Pierre Camberlin
email: camber@u-bourgogne.fr

The climate of the Nile Basin is characterised by a strong latitudinal wetness gradient. Whereas the areas north of 18°N remain dry most of the year, to the south there is a gradual increase of monsoon precipitation amounts. Rainfall regimes can be divided into 9 types, among which summer peak regimes dominate. In the southern half of the basin, mesoscale circulation features and associated contrasts in local precipitation patterns develop as a result of a complex interplay involving topography, lakes and swamps. Precipitation changes and variability show up as 3 distinct modes of variability. Drying trends since the 1950s are found in central Sudan and to some extent the Ethiopian Highlands. The equatorial lakes region is characterised by occasional very wet years (e.g. 1961, 1997). The interannual variations are strongly, but indirectly influenced by El-Nino / Southern Oscillation. Sea surface temperature variations over other ocean basins, especially the Indian and South Atlantic Oceans, also play a significant role. Projections for the late twenty-first century show a 2-4°C temperature increase over the basin, depending on the scenario, but rainfall projections are more uncertain. Most models tend to predict a rainfall increase in the equatorial regions, but there is little consistency between models over the tropical regions.

Introduction

It is well known that the river Nile has the world's longest stretch under arid conditions: along 3000 km of its course, rainfall does not exceed 150 mm annually. However, due to its great latitudinal and altitudinal extent, the Nile basin displays large variations in precipitation receipt. These contrasts, which clearly show up in the mean climate fields, also manifest in time as large year-to-year or longer-term fluctuations.

Mean climate

Drivers of the Nile basin climates

• General circulation and its forcings The Nile basin extends over 35 degrees of latitude, from the equatorial zone (4°S) to the northern subtropics (31°N). This results in highly contrasted climatic conditions, dominated by the Hadley circulation. The Hadley circulation is fuelled by a north-south energy gradient between a zone with excess energy (to the south of the basin, shifting to the central part with the northern summer heating) and a zone with a deficit (to the north of the basin). The excess energy originates from high solar radiation gains, low terrestrial radiation losses due to an extensive cloud cover, and a high atmospheric moisture content (latent heat). The deficit in the north, mainly the Sahara desert, is related to lesser solar radiation gains (in the northern winter), to high terrestrial radiation losses due to cloudless skies, and to a dry atmosphere (low latent heat content).

The excess surface energy induces relative low pressure and rising motion, as well as a lowlevel wind convergence, the Intertropical Convergence Zone (ITCZ). The ITCZ is located southward outside the Nile basin in the northern winter, and gradually shifts to the north to reach the central part of the basin by mid-summer (fig. 1). Rising motion within the ITCZ results into widespread precipitation.

Reciprocally, the energy loss at higher latitudes is accompanied by diverging low-level winds, subsidence and permanent dryness. The northern part of the Nile basin is therefore capped by high-pressure systems, namely the Libyan and Arabian Highs, during much of the year (fig. 1). In summer they weaken and are partly replaced by surface heat lows, though subsidence still prevails higher in the troposphere.

Winds systems reflect the dominant influence of the Hadley circulation. The Libyan high drives north-westerly winds in Egypt, turning to north-easterly in northern Sudan, during most of the year (fig. 1). Occasionally, in winter, the northernmost part of the region along the Mediterranean is affected by disturbances associated with upper troughs in the mid-latitude westerly circulation. To the south, the dry north-easterly trade winds combine with those originating from the Indian Ocean to reach southern Sudan and Uganda. From March, the ITCZ starts shifting northward, and south-westerly monsoon winds, originating from the South Atlantic and the Congo basin, appear in southern Sudan. The lake Victoria basin dominantly remains under the influence of easterlies, gradually getting a southerly component from April onwards. Further north in Egypt, rising temperatures induce surface desert depressions, which drive extremely hot and dusty southerly khamsin winds across Egypt, causing sudden heat waves in spring (e.g, 41°C in Cairo in early April 2003). By June, the monsoon winds reach central Sudan (16°N, fig. 2) and the western slopes of the Ethiopian Highlands. From the end of June, due to the deepening of the Indian monsoon low further east, the south-westerly monsoon spills over the Ethiopian Highlands to reach the southern Red Sea, where it is channelled until it joins the main Indian monsoon flow in the Arabian Sea. However, a distinct feature of the ITCZ in tropical North Africa, including the Nile valley, is that in summer the surface southerly winds tend to penetrate far to the north due to the Saharan heat low. Mean July temperatures range between 26 and 33°C in Egypt and northern Sudan [START_REF] Shahin | Hydrology of the Nile Basin[END_REF]. The surface heat low causes the "Intertropical Front" (ITF), which separates the monsoon winds from the northerly trade winds, to show a tilt with height [START_REF] Hastenrath | Climate Dynamics of the Tropics[END_REF]. In July-August, at the time of its northernmost location, the ITF is found around 17-19°N at the surface in the Nile valley (fig. 1), and a few hundreds of km further south at 1500 m. South of lake Victoria, diverging south-easterly winds from the Indian Ocean induce dry conditions. Starting in September, the retreat of the ITCZ to the south tends to be faster than its northward shift [START_REF] Osman | On the synoptic climatology of summer rainfall over Central Sudan[END_REF].

In the upper troposphere, the winds are also characterised by a seasonal reversal. In winter strong westerlies, forming the subtropical westerly jet (SWJ), are found over much of the Nile basin (fig. 2). The variations of their latitudinal location and the waves which develop in the westerly flow affect winter weather conditions in the northern part of the basin. In particular, cut-off lows in the eastern mediterranean will induce cloudy or possibly rainy conditions over northern Egypt. Cold fronts sweeping behind Mediterranean depressions result into a significant temperature drop, and sometimes the instability created by the passage of cold air over the warm desert surface produces widespread dust-storms [START_REF] Tucker | Synoptic disturbances over the Nile Valley north of the Intertropical Convergence Zone[END_REF]. The SWJ persists until May, after which the westerlies are replaced by easterlies (fig. 2). They take the form of a jet (Tropical Easterly Jet, TEJ), which originates from Asia as a result of the summer monsoon. Maximum winds are found in July-August at 150 hPa near 10-15°N over Sudan, with velocities decreasing from 25 to 10 m.s -1 from east to west [START_REF] Hulme | The tropical easterly jet and Sudan rainfall: a review[END_REF][START_REF] Segele | Characterization and variability of Kiremt rainy season over Ethiopia[END_REF]).

