Tropical Atlantic SST Dipole

The Tropical Atlantic SST Dipole refers to a cross-equatorial sea surface temperature (SST) pattern that appears dominant on decadal timescales. It has a period of about 12 years, with the SST anomalies manifesting their most pronounced features around 10–15 degrees of latitude off of the Equator. The term Tropical Atlantic SST dipole is only one of the characteristic names used to refer to this mode of variability; other definitions include the interhemispheric SST gradient or the Meridional Atlantic mode. This decadal-scale SST pattern constitutes one of the key features of SST variability in the Tropical Atlantic Ocean, with another one being the Atlantic Equatorial Mode or Atlantic Niño, which occurs in the zonal (east-west) direction at interannual timescales, with sea surface temperature and heat content anomalies being observed in the eastern equatorial basin. Its importance in climate dynamics and decadal-scale climate prediction is evident when investigating its impact on adjacent continental regions such as the Northeast Brazil, the Sahel as well as its influence on North Atlantic cyclogenesis.

Early studies have focused their attention on the connection between the enhancement (decrease) of tropical rainfall in regions such as the Northeast Brazil, the Western Sub-Saharan Africa etc. and perturbations in the tropical Atlantic sea surface temperatures (Moura and Shukla (1981), Nobre and Shukla (1996).) Such research efforts have concentrated on the role of the interhemispheric (meridional) SST mode as a dynamical driver of the tropical Atlantic overlying atmosphere, by analyzing perturbations of this coupled tropical ocean-atmosphere system while examining local as well as remote influences (i.e. the tropical -mid-latitude N. Atlantic connection (Tanimoto and Xie (1999), Tourre et al. (1999)), the tropical Pacific influence via El Nino/La Nina events (Enfield and Mayer (1997)). Other studies, however, such as the ones carried out by Houghton and Tourre (1992) and Enfield and Mayer (1997), have questioned the very existence of this interhemispheric gradient (or SST dipole) as a statistical mode of climate variability. These studies suggest that the principal component analysis used to analyze the tropical Atlantic variability (TAV) and extract these statistical SST modes imposes a constraint on the analysis (due to the requirement of the orthogonality of the principal components that are associated with the different SST modes), without which the connection between the SST anomaly patterns in the Northern and Southern hemispheres appears to be insignificant.

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