Bernard Fontaine

Sahel droughts and Enso dynamics

Correlations between summer Sahel rainfall and Southern Oscillation Index has increased during the last thirty years. At high frequency time scale (periods lower than 8 years), an intertropical Atlantic zonal divergent circulation anomaly is forced by the difference of sea surface temperature (SST) anomalies between the eastern equatorial parts of Pacific and Atlantic. This zonal connection worked well during most of the E1 Nino/Southern Oscillation (ENSO) events occurring after 1970; positive/negative SST anomalies in the eastern Pacific/Atlantic led to rainfall deficits over the whole West Africa. At low frequency time scale (periods greater than 8 years), positive SST anomalies in the Indian ocean and in equatorial Pacific existing after 1970 have been associated with decreasing rainfall intensity over West Africa through another zonal divergent circulation. These different time scales remote SST forcings are combined to provide a global zonal divergent circulation anomaly pattern which could explain the strong association between Sahel drought and ENSO dynamics after 1970.

West African Monsoon Dynamics and Eastern Equatorial Atlantic and Pacific SST Anomalies (1970–88)

Abstract The Laboratoire de Meteorologie Dynamique atmospheric GCM is used to investigate relationships between West African monsoon dynamics and SST anomalies in the eastern equatorial Atlantic and Pacific for the period 1970–88. Positive SST anomalies in the eastern equatorial Pacific, mainly associated with a larger east–west divergent circulation over the tropical Atlantic, are found to coincide with negative rainfall anomalies over West Africa. This is the case for the composite ENSO warm episodes of 1972, 1976, 1982, and 1983. By contrast, positive SST anomalies in the eastern equatorial Atlantic are accompanied by a southward shift of the intertropical convergence zone along with negative rainfall anomalies in the Sahel and positive rainfall anomalies in the Guinean region. This was the case in 1987. The ENSO warm event during this year had apparently no significant impact on West African monsoon dynamics. A zonal atmospheric coupling associated with differences of SST anomalies between the eastern...

Sea Surface Temperature Fields Associated with West African Rainfall Anomaly Types

Abstract Four West African rainfall anomaly types are defined in relation to the northern summer rainfall departure signs in the Sahel and in the Guinean region in order to investigate the statistical links between interannual variability of West African rainfall and sea surface temperature (SST) through the period 1950–90. Composite analysis depicts the setup of four different mean SST anomaly fields. Drought over all of West Africa is associated with the growth of positive SST anomalies in the eastern Pacific and in the Indian Ocean, and negative SST anomalies in the northern Atlantic and in the Gulf of Guinea. In contrast, drought limited to the Sahel corresponds mostly to a northward expansion of positive SST anomalies in the southern Atlantic, and negative SST anomalies in the northern Atlantic. Northward expansion of negative SST anomalies in the southern Atlantic, positive SST departures in the northern Atlantic, and development of negative SST anomalies in the eastern Pacific appear to be synchron...

Rainfall anomaly patterns and wind field signals over West Africa in August (1958-1989)

Abstract Relationships between monthly West African rainfall anomaly patterns and monthly tropospheric wind changes are investigated for the 32 months of August (1958–1989) at an interannual time step. Regarding the Sahelian belt alone, results show that wet (dry) months are significantly linked to an increase (decrease) in both upper easterlies and lower southwesterlies, along with weaker (stronger) than usual midtropospheric easterlies south and under the main axis of the African Easterly Jet (AEJ). However, the most interesting signals are obtained with consideration of the West African rainfall anomaly patterns. Large droughts over Sahelian and Guinean areas are significantly associated with a less southward extension of upper easterlies and a decrease of the upper-meridional diffluence in the Hadley circulation. Contrasted rainfall anomaly patterns exhibiting both Sahelian droughts and Guinean floods are mainly linked to easterly anomalies in the equatorial stratosphere and enhanced easterlies south ...

Wind-Field Coherence and Its Variations over West Africa

Abstract The wind-field coherence over West Africa is investigated during two contrasted periods, 1953–63 (abnormally wet) and 1963-75 (abnormally dry) in the Sahel. Zonal and meridional components of the monthly mean wind are computed with radiosonde data relative to the 950-100-hPa layer. A descriptive analysis reveals the main features of the West African atmospheric circulation, and a correlation analysis points out their dynamical coherence. The exiwnce of Hadley-type cells of Northern and Southern hemispheres are confirmed, and a mutual interaction in the lower layers is depicted. The functioning of a Walker-type cell, defined by a coupling between the zonal components of the monsoonal flow and of the tropical easterly jet (TEJ), is shown during northern summer over the whole of West Africa. There is no correlation between the TEJ velocity and the African easterly jet (AEJ) velocity over the Sahelian belt. During the dry period, the coherence of the cell circulations vanishes in concomitance with a ...