• Regional and local factors

Among the factors which regionally modify the general circulation pattern, the most important one is topography. The highlands which bound the Nile basin to the east, from Eritrea to Kenya, restrict the penetration of the easterlies from the Indian Ocean. An exception is the gap between the Ethiopian Highlands and the Kenya Highlands, where strong easterlies prevail throughout the year (Turkana Jet; [START_REF] Kinuthia | A newly found jet in North Kenya (Turkana Channel)[END_REF]. Mountain ranges also set up their own circulation and generate their own climate. A strong daytime horizontal flow is directed towards the heated mass of the Ethiopian Highlands, while it is suggested that the nearby Sudanese plains exhibit subsidence [START_REF] Flohn | Contributions to the synoptic climatology of North-East Africa[END_REF][START_REF] Pedgley | Diurnal incidence of rain and thunder at Asmara and Addis-Ababa[END_REF]. Some of the Great Lakes of East Africa also tend to develop their own circulation, in the form of lake breezes induced by the small diurnal temperature variations of the lakes (around 25°C for lake Victoria) compared to the surrounding areas. This is most evident for lake Victoria, whose circular geometry encourages daytime breezes diverging from the lake to the warmer surrounding land, and night-time land breezes converging to the warmer middle part of the lake [START_REF] Fraedrich | A simple climatological model of the dynamics and energetics of the nocturnal circulation at lake Victoria[END_REF]. These breezes interact with slope circulation, especially to the northeast of the lake which is bordered by the Western Kenya Highlands [START_REF] Okeyo | The influence of lake Victoria on the convective activities over the Kenya highlands[END_REF]. The joint effect of lake and upslope breezes enhances afternoon convection. The strong updraughts are responsible for a high frequency of hail around Kericho and Nandi Hills in Kenya (Alusa, 1986). An important aspect is also the interaction with the large-scale circulation [START_REF] Asnani | Diurnal variation of precipitation in East Africa[END_REF][START_REF] Anyah | Simulated physical mechanisms associated with multi-scale climate variability over lake Victoria Basin in East Africa[END_REF]. The latter in the middle and upper troposphere is predominantly easterly, thus the convective clusters associated with low-level convergence and instability tend to drift westwards. This explains asymmetries in the rainfall distribution on either side of the mountain ranges or lakes.

The influence of regional features (topography, waterbodies, land cover) over atmospheric circulation is best studied by numerical modelling. The overall influence of the East African Great Lakes was assessed by [START_REF] Bonan | Sensitivity of a GCM simulation to inclusion of inland water surfaces[END_REF] using a general circulation model, but regional models are needed to adequately resolve the local topographical features. For East Africa, Sun et al. (1999a-b) assessed the ability of the NCAR RegCM2 model to simulate the region's climate. It was subsequently shown that taking into account three-dimensional lake dynamics is necessary to satisfactorily reproduce climate conditions over lake Victoria itself [START_REF] Song | A coupled regional climate model for the lake Victoria Basin of East Africa[END_REF]. [START_REF] Anyah | Simulated physical mechanisms associated with multi-scale climate variability over lake Victoria Basin in East Africa[END_REF] then performed sensitivity experiments showing the key role played by the Kenya Highlands on the diurnal circulation and associated rainfall distribution over the lake Victoria basin. For the Nile basin as a whole, [START_REF] Mohamed | Hydroclimatology of the Nile: Results from a regional climate model[END_REF] used the RACMO regional model to evaluate the components of the water cycle. The study enabled to confirm that in June-September the low-level moisture advection towards the Ethiopian Highlands and Bahr-el-Ghazal region is mainly of Atlantic origin, whereas in Uganda and east of the Bahr-el-Jebel (White Nile) the Indian Ocean is the major moisture source. However, regional models still have some difficulties in reproducing some aspects of climate variation. For instance, while moisture convergence and associated rainfall is well simulated in June-September over the Ethiopian Highlands and Uganda north of lake Victoria in the RACMO model [START_REF] Mohamed | Hydroclimatology of the Nile: Results from a regional climate model[END_REF], the March-May rains over East Africa are strongly underestimated.

Spatial and temporal distribution of rainfall

• Mean annual rainfall

The mean annual rainfall for the Nile basin is low (630 mm over the period ), but it is spatially very contrasted. As much as 28% of the basin receives less than 100 mm annually (fig. 3), and part of it experiences hyper-arid conditions. However, a substantial area exhibits sub-humid conditions (34% between 700-1300 mm), as displayed on the frequency distribution plot.

Spatially, there is a very gradual decrease of rainfall amounts from south to south in the central part of the basin (about 100-140 mm per degree of latitude; fig. 4). North of about 18°N, from northern Sudan all across Egypt, rainfall is negligible (below 50 mm a year), except for a small increase along the Mediterranean coast (Alexandria 180 mm). Precipitation in excess of 1000 mm is restricted to two areas: the equatorial region from south-western Sudan to most of the lake Victoria basin, and the Ethiopian Highlands. Even in these two areas, precipitation amounts are contrasted, with maxima around 2100-2300 mm near Gore, south-western Ethiopia, 2200 mm over the western part of lake Victoria, 2000 mm on the western slopes of Mt Elgon. Localised values over 2500 mm are found on Mt Rwenzori [START_REF] Osmaston | Rwenzori Mountains National Park, Uganda. Glaciers and Glaciations[END_REF], and perhaps over 3000 mm on the western slopes [START_REF] Leroux | Le climat de l'Afrique Tropicale[END_REF]. Parts of the Western Rift Valley (lake Edward, lake Albert), north-eastern Uganda, and areas to the west and south of lake Victoria get less than 900 mm (fig. 4). This general distribution reflects the latitudinal movement of the ITCZ, which never reaches Egypt and northernmost Sudan, stays only briefly in central Sudan but longer further south. In wet areas, regional and local enhancements of rainfall are found on the west-facing slopes, mainly as a result of orographic lifting of the moist south-westerlies or westerlies. In the Great Lakes region in the south, local wind systems associated with the lakes and the complex topography of the Rift system explain the variable rainfall amounts. Rain shadows are found over lakes Albert and Edward in the Western Rift, while the maximum over the western part of lake Victoria results from the nocturnal convergence which takes place over the lake, generating thunderstorms which subsequently drift westward. The increase of rainfall with elevation is far from being universal; moisture convergence or orographic uplift (the latter possibly resulting into mid-slope maxima or windward / leeward rainfall asymmetries) are actually of much greater importance in both Ethiopia and the Equatorial Highlands.

