions

Fig. 1 :Fig. 2 :

 12 Fig. 1 : Study area and location of the stations quoted in the analysis. Contours show

Fig. 3 :Fig. 4 :

 34 Fig. 3 : Distribution of daily rainfall amounts at 3 stations of the East Africa coast for the

Fig. 5 :

 5 Fig.5 : same as fig.4 but for day d-3.

Fig. 7 :

 7 Fig.7 : same as fig.6 but for day d-3.

Fig. 11 :

 11 Fig.11: Time-Latitude cross-section for zonal wind (m/s) during the period between 15-25

Fig. 14 :Fig. 1 :

 141 Fig. 14: Time-Height section for moisture flux (kg/m² s) for the period 15th -25 th October

Fig. 2 :

 2 Fig. 2 : Mean monthly rainfall over the East Africa Coast (average of Mombasa, Malindi and Lamu stations, solid line). Monthly rainfall for the year 1997 is also shown (dashed line).

Fig. 3 :

 3 Fig. 3 : Distribution of daily rainfall amounts at 3 stations of the East Africa coast for the period October to December 1997.

Fig. 4 :

 4 Fig.4: Composite maps of daily NCEP-DOE reanalysis 850 hPa fields for October-December wet events along the EA Coast(567 events, 1979-1997, day d0). Reanalysis data are deseasonalised but not filtered. Vectors : grid-points with significant (Student's t-test, 95% c.l.) U or V wind anomalies ; vector size is proportionate to the t value and points in the direction of the wind anomaly. Contours : significant geopotential height anomalies ; bold solid line : positive anomalies significant at 95% c.l., heavy bold : at 99% c.l. ; dashed lines : same but for negative anomalies. Shading : significant vertical velocity anomalies ; dark grey : upward motion ; light grey : downward motion.

Fig. 5 :

 5 Fig.5 : same as fig.4 but for day d-3.

  Fig.6 : same as fig.4, for d0 but using 70-day high-pass filtered NCEP-DOE data.

  Fig.7 : same as fig.6 but for day d-3.

  Fig.8 : same as fig.4, but for the onset (d0) of wet spells along the EA coast.

Fig. 9 :Fig. 10 :Fig. 11 :Fig. 12 :

 9101112 Fig. 9: Mean Sea Level Pressure (MSLP) in Hectopascals (hPa) for (a) Long-term mean 1979-2002 (b) 15 th October 1997, (c) 19 th October 1997 and (d) Time-longitude cross section along 20°S at 2hPa intervals for October 1997. On figure d, shaded where MSLP is greater than 1019 hPa.

Fig. 13 :

 13 Fig. 13: Time-Height section for area-average horizontal wind vectors and isotachs (m/s) during 15-25 October 1997 over Mombasa area (2°-5°S, 39°-43°E). The thick contour is 9 m/s. Vectors and figures to the right show the mean October wind direction and speed for 1971-2000 (m/s).

Fig. 14 :

 14 Fig. 14: Time-Height section for moisture flux (kg/m² s) for the period 15th -25th October 1997 averaged over Mombasa area (2°-5°S, 39°-43°E). The direction and intensity of the moisture fluxes are indicated as arrows. The moisture flux is contoured at 25 kg/ (m² s) intervals.

Table 1 :

 1 Correlation between daily rainfall variations at Mombasa-Airport (Kenya) and at four other stations in Kenya, for the OND season(1962-1997, n=2863). Rainfall amounts were square-root transformed. Correlations above 0.40 are underlined.

				Coast (North of		Inland (West of
				Mombasa)		Mombasa)
	Station		MALINDI	LAMU	VOI	MAKINDU
	Distance to Mombasa	100 km	240 km	130 km	280 km
	OND, raw data	0.43	0.30		0.25	0.19
	OND, 3-day moving-average	0.59	0.46		0.33	0.30
	Table 2 : Co-occurrence of heavy daily rainfall events (over 20mm) at Mombasa (Kenya)
	and at other stations in Kenya (1962-1997, OND season).
	Station	Location	Distance	Mean OND	Heavy daily	% of local heavy rainfall
			to	number of		rainfall	events recording :
		Mombasa	heavy daily	events as a %	No rain at	Over 20 mm
			(km)	rainfall events	of wet days	Mombasa	at Mombasa
	MOMBASA Coast	0	16			11	0	100
	MALINDI	Coast (N)	100	11			11	13	35
	LAMU	Coast (N)	240	9			13	22	24
	VOI	Inland	130	14			11	36	13
	MAKINDU	Inland	280	21			17	37	7

Acknowledgments:

The first author's stay at Centre de Recherches de Climatologie, University of Bourgogne was supported by a CNRS associate researcher grant. The first author is also grateful to the University of Nairobi for granting him leave of absence during this work.