David Monkam

Convective available potential energy (CAPE) in Northern Africa and tropical Atlantic and study of its connections with rainfall in Central and West Africa during Summer 1985

Abstract In this paper, we study the distribution of the convective available potential energy (CAPE) in northern Africa and tropical Atlantic in summer. The pattern of this parameter is quite steady and presents three main zones. The first one is located between the equator and 15°N–20°N, with positive values of CAPE. In the second and the third zones located north of the band of latitudes 15°N–20°N and south of the equator, respectively, CAPE has negative values. We examined, in some details, the main tendencies of the CAPE pattern inside the first zone (CAPE >0, in general) and we compared them to the rainfall pattern. We noted that the CAPE pattern in this zone presents three regions of maximum values: west, center, and east of the zone. The maximum at east and the maximum in the center seem to be connected to the orography effects. Between 15°N and 20°N, the CAPE distribution is characterised by its iso-line zero. This iso-line zero seems to define the north limit of the inter tropical convergence zone (ITCZ). In fact, the position of the iso-line zero on the CAPE distribution fluctuates during summer as the position of the ITCZ so that this parameter can also be used as a parameter or an index to define the ITCZ position in northern Africa and tropical Atlantic in the summer. The patterns of the rainfall and CAPE in this zone (5°N–20°N) have a very good similarity. When the rainfall is very weak ( −1 ) towards 12.5°N–15°N, the CAPE tends to become zero. The rainfall pattern presents, in general, high values towards Guinea and around Lake Chad. These regions are not very far from the maximum values of CAPE observed in different figures. The fields of correlation coefficients between the two parameters show that the rainfall and the CAPE are very well correlated around the ITCZ and towards some mountains. So, the strong values of the correlation coefficients seem to indicate that each of these two parameters is influenced by the ITCZ effects and by the orography effects.

Convection activity over the Guinean coast and Central Africa during northern spring from synoptic to intra-seasonal timescales

This study proposes an overview of the main synoptic, medium-range and intraseasonal modes of convection and precipitation in northern spring (March–June 1979–2010) over West and Central Africa, and to understand their atmospheric dynamics. It is based on daily National Oceanic and Atmospheric Administration outgoing longwave radiation and Cloud Archive User Service Tb convection data, daily TRMM and Global Precipitation Climatology Project rainfall products and daily ERA-Interim reanalysis atmospheric fields. It is first shown that mesoscale convective systems can be modulated in terms of occurrences number and intensity at such time scales. Based on empirical orthogonal function analyses on the 2–90-day filtered data it is shown that the main mode of convective and rainfall variability is located along the Guinean coast with a moderate to weak extension over Central Africa. Corresponding regressed deseasonalised atmospheric fields highlight an eastward propagation of patterns consistent with convectively coupled equatorial Kelvin wave dynamics. Then a singular spectrum analysis combined with a Hierarchical Ascendant Classification enable to define objectively the main spectral bands of variability within the 2–90-day band, and highlight three main bands, 2–8-, 8–22- and 20–90-day. Within these three bands, space–time spectral decomposition is used to identify the relative impacts of convectively coupled equatorial Kelvin, Rossby and inertia–gravity waves, as well as Madden–Julian Oscillation (MJO) signal. It confirms that eastward propagating signals (convectively coupled equatorial Kelvin wave and MJO) are highly dominant in these convection and precipitation variability modes over the Guinean coast during northern spring. So, while rain-producing individual systems are moving westward, their activity are highly modulated by sub-regional and regional scales envelops moving to the east. This is a burning issue for operational forecasting centers to be able to monitor and predict such eastward propagating envelops of convective activity.

Variability and trends of local/regional scale surface climate in northern Africa during the twentieth century

Four regions are detected in northern Africa (20° W–40° E, 0–30° N) by applying the cluster analysis method on the annual rainfall anomalies of the period 1901–2000. The first region (R1), an arid land, covers essentially the north of 17.75° N from west to east of the study zone. The second region (R2), a semiarid land with a Sahelian climate, less warm than the dry climate of R1, is centred on Chad, with almost regular extension to the west towards Mauritania, and to the east, including the north of the Central African Republic and the Sudan. The region 3 (R3), a wet land, is centred on the Ivory Coast and covers totally Liberia, the south part of Ghana, Togo, Benin and the southwest of Nigeria. The fourth region (R4), corresponding to the wet equatorial forest, covers a part of Senegal, the Central Africa, the south of Sudan and a part of Ethiopia. An analysis of observed temperature and precipitation variability and trends throughout the twentieth century over these regions is presented. Summer, winter and annual data are examined using a range of variability measures. Statistically, significant warming trends are found over the majority of regions. The trends have a magnitude of up to 1.5 K per century. Only a few precipitation trends are statistically significant. Regional temperature and precipitation show pronounced variability at scales from interannual to multi-decadal. The interannual variability shows significant variations and trends throughout the century, the latter being mostly negative for precipitation and both positive and negative for temperature. Temperature and precipitation anomalies show a chaotic-type behaviour in which the regional conditions oscillate around the long-term mean trend and occasionally fall into long-lasting (up to 10 years or more) anomaly regimes. A generally modest temporal correlation is found between anomalies of different regions and between temperature and precipitation anomalies for the same region. This correlation is mostly positive for temperature in cases of adjacent regions. Several cases of negative interregional precipitation anomaly correlation are found. The El Niño Southern Oscillation significantly affects the anomaly variability patterns over a number of regions, mainly regions 3 (R3) and 4 (R4), while the North Atlantic Oscillation significantly affects the variability over arid and semiarid regions, R1 and R2.

Analysis of return periods and return levels of Yearly July–September extreme droughts in the West African Sahel

This paper aims to model the occurrence of Yearly July–September (YJAS) extreme droughts in the West African Sahel (WAS) and to estimate return periods and return levels of these events through stationary peaks-over-threshold model. For this purpose, the historical gridded monthly rainfall data from Climatic Research Unit for the period 1901–2009 were used. The results show that return levels of YJAS dry extremes have increased since 1970, implying that YJAS extreme droughts are consistently more severe after 1970 than they were before. Approximately

Variability of static stability over West Africa during Northern summer 1979-2005

62\%

The 6-9 day wave and rainfall modulation in northern Africa during summer 1981

62% of the WAS area in the postchange period of 1971–2009 was dominated by dry spells not longer than 1 year. The dynamics of the YJAS extremes drying trend indicate that the changes at the tails of YJAS dry extreme distribution have contributed to the dry trend in mean YJAS rainfall in the WAS. The estimated 40-year return level of these events based on 1971–2009 period was less than the average of dry extremes of the same period, suggesting that droughts could intensify in the future even though with some amelioration. Such a finding could prove helpful in anticipation of climate risks in this region where adaptive capacities are very low.