• Seasonal rainfall distribution

The seasonal distribution of the rains is illustrated by the typology shown in fig. 5, which is based on mean monthly rainfall for the period 1961-1990. The classic division of the basin into three rainfall regimes (arid, tropical and equatorial, from north to south) remains valid, but a further separation into nine types is useful. The northernmost part of Egypt receives some winter rains characteristic of the Mediterranean margins (type 1). The rest of the country, as well as northern Sudan, are totally dry throughout the year (type 2). At around 18°N a summer rainfall peak starts to appear (type 3), but the mean rainfall for the wettest month (August) remains very low. It increases in types 4 and 5, while the rainy season becomes longer (for type 5, 4 to 6 months above 50 mm at 10-13°N in central Sudan). In Southern Sudan (type 6), the rainy season lengthens further, but though the peak is still in August, rainfall tends to level off during summer. This contrasts with type 7, covering much of western Ethiopia, where the rainfall regime is very similar to type 6, except for a strong increase in the middle of the rainy season, in July-August. This increase is likely to be related to the moist monsoon south-westerlies becoming thicker [START_REF] Segele | Characterization and variability of Kiremt rainy season over Ethiopia[END_REF], and overflowing the warm surface of the Ethiopian plateau, which results into enhanced convective instability. Equatorial regimes are found further south, with two peaks (the main one in April) separated by two drier seasons, in connection with the twice-a-year passage of the ITCZ over the region [START_REF] Nicholson | A review of climate dynamics and climate variability in Eastern Africa[END_REF]. In much of Uganda and Western Kenya (type 8), the northern winter is the driest season. After the March-May rains, rainfall remains quite high for 6 consecutive months till November. Over lake Victoria and its catchment in Tanzania, Rwanda and Burundi (type 9), it is the northern summer which becomes the driest season, whereas there is only a relative decrease between the "short rains", peaking in November, and the "long rains", peaking in April.

• Small-scale temporal organisation of rainfall

The modalities of within-season variations in rainfall depend on the region. Apart from northern Egypt, the effect of extra-tropical disturbances on rainfall in the Nile basin is limited. Upper troughs remain generally rainless over Sudan, though over eastern equatorial Africa, cases of interactions with moist easterlies have been reported, giving enhanced rainfall. In Sudan during the summer rains, a pronounced periodicity around 4-5 days, except in dry years, is found in daily precipitation [START_REF] Hammer | Rainfall patterns in the Sudan[END_REF][START_REF] Hammer | Rainfall characteristics in Eastern Sahel[END_REF], and is suggested to be associated with easterly waves activity. As recently demonstrated by [START_REF] Mekonnen | Analysis of Convection and Its Association with African Easterly Waves[END_REF], dynamic components of easterly waves are weak in the region, compared to West Africa. However, a 2-6 day convective variance as strong as in West Africa suggests an initiation of the waves by convective activity, on the western side of the mountains of Sudan (Darfur) and Ethiopia (see also an example in [START_REF] Desbois | Characterisation of some elements of the Sahelian climate and their interannual variations for July 1983, 1984 and 1985 from the analysis of METEOSAT ISCCP data[END_REF]. Though most storms developed over Ethiopia drift westward due to the Tropical Easterly Jet of the upper troposphere, many decay before reaching the Nile plains [START_REF] Pedgley | Diurnal variation of the incidence of monsoon rainfall over the Sudan[END_REF]. In equatorial East Africa, organised weather systems are even more difficult to detect. Synoptic disturbances have been identified, but little is still known about their behaviour and relationship with rainfall, partly because the effects of topography make them difficult to follow [START_REF] Nicholson | A review of climate dynamics and climate variability in Eastern Africa[END_REF]. Emphasis has been given to incursions of moist westerlies from the Congo basin, to explain wet spells in the region. However, this is challenged by [START_REF] Anyah | Simulated physical mechanisms associated with multi-scale climate variability over lake Victoria Basin in East Africa[END_REF] who demonstrated through numerical modelling that rainfall over the lake Victoria basin is much less sensitive to changes in moisture entering the region from the west than from the east. It is likely that it is more the zonal wind convergence associated with westerly wind anomalies propagating from the Congo basin, rather than the moisture amount, that is instrumental in rainfall variability. At longer time-scales, the 30-60 day "Madden-Julian" oscillation (MJO), which strongly modulates convection over the Indo-Pacific region, has also a moderate effect on intraseasonal rainfall variability in Eastern Africa [START_REF] Pohl | Influence of the Madden-Julian Oscillation on East African rainfall. Part I: Intraseasonal variability and regional dependency[END_REF].

