The Atlantic and Pacific oceans both have an Equatorial Undercurrent, which is a strong eastward current lying near the equator. Its main role in both oceans is to supply thermocline waters from the shallow subduction zones in the subtropics to the main upwelling zones in the central and eastern part of the equatorial basins and the off-equatorial upwelling domes near the Eastern boundary. In the Pacific Ocean, this current is well-described from over a decade of intensive shipboard and time-series programs. The variability of the Equatorial Undercurrent has been closely linked to sea surface temperature variations in the eastern cold tongue region on both seasonal to interannual time scales and thus the system has important consequences for El Nino events. In the Atlantic Ocean, the Equatorial Undercurrent has received much less scientific attention. Consequently, its mean structure across the basin and its seasonal-to-interannual variability is not well understood. Particularly in the eastern part of the basin, where the Equatorial Undercurrent decays and appears to exhibit strong variability in its eastward penetration, observations are sparse and virtually no time series measurements are available.
In this study, scientists from the University of Miami will study the seasonal-to-interannual variability of the current in the eastern Atlantic through moored current observations, using ADCPs, for a three year period. With these data, they will provide a description of the mean state and the variability of the current in the central and eastern Atlantic including how far the current penetrates into the Gulf of Guinea. The study will be conducted at the same time as European scientists are also investigating the circulation of the equatorial Atlantic, and thus one of the broader impacts of this study is the international collaboration. The study also will greatly improve our understanding of how oceanic processes affect sea surface temperature variability in the eastern tropical Atlantic. This variability influences the climate throughout the region. They study contributes to the goals of TACE (Tropical Atlantic Climate Experiment), which is a CLIVAR program that is designed to improve climate predictions.
In recent years the eastern tropical Atlantic cold-tongue region has come under intense focus within the framework of Atlantic climate predictability studies. Many studies indicate that a high predictability of seasonal rainfall variability in surrounding areas of the tropical Atlantic can be realized, particularly over west Africa and northeast Brazil, if accurate sea surface temperature (SST) forecasts can be made several months in advance. An improved predictive capability in this region is important as it will have a direct impact on the economies of some of the world’s most densely populated and poorest countries, which rely heavily on agriculture, and for mediating the development of tropical diseases such as dengue, malaria, cholera, and meningitis, which are prevalent in the region. The Equatorial Undercurrent (EUC) is a key part of the coupled climate system in the tropical Atlantic owing to its role in supplying cold thermocline waters to the equatorial upwelling regions in the central and eastern Atlantic. While in the Pacific Ocean the EUC is rather well-described from nearly two decades of intensive shipboard and time-series observations in the TOGA and TAO/TRITON programs, the EUC in the Atlantic has until recently been relatively poorly sampled and neither its mean structure across the basin or its seasonal-to-interannual variability is understood. Its linkage to warm and cold phases of the boreal summer equatorial cold tongue, referred to as "Atlantic Niños", is also poorly understood. During 2007-2011, a coordinated set of moored current meter observations was collected across the central and eastern equatorial Atlantic during 2007-2011 by German, French, and U.S. investigators. These observations contributed to the goals of TACE (the "Tropical Atlantic Climate Experiment"), a 2007-2012 international CLIVAR program designed to improve climate predictions for the tropical Atlantic region, with a particular focus on the eastern equatorial Atlantic. The U.S. part of this effort consisted of moored Acoustic Dioppler current profiler (ADCP) measurements at four sites along 10ºW and 0ºE, complementing a French ADCP mooring at 10ºW, and an array of moorings along 23ºW maintained by IFM-GEOMAR (Germany). The data have been used to document the variability of the EUC across the basin, its linkages to wind forcing and SST variability, and the nature of the intraseasonal variability along the equator. The results are being used with numerical models to understand the dynamics controlling the EUC variability and the fate of EUC waters in the eastern Gulf of Guinea. The results of this analysis have led to a vastly improved understanding of the EUC in the Atlantic, which forms the primary intellectual merit of this study. The main broader impacts of the study lie in contributing to an improved understanding of processes influencing SST variability in the eastern tropical Atlantic and associated climate variability in the tropical Atlantic sector. The principal results of the study can be summarized as follows. The EUC shows a differing seasonal cycle moving eastward across the basin: at 23 W, the strongest EUC transport occurs in boreal fall in association with maximum easterly wind stress; at 10 W the EUC transport shows a semiannual cycle with a deep extension in boreal summer/fall; and at 0 E the EUC has a spring maximum, while it is weakest during the boreal summer cold tongue phase. At all locations the EUC core exhibits a similar seasonal vertical migration, with shallowest core depths occurring in boreal spring and deepest core depths in boreal fall. The maximum core intensity occurs in boreal spring all across the basin, when the EUC is shallow, during the annual wind relaxation. At this time the EUC transport is almost uniform across the basin. Using PIRATA and ARGO temperatire/salinity profile data, the observed EUC transports at the three longitudes are related to surface temperature and salinity changes on seasonal and interannual time scales. The EUC is found to be stronghest in the west (23 W) during years with anomalously strong cold tonges, but is actually weaker in the central (10 W) and eastern (0 E) part of the basin during these years, due to enhanced upweling and mass loss to the mixed layer. The data are presently being used to validate ocean models for the region and to help diagnose the causes of systematic eastern Atlantic SST biases in coupled models used for climate prediction.
