Mean Kinematic Characteristics of Synoptic Easterly Disturbances over the Atlantic

This study investigates the mean kinematic characteristics of the tropical Atlantic easterly disturbances in January-March(JFM),April-June(AMJ),July-September(JAS)and October-December(OND)from 1968-1998.For each season,the preferential tracks of these disturbances in the 3-10-day band periods were computed and spatialized,as well as their associated wavelength,velocity and main period,which lies between 3-5 days and between 6-9 days depending on the track and the season. Two main tracks are highlighted over the Atlantic Ocean.During OND and JFM these two tracks are well separated and located in each hemisphere around 15°S and 12.5°N.From AMJ to JAS these tracks migrate northward;in JAS,they merge into one over the northern tropical Atlantic along 17.5°N.The associated wavelength fields exhibit a meridional gradient,with large wavelengths(greater than 4000 km) around the equator,between 5°N and 5°S,and smaller wavelengths outside of this latitude band(between 2500-3500 km).The phase speed is also found to exhibit poleward decreasing values from 12-6 m s-1. Over the north Atlantic track,6-9-day disturbances were found to occur from January to May and approximately from October to December.From June to September,the 3-5-day waves dominate the synoptic activity.Over the south Atlantic track,between May and August the synoptic variability is mainly explained by the 3-5-day disturbances but from January to April and from September to December both 3-5-day waves and 6-9-day waves can occur.     

Comparative influence of land and sea surfaces on the Sahelian drought: a numerical study

The aim of this work is to compare the relative impact of land and sea surface anomalies on Sahel rainfall and to describe the associated anomalies in the atmospheric general circulation. This sensitivity study was done with the Météo-France climate model: ARPEGE. The sensitivity to land surface conditions consists of changes in the management of water and heat exchanges by vegetation cover and bare soil. The sensitivity to ocean surfaces consists in forcing the lower boundary of the model with worldwide composite sea surface temperature (SST) anomalies obtained from the difference between 4 dry Sahel years and 4 wet Sahel years observed since 1970. For each case, the spatiotemporal variability of the simulated rainfall anomaly and changes in the modelled tropical easterly jet (TEJ) and African easterly jet (AEJ) are discussed. The global changes in land surface evaporation have caused a rainfall deficit over the Sahel and over the Guinea Coast. No significant changes in the simulated TEJ and an enhancement of the AEJ are found; at the surface, the energy budget and the hydrological cycle are substantially modified. On the other hand, SST anomalies induce a negative rainfall anomaly over the Sahel and a positive rainfall anomaly to the south of this area. The rainfall deficit due to those anomalies is consistent with previous diagnostic and sensitivity studies. The TEJ is weaker and the AEJ is stronger than in the reference. The composite impact of SST and land surfaces anomalies is also analyzed: the simulated rainfall anomaly is similar to the observed mean African drought patterns. This work suggests that large-scale variations of surface conditions may have a substantial influence on Sahel rainfall and shows the importance of land surface parameterization in climate change modelling. In addition, it points out the interest in accurately considering the land and sea surfaces conditions in sensitivity studies on Sahel rainfall.     

African Easterly Waves and convective activity in wet and dry sequences of the West African Monsoon

Janicot and Sultan (Geophys Res Lett 28(3):523–526, 2001) and Sultan et al. (J Clim 16(21):3389–3406, 2003) showed evidence of an intraseasonal signal of Sahelian rainfall corresponding to wet and dry sequences of the West African Monsoon. Using NCEP/NCAR reanalysis, NOAA outgoing longwave radiation (OLR) and observed daily rainfall over West Africa from 1968 to 1990, this paper investigates the variability of 3 to 5-day African Easterly Waves (AEWs), convection and their relationship with rainfall in these wet and dry sequences. The mean daily value rainfall during wet sequences is twice the mean value during dry sequences but the number of dry or wet sequences per year is not correlated with the annual rainfall. Wet sequences account for 39% of the annual accumulated rainfall while dry sequences account for 22%. The number of 3 to 5-day AEWs increases during wet years in wet sequences and the activity tends to be larger during wet years in both wet and dry sequences. These AEWs explain 40% of the accumulated rainfall during wet sequences whereas they contribute to 26% of the accumulated rainfall observed during dry sequences. Generally, they contribute to the increase of rainfall during these sequences. Mean convection is stronger and there are twice as many low OLR days (<225W/m²) during wet than dry sequences. The mean rainfall for days with high convective activity (convective days) is also twice as great during wet sequences. Rainfall that occurs during days without low OLR (weak convection with warm cloud tops or isolated deep convection) contributes to 69% of the total rainfall during dry sequences and 45% during wet sequences. A composite study was performed from day D ₀−10 to day D ₀+10 in each sequence. Wet (dry) sequences of the African monsoon start with a decrease (slight increase) of the negative meridional Ertel Potential Vorticity (PV) gradient at 700 hPa, associated with an increase (decrease) of the spectral density of AEWs. During the wet sequence, the African Easterly Jet (AEJ), detected by 700 hPa zonal wind, decreases and moves northward, whereas the Tropical Easterly Jet (TEJ), detected at 200 hPa, increases and shifts southward. Convective activity increases from D ₀−6 to D ₀−3 and remains high for 4 days in wet sequences. The daily rainfall increases (decreases) between D ₀−6 and D ₀ and returns to the mean value at D ₀+4 for wet (dry) sequences.