Northeastern Tropical Atlantic SST and Sahel Rainfall Variability

The SST variability during the summer period in the northeastern tropical Atlantic region (NTA) is characterized by an alternation of warming/cooling which represents 87% of the total variability. The aim of this paper is to study the atmospheric responses as well as the precipitation associated with these oceanic conditions. Based on Reynolds’s SST from 1982 to 2019, a normalized Northern Tropical Atlantic index (NTAI) is computed into the region between 15° - 25°W;12° - 16°N and a composite analysis is then performed. It is shown that the NTAI is significantly correlated with the SST’s first principal component mode (PC1) in this region. Moreover, the composite of SST anomalies and atmospheric parameters exhibits a strong local ocean-atmosphere interaction which highly impacts the large-scale atmospheric circulation in West Africa, particularly in the western Sahel. An in-depth analysis shows that the atmospheric response to the warm (cold) SST is a cyclonic (anticyclonic) circulation in the lower layers near the West Africa Coast. This cyclonic (anticyclonic) circulation strengthens/reduces the moisture transport towards the continent in the low levels. In the middle layers of the atmosphere (500 hPa), the warm (cold) composite is associated with a decrease (increase) in the intensity of the African Easterly Jet (AEJ) whereas, in the upper atmosphere (200 hPa), the strengthening (weakening) of the Tropical Easterly Jet (TEJ) is observed. With regard to the composite precipitation field, a positive/negative SST anomaly is associated with significantly enhanced/reduced rainfall in the western Sahelian region. It is found that this relationship (correlation) increases as we are closer to the coasts.

Formation and Transport of a Saharan Dust Plume in Early Summer

This research studies the capability of the Weather Research and Forecasting model coupled with the Chemistry/Aerosol module(WRF-Chem)with and without parametrization to reproduce a dust storm,which was held on 27th June 2018 over Sahara region.The authors use satellite observations and ground-based measurements to evaluate the WRF-Chem simulations.The sensitivities of WRF-Chem Model are tested on the replication of haboob features with a tuned GOCART aerosol module.Comparisons of simulations with satellite and ground-based observations show that WRF-Chem is able to reproduce the Aerosol Optical Depth(AOD)distribution and associated changes of haboob in the meteorological fields with temperature drops of about 9℃and wind gust 20 m·s–1.The WRF-Chem Convec-tion-permitting model(CPM)shows strong 10-meter winds induced a large dust emission along the leading edge of a convective cold pool(LECCP).The CPM indicates heavy dust transported over the West African coast(16°W-10°W;6°N-21°N)which has a potential for long-distance travel on 27th June between 1100 UTC and 1500 UTC.The daily precipitation is improved in the CPM with a spatial distribution similar to the GPM-IMERG precipitation and maxi-mum rainfall located at the right place.As well as raising a large amount of dust,the haboob caused considerable dam-age along its route.

Case Study of Coastal Fog Events in Senegal Using LIDAR Ceilometer

This study aims to examine the atmospheric conditions characterising fog phenomena on the Senegalese coast focusing on two specific instances that occurred on April 3 and April 30,2023.These events were detected by the LIDAR Ceilometer installed at LPAOSF/ESP/UCAD and confirmed on the METARs of the meteorological stations at Dakar and Diass airports.The LIDAR’s backscatter signal showed that the fog of April 3 started around midnight with a vertical extension at 100 m altitude and dissipated around 10 a.m.The April 30 event characterized by a good vertical extension from the surface up to 300 m above sea level,was triggered just after 2 a.m.and lasted around 3 hours.The results showed that a decrease in temperature,accompanied by an increase in humidity and light wind,is favorable for the triggering and persistence of fog.Sea Level Pressure(SLP)anomaly fields show two distinct configurations.The April 3 event was characterized by a zonal dipole of SLP anomalies between the Sahara and the northern Senegalese coast,while the April 30 event was characterized by a meridional dipole between the Sahara and the Gulf of Guinea area as far as the equatorial Atlantic.A weakening of the pressure around the study area was observed in both cases,allowing moisture advection to favor the onset of fog.The hovmoller diagrams of relative humidity and wind show that a good vertical extension of humidity associated with a westerly wind in the lower layers plays an important role in the formation and persistence of fog.The presence of dry air associated with a weak easterly wind in the middle layers could explain the low vertical extension of the fog on April 3.A strong wind in the lower layers would be responsible for the premature dissipation of the April 30 fog.

Study of Boundary Layer Height over West Africa

Monthly means of boundary layer height (BLH) over West Africa are presented based on 36 years (1979-2014) of six-hourly ERA-Interim reanalysis. In this region, we found that there is a link between the West Africa Monsoon (WAM) and the monthly means of BLH in the summer. The trend and empirical orthogonal function (EOF) of BLH are presented, including the mid July variability of BLH with the precipitation. The dominant EOF of BLH accounts for around 42% of the variance with slightly large amplitude in the north while relatively small in the equatorial band. The second EOF which accounts for 16.4%, describes a longitudinal contrast with a zonal gradient. The relationship between BLH and precipitation is found using the canonical correlation analysis (CCA). Significant trends of the first and second pairs of BLH with precipitation are shown. The first and second CCA pair has a correlation of 68% and 60% with 12.2 and 10.8 degrees of freedom respectively. The critical correlation coefficients at the 95% level are 0.21 and 0.65 for the first and second CCA pairs respectively. This first CCA pair mostly determines the arid and semi-arid areas where the rate of explained regional variance is about 78% in the arid area and 73% in the semi-arid area. For the second pair of CCA, the rate of explained regional variance is more than 60% in the Guinea coast and wet equatorial area.

Transport and Deposition of Saharan Dust Observed from Satellite Images and Ground Measurements

Haboob occurrence strongly impacts the annual variability of airborne desert dust in North Africa.In fact,more dust is raised from erodible surfaces in the early summer(monsoon)season when deep convective storms are common but soil moisture and vegetation cover are low.On 27 June 2018,a large dust storm is initiated over North Africa associated with an intensive westward dust transport.Far away from emission sources,dust is transported over the Atlantic for the long distance.Dust plume is emitted by a strong surface wind and further becomes a type of haboob when it merges with the southwestward deep convective system in central Mali at 0200 UTC(27 June).We use satellite observations to describe and estimate the dust mass concentration during the event.Approximately 93%of emitted dust is removed the dry deposition from the atmosphere between sources(10°N–25°N;1°W–8°E)and the African coast(6°N–21°N;16°W–10°W).The convective cold pool has induced large economic and healthy damages,and death of animals in the northeastern side of Senegal.ERA5 reanalysis has shown that the convective mesoscale impacts strongly the climatological location of the Saharan heat low(SHL).