Rainfall intensities may be high, though not as extreme as in many parts of the tropics. Maximum 24-hour precipitation, though an imperfect indicator, shows that highland locations do not necessarily tend to have the highest intensities, despite their high mean rainfall amounts [START_REF] Potts | Maximum daily point rainfall in Uganda: A sample survey[END_REF][START_REF] Griffiths | Eastern Africa; Ethiopian Highlands[END_REF]. A generalised map of 100-yr 24-hour extreme rainfall for the lake Victoria catchment area, based on Gumbel-I distribution, provides values generally comprised between 110 and 150 mm (WMO, 1974). As an illustration of the poor relationship between mean and extreme rainfall, in Uganda, the return period for a 100 mm daily fall is 4 years at the dry station of Moroto, and between 10 and 30 years for most wetter stations across the country, while at the highland station of Kabale (1800 m) it is even as long as 170 years [START_REF] Potts | Maximum daily point rainfall in Uganda: A sample survey[END_REF]). The differences result from the respective parts played by convective and orographic rainfall, and the lesser precipitable moisture over high ground. In the northern part of the Nile basin, the computation of extreme rainfall probabilities is difficult due to rainfall events becoming much less frequent. However, very high intensities are possible. In the Nile Plains at Khartoum, a 201 mm 24-hour record, resulting into extensive damages and human deaths, was recorded on 4 August 1988 [START_REF] Hulme | The August 1988 storm over Khartoum: its climatology and impact[END_REF], while the record of the previous 90 years was only 88 mm. On shorter time spans, very high peak rainfall rates may be found throughout the basin.

As in most other continental regions, rainfall generally peaks in the afternoon, especially over mountain areas [START_REF] Desbois | Characterisation of some elements of the Sahelian climate and their interannual variations for July 1983, 1984 and 1985 from the analysis of METEOSAT ISCCP data[END_REF][START_REF] Ba | Analysis of convective activity and its relationship to the rainfall over the Rift Valley lakes of East Africa during 1983-90 using the METEOSAT infrared channel[END_REF], but quite contrasted patterns are found across the basin. These diurnal regimes reflect the regional wind systems and their daytime evolution. The afternoon maximum is sharp over the Ethiopian Highlands and in northern Uganda and western Kenya (fig. 6). Asmara (Eritrea), on the plateau, gets one third of its rainfall between 1500 and 1800 local time. This maximum shifts to the evening hours further west, as the main convective cloud clusters move westward in the direction of the prevailing upper easterlies. An early morning maximum is even found in the central plains of Sudan. At Kosti 52% of the August rainfall occurrences occur between 0000 and 0900 LT [START_REF] Pedgley | Diurnal variation of the incidence of monsoon rainfall over the Sudan[END_REF]. The weakness of the afternoon maximum is suggested to be due to subsidence, as a result of the daytime circulation which develops between the Nile Plains and the Ethiopian Highlands [START_REF] Pedgley | Diurnal incidence of rain and thunder at Asmara and Addis-Ababa[END_REF]. Further south over the western part of lake Victoria, the morning peak is even more pronounced (fig. 6). It results from the build up of the nighttime convergence in the middle of the lake, the associated thunderstorms then drifting westward [START_REF] Asnani | Diurnal variation of precipitation in East Africa[END_REF][START_REF] Ba | Analysis of convective activity and its relationship to the rainfall over the Rift Valley lakes of East Africa during 1983-90 using the METEOSAT infrared channel[END_REF][START_REF] Anyah | Simulated physical mechanisms associated with multi-scale climate variability over lake Victoria Basin in East Africa[END_REF].

Climate variability and change: present and future

Space-time patterns of rainfall variability

The region shows relatively coherent areas of rainfall variability. For the period 1951-2000, 44 % of the space-time variance (for all areas receiving at least 100 mm annual rainfall) can be accounted for by 3 regional modes of variability, as deduced from principal component analysis (fig. 7). Region 1 corresponds to the northern part of the summer rainfall belt (centred around 15-16°N), from northern Darfur to Kordofan and the Ethiopian-Eritrean border, with a southward extension along the Nile Valley. It is characterised by heavy rains in the 1950s and early 1960s, followed by much drier conditions throughout the last three decades of the twentieth century, except for relatively isolated wet years like 1980, 1988 and 1998-1999. The second region is located further south, describing the latitudes 7-14°N but not continuously from Sudan to Ethiopia. Areas best represented are Darfur and the Ethiopian-Sudanese border area. Like for region 1, a marked rainfall decrease is evident, but the shift from wetter to drier conditions is delayed to the late 1970s, and a partial recovery occurs in the late 1990s. Region 3 comprises much of the equatorial regions, as well as the southwesternmost part of Ethiopia. It is dominated by a skewed distribution of annual rainfall, with some occasional very wet years like 1961, followed by 1951, 1963, 1982 or 1997. There is a tendency for the 1960s to stand out as wetter than average. This description of space-time rainfall variations leaves out smaller size areas, like the central part of the Ethiopian Highlands and parts of southern Sudan. This is partly because the timescale retained (annual) combines different seasons and is not fully relevant for those areas where different climate signals, with a different spatial extension, are consecutively found during the year. For instance, part of the Ethiopian Highlands displays spring rains which are unrelated to those found at the same time in equatorial regions. Additionally, the main Ethiopian rains in summer show similarities with both the sudano-sahelian belt further west, and the equatorial regions, at this time of the year. However, on an annual basis, the equatorial region is dominated by the interannual variations of the northern autumn "short rains". On average the amount of rain during the latter is equal or less than that of the spring "long rains". However, the "short rains" display a much larger amplitude in their interannual variability than the rains falling during the rest of the year [START_REF] Nicholson | A review of climate dynamics and climate variability in Eastern Africa[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]. In addition, it is spatially much more coherent than the "long rains" [START_REF] Camberlin | The east African March-May rainy season : associated atmospheric dynamics and predictability over the 1968-97 period[END_REF]. Therefore, the "short rains" dominate the interannual signal in Equatorial rainfall and lake levels [START_REF] Bergonzini | Zonal Circulations over the Indian and Pacific Oceans and the Level of Lakes Victoria and Tanganyika[END_REF].

