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Bernard Fontaine
and
Serge Janicot

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 synchronous of flood limited to the Sahel. Flooding over all of West Africa is mainly associated with positive SST anomalies in the northern Atlantic. This approach complements previous papers dealing with rainfall anomalies located in the Sahel alone. In particular, it points out different associations between Atlantic/Pacific SST anomalies and Sahel rainfall variability. Individual cases are also discussed, especially the case of 1972 when a West African rainfall deficit was concomitant to a southward location of the intertropical convergence zone over the tropical Atlantic.

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Bernard Fontaine
and
Serge Janicot

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 more southward position of the intertropical convergence zone, a reduced monsoonal flow, an increased AEJ velocity, and a reduced TEJ velocity. The vertical sheer involved by the thermically enhanced AEJ is concomitant at the monthly time scale, with the decrease of the vertical coupling between the lower Layers' convergence of the monsoons flow and the lower layers' divergence associated with the TEJ.

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Bernard Fontaine
,
Serge Janicot
, and
Vincent Moron

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 and under the AFJ, associated with a reduction of westerlies in the monsoon flow and midtropospheric southerly anomalies north of 15°N. The opposite-contrasted rainfall patterns (Sahelian floods and Guinean droughts) are characterized by a southward extension of upper easterlies, a decrease of the easterlies extending south of the AEJ, an increase of wind components in the monsoon flow, and an increased confluence of the meridional components in the midtroposphere above 5°N. Two indexes taking into account the vertical (200 hPa/850 hPa) and the 850 hPa latitudinal (10°N/20°N) relationships in the zonal wind component discriminate the two contrasted rainfall anomaly patterns. A third index, using the meridional components in high levels for documenting diffluence variability in the Hadley circulation, characterizes large-scale West African droughts.

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Benjamin Pohl
,
Serge Janicot
,
Bernard Fontaine
, and
Romain Marteau

Abstract

Madden–Julian oscillations (MJOs) are extracted over the Indo-Pacific basin using a local mode analysis. The convective perturbations are then projected over a larger domain to evaluate their remote consequences over the West African monsoon (WAM) intraseasonal variability. Rather weak (4–6 W m−2) convective fluctuations occurring in phase with those over the southern Indian basin are found over Africa, confirming the results of Matthews. In reverse, 40-day fluctuations in the WAM, similarly detected and projected over a widened area, demonstrate that a large majority of these events are embedded in the larger-scale patterns of the MJO. The regional amplitude of intraseasonal perturbations of the West African convection is not statistically associated with the amplitude of the MJO over the Indian basin but is instead closely related to background vertical velocity anomalies over Africa, possibly embedded in changes in the regional Walker-type circulation. Subsiding motion over Africa is recorded during the most energetic convective perturbations in the WAM.

Composites analyses over the MJO life cycle, as depicted by the real-time daily indices developed by Wheeler and Hendon, show that positive outgoing longwave radiation (OLR) anomalies during the dry phase are of larger amplitude and spatially more coherent than negative anomalies during the wet phase, especially over the Sahel region. Over West Africa, the phase of suppressed convection is thus of greater importance for the region than the phase of enhanced convection. Rain gauge records fully confirm these results. The MJO appears to be significantly involved in the occurrences of dry spells during the monsoon over the Sahel, whereas large-scale convective clusters are only restricted to the equatorial latitudes and thus affect the Guinean belt, which experiences its short dry season at this time of the year.

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Irene Polo
,
Albin Ullmann
,
Pascal Roucou
, and
Bernard Fontaine

Abstract

Weather regimes (WRs) have been defined over the Euro-Mediterranean region (15°–70°N, 60°W–60°E) from May to October using the daily sea level pressure, 700-hPa geopotential height, and specific humidity from the European Centre for Medium-Range Weather Forecasts Re-Analysis (ERA)-Interim over the 1989–2008 period. Computations are based on a neural network classification technique referred to as self-organizing maps, and the WRs produced can be used by the scientific community for comparison with other periods, projection onto model outputs, seasonal prediction, or teleconnection studies. The article particularly examines the relationship between WRs and West Africa (WA) rainfall, and the study’s results suggest that changes in particular WR frequencies can account for a part of the WA’s interannual rainfall variability. Thus, during anomalous wet (dry) years in WA rainfall, both more occurrences of WRs related to the negative (positive) summer North Atlantic Oscillation (NAO)–like pattern and fewer occurrences of WRs related to the positive (negative) summer NAO-like pattern are attested in July and August (SN− and SN+, respectively). This is associated with a zonal symmetric pattern, consistent along the midtroposphere—that is, a low pressure anomaly centered over 50°N, 20°W and Eurasia (Greenland) and a high pressure anomaly centered over Iceland (central Europe) for SN− (SN+). Another striking characteristic of SN− (SN+) are southeastward (southwestward) anomalous surface winds flowing from (to) the Atlantic Ocean at 20°N; therefore able to enhance (weaken) wet convection. That sea surface temperature associated with SN− shows a warming of the Mediterranean in July and the opposite with SN+ in August suggests that temperature anomalies could be a precursor in the change of frequency of SN− and SN+.

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Serge Janicot
,
Ali Harzallah
,
Bernard Fontaine
, and
Vincent Moron

Abstract

The Laboratoire de Météorologie 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 equatorial Pacific and the Atlantic is evident in the period 1970–88. Positive (negative) phases of this coupling could enhance the impact of ENSO warm (cold) events on West African monsoon dynamics.

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Belen Rodríguez-Fonseca
,
Elsa Mohino
,
Carlos R. Mechoso
,
Cyril Caminade
,
Michela Biasutti
,
Marco Gaetani
,
J. Garcia-Serrano
,
Edward K. Vizy
,
Kerry Cook
,
Yongkang Xue
,
Irene Polo
,
Teresa Losada
,
Leonard Druyan
,
Bernard Fontaine
,
Juergen Bader
,
Francisco J. Doblas-Reyes
,
Lisa Goddard
,
Serge Janicot
,
Alberto Arribas
,
William Lau
,
Andrew Colman
,
M. Vellinga
,
David P. Rowell
,
Fred Kucharski
, and
Aurore Voldoire

Abstract

The 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 decadal forecasting. The agreement among future projections has improved from CMIP3 to CMIP5, with a general tendency for slightly wetter conditions over the central part of the Sahel, drier conditions over the western part, and a delay in the monsoon onset. The role of the Indian Ocean, the stationarity of teleconnections, the determination of the leader ocean basin in driving decadal variability, the anthropogenic role, the reduction of the model rainfall spread, and the improvement of some model components are among the most important remaining questions that continue to be the focus of current international projects.

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