PHYSICAL OCEANOGRAPHY DIVISION

 

 

 

Intorduction

| The Tropical Atlantic | The workshop |

The TAV affects directly the climate of Northwestern Africa, northern South America, Central America, the Caribbean and the southern United States and produces climate responses over the inter-American region that are comparable to those of ENSO (Enfield, 1996). The TAV is also associated with the frequency of land-falling hurricanes in the Americas and Caribbean (Gray, 1990; Landsea et al., 1999). In addition to the relation with the climate of the surrounding regions, understanding of the TAV is ecessary to successfully validate the large-scale climate modes in numerical models (e.g. Lau and Nath, 1994; Saravanan and Chang, 1999).

A number of studies have identified two primary modes of interannual climate variability in the tropical Atlantic (see, for example, Servain and Merle, 1993). One of them, the "equatorial" mode, has a time scale of 2-4 years. It is similar to, albeit much weaker than, the Pacific El Niño Southern Oscillation (ENSO) in that it relates changes in the tropical ocean's thermal structure to trade-wind anomalies in the western equatorial ocean. Specifically, when the trades intensify (weaken) in the western Atlantic, the equatorial thermocline slope increases (decreases), and negative (positive) SST anomalies develop in the eastern equatorial ocean, particularly in the Gulf of Guinea. The second mode, the "dipole" mode, has no Pacific counterpart. It is characterized by a north-south interhemispheric gradient of SST anomalies, and its time scale ranges from interannual to decadal (Moura and Shukla, 1981; Servain, 1991). During typical dipole episodes, anomalies appear with opposite signs on either side of the ITCZ, although the development is not always simultaneous (Houghton and Tourre, 1992). So far, this mode has been observed mainly in SST and surface wind fields, and little is known about its subsurface manifestations. Recently, it was claimed that these two modes are related to each other and both linked to the west equatorial wind stress variability (Servain et al., 1999, 2000; Murtugudde et al., 2001). Other studies analyzed the departures of sea surface temperatures from climatology to determine the degree to which SST anomalies of opposite sign in the tropical North and South Atlantic occur (Enfield et al., 1999). Results indicate that anti-symetric ("dipole") configurations of SST anomalies on basin scales are not ubiquitous in the tropical Atlantic. Unless the data are stratified by both season and frequency, inherent dipole behavior cannot be demonstrated. Upon removing the global ENSO signal in SST anomaly from the data, the regions north or south of the Atlantic ITCZ have qualitatively different temporal variability and are poorly correlated. Dipole configurations do occur infrequently (12-15% of the time), but no more so than expected by chance for stochastically-independent variables. Non-dipole configurations that imply significant meridional SST anomaly gradients occur much more frequently, nearly half of the time.

All studies agree that large-scale SST gradients are key factors in driving the climate response in the tropical Atlantic sector. The temporal variability in the meridional SST gradient across or south of the ITCZ has strong effects on the climates of northeast Brazil and West Africa. There are some indications that the off-equatorial regions play a dominant role in the establishment of the meridional SST gradients. Studies attest to the role of positive feedback mechanisms through surface fluxes for providing persistence to the SST anomalies forcing SST through surface fluxes. Therefore while the SST is an important variable to measure for understanding TAV, surface fluxes measurements are also crucial. Wind fluctuations result in both local surface flux anomalies and in thermocline-depth variations that also modify the effect of the surface fluxes. On the larger scale, recent research indicates that TAV is correlated with the North Atlantic Oscillation (NAO) (Yang, 1999) and therefore, directly or indirectly to the meridional overturning circulation (MOC). The role that the MOC plays in giving persistence to the NAO climate phases is still not fully understood. Nor are the mechanisms for interhemispheric exchange of mass and heat. In addition, there is not enough observational evidence at present to trace the time-dependent pathways of the upper limb of the MOC across the tropic Atlantic.

Several years of research efforts have offered prospects for improving climate prediction based on the tropical Atlantic. Nevertheless, many fundamental questions of crucial importance for achieving predictability remain unanswered. Modeling of the tropical Atlantic has not yet achieve a minimum level of predictability. The reasons are, among others, the lack of observations and the poor understanding of the dynamics of the tropical Atlantic. There is also a need to better define the predictability limits of TAV and related phenomena through diagnostics of data and model experiments.