A key feature of the last decades over part of the Nile basin is the downward rainfall trend, whose spatial pattern is shown in fig. 8. Over the period 1951-2000, the linear trend is particularly pronounced in most of northern Sudan (around 50% precipitation decrease). It is also highly significant in Darfur (15-50% decrease). Other regions of significant decrease include central Sudan (10-40%) and south-western Ethiopia (20-35%). The equatorial region exhibits localised areas of rainfall decrease, like in West Nile (Uganda), but the overall rainfall is stable during 1951-2000. Small increases are restricted to isolated pockets around Addis-Ababa and the lake Victoria shorelines.

In Sudan, the rainfall decrease which started in the second half of the 1960s resulted into a much shorter rainy season in the 1970s and 1980s, particularly due to an earlier end, while all classes of daily rainfall amounts have seen a lesser occurrence [START_REF] Hulme | Changes in wet season structure in Central Sudan, 1900-86[END_REF]. As in the West African Sahel, the rainfall recovery in the 1990s is only a very partial one, this decade remaining drier than the 1950s and early 1960s. In Ethiopia, previous evidence of decreasing rainfall is mixed [START_REF] Hulme | Recent fluctuations in precipitation and runoff over the Nile sub-basins and their impact on main Nile discharge[END_REF][START_REF] Conway | Some aspects of climate variability in the north east Ethiopian highlands -Wollo and Tigray[END_REF][START_REF] Funk | Estimating Meher crop production using rainfall in the 'Long Cycle' region of Ethiopia[END_REF][START_REF] Seleshi | Recent changes in rainfall and rainy days in Ethiopia[END_REF] : several studies failed to detect any systematic downward trend while others did. [START_REF] Mould | Over one century of rainfall and temperature observations in Addis Ababa, Ethiopia[END_REF] noted that Addis-Ababa rainfall shows little trends, but this station was found to be unrepresentative of the rest of the Ethiopian Highlands. In the latter, a drying trend is actually shown if only the summer monsoon rains are considered, and the 1950s and 1960s included. Several wet years in the 1990s additionally contribute to dampen the trend, but it remains very significant in south-western Ethiopia. The coherence between Central Sudan rainfall and that recorded over part of the Ethiopian Highlands (Atbara basin), as shown by region 1, replicates the correlation found in annual rainfall for the corresponding sub-basins in [START_REF] Hulme | Recent fluctuations in precipitation and runoff over the Nile sub-basins and their impact on main Nile discharge[END_REF]. Further south, the association between Western Ethiopia and south-western Sudan precipitation (region 2), corroborates the correlation found by [START_REF] Sutcliffe | The Hydrology of the Nile[END_REF] between the river flows of the Blue Nile and the Jur at Wau (Sudan).

Causes of rainfall variability

One of the main causes of climate variability in the tropics, the El-Nino Southern Oscillation (ENSO), has for long been found to affect the Nile basin, as demonstrated by Nile flows interannual variations [START_REF] Bliss | The Nile flood and world weather[END_REF]. However, studies on the regional rainfall response to ENSO did not emerge until the late 1980s. This response is seasonally dependant (fig. 9).

The main ENSO signal is found during the northern summer, at which time a negative correlation is found with the Nino 3+4 index, depicting lower than normal rainfall in years of higher sea-surface temperatures (SST) in the eastern equatorial Pacific (i.e., El Nino years). The summer signal is consistent over most parts of the basin where rains are found during this season (fig. 10). This includes most of Sudan and Ethiopia, but the largest regional correlations (-0.71 for July-September 1951[START_REF] Conway | Some aspects of climate variability in the north east Ethiopian highlands -Wollo and Tigray[END_REF] are found further south in Uganda and Western Kenya. Significant correlations between ENSO and rainfall have been documented by [START_REF] Osman | Qualitative rainfall prediction models for central and southern Sudan using El Niño-southern oscillation and Indian Ocean sea surface temperature Indices[END_REF] for Sudan (annual rainfall), by [START_REF] Haile | Drought in Ethiopia[END_REF], Beltrando & Camberlin (1993), [START_REF] Seleshi | Rainfall variability in the Ethiopian and Eritrean highlands and its links with the Southern Oscillation Index[END_REF], [START_REF] Seleshi | Recent changes in rainfall and rainy days in Ethiopia[END_REF], [START_REF] Segele | Characterization and variability of Kiremt rainy season over Ethiopia[END_REF] and [START_REF] Korecha | Predictability of June-September Rainfall in Ethiopia[END_REF] for Ethiopia (main July-September rains as well as rainy season length), and by [START_REF] Ogallo | Relationships between seasonal rainfall in East Africa and the Southern Oscillation[END_REF], [START_REF] Camberlin | June-September rainfall in North-Eastern Africa and atmospheric signals over the tropics: a zonal perspective[END_REF], [START_REF] Indeje | ENSO signals in East African rainfall seasons[END_REF] and Phillips & McIntyre (2000) for Uganda and Western Kenya (July-September). All conclude to some summer rainfall reduction in El-Nino years.

