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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.〔Chang et al., ''A decadal climate variation in the tropical Atlantic Ocean from thermodynamic air-sea interactions'', Nature Vol. 385 , 1997〕 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 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.〔A.D. Moura and J. Shukla,''On the Dynamics of Droughts in Northeast Brazil: Observations, Theory and Numerical Experiments with a General Circulation Model'', Journal of the Atmospheric Science, Vol. 38, 1981〕 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.
==Structure and Key Features of the Interhemispheric SST Dipole==
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 SubSaharan 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.
Ruiz-Barradas et al. (2001) proposed that the aforementioned controversy potentially exists due to the fact that most of the previously-cited studies have attempted to identify tropical Atlantic SST patterns of variability based solely on ocean data, instead of making use of a combined analysis of observed or modeled perturbations in both the ocean and the atmosphere. Following such an additive approach, Ruiz-Barradas et al.〔Ruiz-Barradas et al., ''Structure of Interannual-to-Decadal Climate Variability in the Tropical Atlantic Sector'', Journal of Climate, Vol. 13, 2000〕 noted that a positive Northern Hemisphere (NH) SST anomaly is associated with a northward, wind-stress anomaly and a subsequent cyclonic (counterclockwise) circulation in the subtropics, which interferes with the background flow easterlies; the opposite is true for the southern hemisphere, where the wind stress anomaly acts to enhance the south-easterlies. Such interference with the background flow subsequently leads to a decrease (enhancement) of the heat fluxes from the ocean toward the atmosphere and therefore an intensification of the positive(negative) SST anomalies, in the warm (cool) hemisphere.
Other discernible features of the interhemispheric dipole noted in the aforementioned study, include a strengthened, anomalous downwelling in the hemisphere that is characterized by positive SST anomalies and a respective, less prevalent upwelling in the negative SST anomalies-hemisphere. Furthermore, diabatic heating perturbations are also shown to be linked to cross-equatorial SST changes, with positive anomalies being observed over the warmer Northern hemisphere waters and negative ones over the Southern hemisphere. A strong connection is also found between the tropical Atlantic SST dipole and the overlying atmosphere; enhanced convective patterns and anomalous precipitation appear correlated with warm NH SSTs, whereas the opposite phase is observed across the equator, over the cooler SH waters. Finally, a connection is discerned between the surface and sub-surface interhemispheric pattern-related SSTs. Northern SST anomalies appear to permeate vertically below the ocean surface reaching thermocline (THC) depths and inciting in this way THC anomalies of up to 3m.〔Ruiz-Barradas et al. Structure of Interannual-to-Decadal Climate Variability in the Tropical Atlantic Sector, Journal of Climate, Vol. 13, 2000〕 This provides additional evidence of the critical role of surface ocean currents in transporting warm waters and heat meridionally, since surface wind-induced Ekman puming appears to be the key contributor for such THC and heat content anomalies.
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