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.
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