However in October-December, the correlation with ENSO switches signs, becoming positive (fig. 9). At this time of the year, precipitation, mostly restricted to the equatorial regions, is higher than normal in El-Nino years. This pattern is typical of Eastern Equatorial Africa during the "short rains" of October-December [START_REF] Ogallo | Relationships between seasonal rainfall in East Africa and the Southern Oscillation[END_REF][START_REF] Hastenrath | Atmospheric-hydrospheric mechanisms of climate anomalies in the western equatorial Indian Ocean[END_REF][START_REF] Nicholson | The relationship of the El Nino-Southern Oscillation to African rainfall[END_REF][START_REF] Nicholson | The Influence of La Nina on African Rainfall[END_REF][START_REF] Mutai | East African rainfall and the tropical circulation / convection on intraseasonal to interannual timescales[END_REF][START_REF] Indeje | ENSO signals in East African rainfall seasons[END_REF]. Following [START_REF] Saji | A dipole mode in the tropical Indian Ocean[END_REF] and [START_REF] Behera | Paramount Impact of the Indian Ocean Dipole on the East African Short Rains: A CGCM Study[END_REF], the ENSO signal in the East African "short rains" may only be an indirect manifestation of anomalous Walker-type (eastwest) circulation in the Indian Ocean, associated with zonal SST gradients. Though most of the years which record abundant "short rains" in Eastern Africa are El-Nino years, like 1965, 1972[START_REF] Wood | A preliminary chronology of Ethiopian droughts[END_REF][START_REF] Kinuthia | A newly found jet in North Kenya (Turkana Channel)[END_REF][START_REF] Camberlin | Rainfall anomalies in the source region of the Nile and their connection with the Indian summer monsoon[END_REF]and 2006 in the last 4 decades, there exist a few cases of high rainfall which can solely be explained by anomalous SST and circulation in the Indian Ocean. This is best shown in the year 1961 [START_REF] Flohn | East African rains of 1961/62 and the abrupt change of the White Nile discharge[END_REF][START_REF] Kapala | Monsoon anomaly or an El-Nino event at the equatorial Indian Ocean ? Catastrophic rains 1961/62 in East Africa and their teleconnections[END_REF][START_REF] Hastenrath | Circulation mechanisms of climate anomalies in the equatorial Indian Ocean[END_REF], whose exceptionally heavy rains resulted from a strong warming (cooling) in the western (eastern) Indian Ocean, in the absence of El-Nino conditions. Thus the October-December rains exhibit a higher correlation with the zonal SST gradient in the Indian Ocean than with the Nino 3+4 index (fig.9-10), in agreement with [START_REF] Saji | Possible impacts of Indian Ocean Dipole mode events on global climate[END_REF], though in many cases it is ENSO which originally triggers off the Indian Ocean dipolar events. During other seasons (including summer), zonal SST gradients in the Indian Ocean have virtually no role in rainfall variability over the Nile basin.

Though the summer rains in the Nile basin are partly dependant on ENSO, a stronger connection exists with the Indian Monsoon [START_REF] Camberlin | June-September rainfall in North-Eastern Africa and atmospheric signals over the tropics: a zonal perspective[END_REF][START_REF] Camberlin | Rainfall anomalies in the source region of the Nile and their connection with the Indian summer monsoon[END_REF]. Averaged over Ethiopia, Eritrea, Sudan, Uganda and Western Kenya (i.e., roughly the part of the Nile basin receiving summer rains), July-September rainfall during 1953-1988 is correlated at -0.89 with Bombay pressure, an indicator of the Indian monsoon intensity. Droughts (heavy rains), and not only those occurring at times of El-Nino (La-Nina) events, therefore tend to occur simultaneously in the Nile basin and in India. This is because a deepened monsoon low increases the pressure gradient with the South Atlantic Ocean, resulting into strengthened moist south-westerlies / westerlies in Ethiopia, Sudan and Uganda. An additional factor is the strengthened Tropical Easterly Jet associated with an intense Indian monsoon, known to enhance rainfall in the sahelian belt [START_REF] Hulme | The tropical easterly jet and Sudan rainfall: a review[END_REF][START_REF] Fontaine | Wind-field coherence and its variations over West Africa[END_REF][START_REF] Grist | A Study of the Dynamic Forces Influencing Rainfall Variability in the West Africa Sahel[END_REF].

Both teleconnections with ENSO and the Indian Monsoon fail to account for the downward rainfall trend experienced in the central part of the basin in the 1970s and 1980s. This trend is actually similar to that found further west in the sudano-sahelian belt of West Africa. Land cover changes, associated in particular with overgrazing, and which was earlier thought as being a possible trigger of the drought [START_REF] Charney | Dynamics of desert and drought in Sahel[END_REF], are now considered only as an amplifier of ocean-induced rainfall variations. Inter-hemispheric SST variations are actually considered as the key player in the long-term rainfall evolution (e.g., [START_REF] Folland | Sahel rainfall and worldwide sea temperatures (1901-1985)[END_REF][START_REF] Ward | Diagnosis and short-lead time prediction of summer rainfall in tropical north Africa at interannual and multidecadal timescales[END_REF][START_REF] Giannini | Oceanic forcing of Sahel rainfall on interannual to interdecadal time scales[END_REF]. In particular, the cooling of the North Atlantic ocean in the late 1960s, combined with the warming of the Southern Hemisphere oceans (including the Indian Ocean) is likely to have resulted into a reduced northward excursion of the ITCZ and/or more rainfall over the tropical oceans than over the African continent. A negative correlation was noted between and Indian Ocean SST and both Sudan rainfall [START_REF] Osman | Qualitative rainfall prediction models for central and southern Sudan using El Niño-southern oscillation and Indian Ocean sea surface temperature Indices[END_REF] and Ethiopia rainfall [START_REF] Seleshi | Rainfall variability in the Ethiopian and Eritrean highlands and its links with the Southern Oscillation Index[END_REF][START_REF] Gissila | Seasonal forecasting of the Ethiopian summer rains[END_REF][START_REF] Korecha | Predictability of June-September Rainfall in Ethiopia[END_REF]. This prompted these authors to define seasonal prediction models, including Indian Ocean SST as well as ENSO as predictors. However it should be noted that the negative correlation with Indian Ocean SST is mainly a manifestation of common trends, as evidenced for the Sahel as a whole [START_REF] Giannini | Oceanic forcing of Sahel rainfall on interannual to interdecadal time scales[END_REF]. South Atlantic SST also exhibits a warming trend. A small contribution of this basin to summer rainfall variability is demonstrated for Sudan and Ethiopia. The South Atlantic warming is not only a component of that of the southern hemisphere, but it also occasionally has a separate incidence on monsoon depth, like in 1984 where the devastating drought which affected Ethiopia and Sudan can be related to exceptionally high SST in the Gulf of Guinea. The main evaporative regions that contribute to summer rainfall in the eastern Sahel are actually central Africa and the Gulf of Guinea, and not the Indian Ocean (Druyan & Koster, 1989). The higher moisture content which prevails in warm years is largely counterbalanced by the reduced northward excursion / monsoon depth which results from the unfavourable meridional energy gradients.

