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.

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).