Influence of the South Atlantic Ocean Dipole on West African Summer Precipitation

• Published in: Journal of climate Vol. 24; no. 4; pp. 1184 - 1197
• Main Authors: Nnamchi, Hyacinth C., Li, Jianping
• Format: Journal Article
• Language: English
• Published: Boston, MA American Meteorological Society 15-02-2011
• Subjects: Climate; Correlations; Datasets; Drought; Earth, ocean, space; Exact sciences and technology; External geophysics; General circulation models; Marine; Meteorology; Monsoons; Ocean-atmosphere interaction; Oceans; Precipitation; Rain; Rainy season; Sahel; Sea surface temperature; Summer; Time series; Wind; South America; Argentina; Guinea; Uruguay; Atlantic Ocean; West Africa; Africa; Brazil; South Atlantic; Sahel; PSW, Africa; ASE, Africa; AS, South Atlantic; rainfall; Divergence; atmospheric precipitation; Summer; Sea surface temperature; ocean-atmosphere interaction; anomalies; Summer monsoon; Convergence; Temperature gradient; Interannual variation; Correlation analysis; Atlantic Niño; Observation data; Rainy season; Composite analysis
• ISSN: 0894-8755; 1520-0442
• DOI: 10.1175/2010jcli3668.1

This paper demonstrates a causal link between the air–sea phenomenon referred to as the South Atlantic Ocean (SAO) dipole (SAOD) and the interannual variability of precipitation over West Africa during the boreal summer monsoon rainy season in which most of the annual rainfall is recorded using an array of observational datasets. Analyses show that positive precipitation anomalies exceeding 40 mm month−1over most locations at the Guinea Coast are associated with the positive phase of the SAOD, which is characterized by warm sea surface temperature anomalies (SSTAs) in the northeastern part of the SAO or the northeast pole (NEP)—that is, the Atlantic Niño sector—and cool SSTA in the southwestern part [southwest pole (SWP)] off the Argentina–Uruguay–Brazil coast. On the other hand, interannual variability in the Sahel is closely connected to the West African summer monsoon and the Atlantic Niño.
The results of this study reveal that the well-known influence of the Atlantic Niño on Guinea Coast precipitation in the literature represents only a component of the ocean–atmosphere interactions in the SAO causing the precipitation anomalies. Indeed, correlation and composite analyses using Guinea Coast precipitation indices consistently yield significant links to both the NEP and SWP centers of action. The hypothesized physical mechanism through which the SAOD-type SST gradients could induce Guinea Coast precipitation anomalies is the Lindzen–Nigam process. During the positive phase of the SAOD, the imprint of SST gradients gives rise to divergence over the SWP linked to convergence and vigorous upward motion over the NEP thereby leading to enhancement of precipitation over the Guinea Coast.