Other causes of rainfall variability are not well known. This particularly applies to the northern spring (March-May) rainy season in the equatorial part of the basin. Its interannual variability is spatially poorly organised, and the above-mentioned sources of climate variability account for only a small part of rainfall fluctuations [START_REF] Camberlin | The east African March-May rainy season : associated atmospheric dynamics and predictability over the 1968-97 period[END_REF]. The zonal gradient between the Atlantic and Indian oceans and the associated anomalous westerlies play a part, and so do interactions with the upper tropospheric extratropical circulation. A North Atlantic Oscillation (NAO) signal is found over the southernmost part of the basin during the December-February dry season [START_REF] Mchugh | North Atlantic Oscillation influence on precipitation variability around the Southeast African Convergence Zone[END_REF], suggesting unseasonable rainfall in negative NAO years, though the teleconnection mechanisms are not well understood. In Ethiopia, an 11-yr cycle was previously identified in rainfall and associated with sunspot numbers and solar activity [START_REF] Wood | A preliminary chronology of Ethiopian droughts[END_REF][START_REF] Seleshi | Sunspot numbers as a possible indicator of annual rainfall at Addis-Ababa, Ethiopia[END_REF]. Evidence of similar oscillations in the equatorial lakes region is still inconclusive.

Projected climate changes

The drying trend which affected much of the Nile basin in the 1970s and 1980s, added to potential climate effects associated with increasing greenhouse gases concentrations in the global atmosphere, have prompted speculations about the future of the Nile basin climate.

In the 1990s, twenty-first century projected climate changes for the Nile basin were focused upon in a series of studies dedicated to the assessment of water resources, based on a direct use of General Circulation Models (GCM) experiments. [START_REF] Strzepek | Vulnerability assessment of water resources in Egypt to climatic change in the Nile Basin[END_REF] used 3 GCM (UKMO, GISS and GFDL) to evaluate future changes in the Nile water resources, under a doubled CO 2 hypothesis. A 15-17% increase in the basin-averaged rainfall was found for the first two models, and a 5% increase for the latter. Expected temperature rises ranged from 3.1 to 4.7°C. A more detailed assessment, based on interpolated data from 6 GCMs, provided contrasted projections [START_REF] Yates | Modeling the Nile basin under climatic change[END_REF]. While for the equatorial lake region almost all the models gave an increase in annual precipitation (0 to 26%), the projections for the Blue Nile catchment were between -9% and +55%. Temperature rises were more consistent among the models, as well as spatially. Inter-model differences in precipitation were also pointed out by [START_REF] Hulme | The impacts of climate variability and future climate change in the Nile Basin on water resources in Egypt[END_REF], who used the GFDL and GISS models as well as a weighted ensemble mean from 7 GCM experiments. Whereas all the experiments showed a temperature increase slightly under 1°C by 2025 with respect to 1961-1990, and fairly identical in the Blue Nile and lake Victoria areas as well as in all the four trimesters considered, precipitation projections were more contrasted. Increases dominated over lake Victoria, but large disparities in the Blue Nile region highlighted the difficulty for most models to simulate precipitation. More recently, [START_REF] Mchugh | Multi-model trends in East African rainfall associated with increased CO2[END_REF], based on the 4 models out of 19 which best simulate East African rainfall, showed a projected rainfall increase in the equatorial regions, mainly due to a rise in DJF and MAM rainfall.

Other studies considered more diverse greenhouse gases emission scenarios (SRES scenarios), but the outcome is not markedly different from that derived from earlier studies. [START_REF] Hulme | African Climate Change: 1900-2100[END_REF], analysing 10 experiments from 7 coupled models, obtained a median temperature rise of 0.12-0.18°C per decade under scenario B1 (low emission scenario) to 0.3-0.6°C per decade under scenario A2 (high emission scenario) for the Nile basin region. In each case the rise is less in the equatorial region, and higher in northern Sudan. Inter-model differences are moderate for temperatures, but not for precipitation. This is particularly so for the summer rains. While a majority of decreases (exceeding natural variability) are projected for much of eastern Africa, in Ethiopia the trend is negative in some models, and positive in others [START_REF] Hulme | African Climate Change: 1900-2100[END_REF]. In the northern winter (DJF), whatever the scenario, there is a more robust tendency for increased rainfall in Ethiopia and Uganda, but this is during the low rainfall season in most parts of the region.

The projections shown in the third assessment report (IPCC, 2001), and confirmed in the 2007 fourth assessment report, give an average temperature increase for the Nile basin region of around 2°C (B1 scenario) to 4°C (A2 scenario) for the years 2090-2099. Confidence in the projections remains lower for precipitation. The only two consistent trends projected using ensemble model experiments correspond to an increase in northern winter (DJF) rainfall in eastern equatorial Africa, and a decrease in the Eastern Mediterranean (in Egypt, from already very low rainfall amounts).

The expected temperature rise is to be compared with the actual changes during the twentieth century. Studying a greater eastern Africa region, from Sudan to Mozambique, [START_REF] King'uyu | Recent trends of minimum and maximum surface temperatures over Eastern Africa[END_REF] found a general minimum (night-time) temperature warming, especially in the northern part of the region, which corresponds to the Nile basin. The geographical patterns of the trends are very complex. However, for East Africa as a whole, a rise of about 0.4°C is found in the last four decades of the twentieth century [START_REF] Hulme | African Climate Change: 1900-2100[END_REF], though it should be recalled that the 1960s stand out as an unusually wet and cold decade. A significant rise (decrease) in hot (cold) extremes is also found in Uganda (New et al., 2006). For Egypt, decreasing temperature trends were found over the observation period 1941-2000 for annual, maximum, winter and autumn temperatures, and increasing trends for minimum, winter and spring temperatures [START_REF] Domroes | Recent temporal and spatial temperature changes in Egypt[END_REF]. Increasing trends prevailed in summer [START_REF] Hasanean | Variability of summer temperature over Egypt[END_REF]. For the recent period 1971-2000 all trends were positive except maximum temperature [START_REF] Domroes | Recent temporal and spatial temperature changes in Egypt[END_REF]. In Addis-Ababa in Ethiopia, [START_REF] Mould | Over one century of rainfall and temperature observations in Addis Ababa, Ethiopia[END_REF] showed increasing trends in annual minimum and maximum temperatures from 1951 to 2002 (0.4°C/decade and 0.2°C/decade, respectively). On the whole, while a majority of warming trends are found in the region in the last 50 years, the patterns are spatially and temporally variable, though becoming increasingly clear in the last decades. Observed rainfall trends have been discussed above. They are characterised by the pronounced decrease of the 1970s and 1980s in Sudan and parts of the Ethiopian Highlands, with a partial recovery since then, and have been shown to result from hemispheric SST variations.

