Zonal structure and seasonal variability of the Atlantic Equatorial Undercurrent

Abstract

Simultaneous mooring arrays were maintained along the path of the Equatorial Undercurrent (EUC) at three longitudes (23°W, 10°W, and 0°E), from October 2007 to June 2011, as part of the CLIVAR Tropical Atlantic Climate Experiment. The measurements allow for the first time a description of the seasonal cycle and interannual variability of the EUC across the Atlantic basin. The mean transport of the EUC at 23°W is 14.3 ± 0.6 Sv, decreasing to 12.1 ± 0.9 and 9.4 ± 0.6 Sv at 10°W and 0°E, respectively. The EUC shows a changing seasonal cycle 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 maximum in boreal spring and fall, while at 0°E the EUC has a single spring maximum. 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. The weakest EUC core intensity occurs during the boreal summer cold tongue phase, especially in the eastern part of the basin. At both 23°W and 10°W, a deep extension of the EUC occurs in boreal summer, which increases the transport in the lower thermocline and partially offsets the weaker upper EUC transport during boreal summer. No clear linkage could be established between the interannual variability of the EUC in the eastern part of the basin and the intensity of the summer cold tongue, despite evidence for such a linkage in the western part of the basin.

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Acknowledgments

This research was supported by the U. S. National Science Foundation under awards OCE0623552 and OCE1129874, and by the Deutsche Bundesministerium für Bildung und Forschung (BMBF) as part of the projects NORDATLANTIK (03F0443B), RACE (03F0651B), MIKLIP (01LP1114A) and by the Deutsche Forschungsgemeinschaft through several research cruises with RV Meteor and RV Maria S. Merian, and as part of the Sonderforschungsbereich 754 ‘‘Climate–Biogeochemistry Interactions in the Tropical Ocean’’. Moored velocity observations were acquired in cooperation with the PIRATA project. The authors thank the PIRATA program for their timely and free provision of data to the scientific community. Special thanks go to Mark Graham and Robert Jones (RSMAS), and Jacques Grelet and Fabrice Roubaud (IRD) who contributed to the RSMAS ADCP mooring maintenance at 0°E and 10°W during PIRATA-FR and US/RSMAS cruises.

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Correspondence to W. E. Johns.

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This paper is a contribution to the special issue on tropical Atlantic variability and coupled model climate biases that have been the focus of the recently completed Tropical Atlantic Climate Experiment (TACE), an international CLIVAR program (http://www.clivar.org/organization/atlantic/tace). This special issue is coordinated by William Johns, Peter Brandt, and Ping Chang, representatives of the TACE Observations and TACE Modeling and Synthesis working groups.

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Johns, W.E., Brandt, P., Bourlès, B. et al. Zonal structure and seasonal variability of the Atlantic Equatorial Undercurrent. Clim Dyn 43, 3047–3069 (2014). https://doi.org/10.1007/s00382-014-2136-2

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Keywords

  • Tace
  • Acoustic Doppler Current Profiler
  • Boreal Summer
  • Cold Tongue
  • Boreal Spring