An improvement of June–September rainfall forecasting in the Sahel based upon region April–May moist static energy content (1968–1997)

This study provides statistical evidence that June–September Sahelian rainfall hindcasts currently based on oceanic thermal predictors apprehend more the negative trend than the interannual rainfall variations. Four physically meaningful predictors of June–September Sahel rainfall are first selected through the near-surface April–May information and several experimental hindcasts provided. We then discuss the skills achieved using regression techniques and cross-validated discriminant functions. In that context, 8/11 of the driest seasons and 8/10 of the wettest are correctly predicted. Finally using completely independent training and working periods we show that better and significant hindcast skills are obtained by adding regional moist static energy predictors to the large-scale sea surface temperature information.

Variability and Predictability of West African Droughts: A Review on the Role of Sea Surface Temperature Anomalies

AbstractThe Sahel experienced a severe drought during the 1970s and 1980s after wet periods in the 1950s and 1960s. Although rainfall partially recovered since the 1990s, the drought had devastating impacts on society. Most studies agree that this dry period resulted primarily from remote effects of sea surface temperature (SST) anomalies amplified by local land surface–atmosphere interactions. This paper reviews advances made during the last decade to better understand the impact of global SST variability on West African rainfall at interannual to decadal time scales. At interannual time scales, a warming of the equatorial Atlantic and Pacific/Indian Oceans results in rainfall reduction over the Sahel, and positive SST anomalies over the Mediterranean Sea tend to be associated with increased rainfall. At decadal time scales, warming over the tropics leads to drought over the Sahel, whereas warming over the North Atlantic promotes increased rainfall. Prediction systems have evolved from seasonal to decada...

Influence of the Mediterranean Sea on the West African monsoon: Intraseasonal variability in numerical simulations

[1] The influence of May to September sea surface temperature (SST) anomalies in the Mediterranean Sea on the West African monsoon is investigated, analyzing the outputs of numerical sensitivity experiments performed using three atmospheric general circulation models (Action de Recherche Petite Echelle Grande Echelle, European/Hamburg, and University of California, Los Angeles) in the framework of the African Monsoon Multidisciplinary Analysis project. The precipitation and atmospheric dynamics response to the SST forcing is explored, in terms of intraseasonal variability, evaluating the results from the individual models and from the multimodel mean. A positive precipitation response to warmer than average conditions in the Mediterranean Sea is found in the Sudano-Sahelian belt in August-September. The proposed dynamic mechanism underlying the Mediterranean action on the West African monsoon is based on the modifications produced by the SST forcing in the moisture content in the lower troposphere. A warmer eastern Mediterranean in August-September feeds the lower troposphere with additional moisture, with a consequent reinforcement of northerly moisture transport toward the Sahel. Furthermore, warmer SST is linked to a strengthening of the Saharan heat low and to an enhancement of the moist static energy meridional gradient over West Africa, favoring the northward displacement of the monsoonal front.

Analysis of the plasma expansion dynamics by optical time-of-flight measurements

Abstract The laser plasma plume expansion induced by UV nanosecond and IR picosecond irradiation of an Y-Ba-Cu-O target has been studied using optical time-of-flight spectra. Nanosecond ablation leads to a broad particle velocity distribution already at the first stage of expansion because of a temperature dispersion of the ejected species. Picosecond irradiation reduces this kind of dispersion and allows one to observed properly the particle expansion dynamics. The velocities of the charged particles are inversely proportional to the square root of the atomic mass. The velocities of the neutral atoms exhibit much weaker dependence on the atomic mass and are determined by collisional expansion. The time-of-flight spectra reveal a double population of YO + molecules with different velocity distribution.

Sudan‐Sahel rainfall onset: Definition of an objective index, types of years, and experimental hindcasts

Using rainfall estimates from the 5-day version of CPC Merged Analysis of Precipitation (CMAP) and Global Precipitation Climatology Project data along with multiscale spatial key descriptors of atmospheric dynamics from National Center for Environmental Prediction/Department of Energy reanalysis 2 (NCEP/DOE), we first define a West African monsoon onset index to determine its successive dates of occurrence over the period 1979-2004 (28-29 June in mean with a standard deviation of 8.5 days). Then we focus on the three main types of time evolutions of that index at the moment of the monsoon onset in terms of precipitation, pressure, temperature, and winds at different levels to detect the most robust associated signals and select on the mid-May, mid-June period different sets of potential predictors for onset dates. Basically, late (early) onsets are preceded by more (less) rainfall southward to the equator by the end of May, then by a clear decrease (increase) of the normal northward cross-equatorial gradient in mid-June. Finally, we present experimental cross-validated hindcasts of the dates of onset always based on four predictors using both reanalyzed atmospheric data and observed rainfall estimates. Accurate forecasting schemes are obtained, especially with CMAP rainfall (time tendencies and cross-equatorial gradients) over the Gulf of Guinea. In this context the hindcasted series are highly significant when compared to observations (75% of explained common variance with the sole rainfall predictors) and can reach 82% of variance when a zonal wind descriptor of regional scale is added. Basically, a late (early) onset is preceded by more (less) rainfall southward to the equator by the end of May, then by a clear decrease (increase) of the normal northward cross-equatorial gradient by mid-June.