The fact that the region is subject to large natural climate variability makes the observation of climatic trends of the last few decades a poor indicator of the climate conditions expected for the Nile basin during the twenty-first century. This can only be assessed through numerical modelling. However, the problem with GCM projections is that they are made at coarse resolutions (typically at a scale of 2.5° latitude and longitude) so that, in a region like the Nile basin which has a complex topography and contrasted land surface properties, the associated regional circulation features are very poorly resolved. To date, there have been few attempts to address this problem, though the current development of a regional climate model tuned to North-East Africa (e.g., [START_REF] Mohamed | Hydroclimatology of the Nile: Results from a regional climate model[END_REF][START_REF] Anyah | Simulated physical mechanisms associated with multi-scale climate variability over lake Victoria Basin in East Africa[END_REF] will be an important step towards higher resolution climate projections. Alternatives are the use of statistical downscaling methods, which rely on the definition of empirical models relating present-day rainfall and temperature variations to large-scale atmospheric patterns. The latter are generally better simulated than precipitation by the GCMs, hence by using the projections of dynamical variables under enhanced greenhouse conditions, it might be feasible to estimate rainfall changes and their spatial distribution. However, this requires a prior understanding of the factors which determine current rainfall variations in the Nile basin region. Data : Long-term mean NCEP-NCAR reanalysis [START_REF] Kalnay | The NCEP/NCAR 40-year Reanalysis Project[END_REF]. Data over areas of complex topography should be considered as approximate. Bold line : July InterTropical Front surface location as deduced from the shift from northerly to southerly winds. Shading : regions of mean ascending motion at 400 hPa. . Data : Climate Research Unit CRU CL 2.0 climatology at a 10' resolution (New et al., 2002). Isohyets in mm.

Fig. 1 :

 1 Fig.1 : Mean surface winds and sea-level pressure over North-East Africa in January and July.Data : Long-term mean NCEP-NCAR reanalysis[START_REF] Kalnay | The NCEP/NCAR 40-year Reanalysis Project[END_REF]. Data over areas of complex topography should be considered as approximate. Bold line : July InterTropical Front surface location as deduced from the shift from northerly to southerly winds. Shading : regions of mean ascending motion at 400 hPa.

Fig. 2 :

 2 Fig.2 : Time-height distribution of mean zonal winds above central Sudan (15°N, 32.5°E), in m.s -1 . Data : Long-term mean (1968-1996) NCEP-NCAR reanalysis. Positive (negative) values indicate westerlies (easterlies).

Fig. 3 :Fig. 4 :

 34 Fig.3 : Frequency distribution of mean annual rainfall in the Nile basin (1961-1990 average), as a percentage of the total surface area. Data : Climate Research Unit CRU CL 2.0 climatology at a 10' resolution (New et al., 2002).

Fig. 5 :

 5 Fig.5 : Typology of rainfall regimes for the Nile basin. Based on a hierarchical cluster analysis of mean monthly rainfall for 1961-1990 (CRU CL 2.0 data). Figures in each panel indicate the maximum monthly rainfall. For type 7, type 6 is shown as a dashed line for comparison.

Fig. 6 :

 6 Fig.6: Mean diurnal distribution of precipitation along west-east cross-sections. Top : crosssection from the Nile plains to the Ethiopian Highlands, around 15°N (3-hourly rainfall amount or occurrence as percentage of the total for May-October). Bottom : cross-section from the lake Victoria west coast to the Indian Ocean, around 0-5°S (3-hourly rainfall occurrence as percentage of the annual total). Data from[START_REF] Pedgley | Diurnal variation of the incidence of monsoon rainfall over the Sudan[END_REF] 1971[START_REF] Tomsett | The diurnal variation of precipitation in East Africa[END_REF].

Fig. 7

 7 Fig.7 Space-time patterns of the interannual variability of annual rainfall amounts, for 1951-2000. First 3 principal components (with a varimax rotation) of the correlation matrix. Map shadings : areas whose rainfall is significantly correlated (5% confidence level) with the corresponding time-series (bottom). Figures are correlation values.

  Fig.7 Space-time patterns of the interannual variability of annual rainfall amounts, for 1951-2000. First 3 principal components (with a varimax rotation) of the correlation matrix. Map shadings : areas whose rainfall is significantly correlated (5% confidence level) with the corresponding time-series (bottom). Figures are correlation values.

Fig. 8 Fig. 9 :

 89 Fig.8 Percentage change in annual rainfall between 1951 and 2000 (slopes of the linear trend as a percentage of the mean rainfall). Data : interpolated rainfall series based on the CRC rainfall data base and CRU rainfall climatology. Dashed : rainfall decrease ; solid : rainfall increase. Dots : significant decrease ; cross-hatching : significant increase (Spearman's ranks correlation, 95% confidence level).

Fig. 10 :

 10 Fig.10 : Correlation maps between interpolated rainfall and climate indices. (a) Nino 3+4 and July-September rainfall ; (b) Nino 3+4 and October-December rainfall ; (c) Indian Ocean Dipole Mode Index and October-December rainfall. Period : 1951-2000. Dashed (solid) lines : negative (positive) correlations. Shading : correlations significant at the 95% level.