Mean meridional currents in the central and eastern equatorial Atlantic
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Ship-based acoustic Doppler current profiler (ADCP) velocity measurements collected by several major field programs in the tropical Atlantic are averaged and combined with estimates of the mean near-surface velocity derived from drifters and Argo float surface drifts (ADCP+D) to describe the mean cross-equatorial and vertical structure of the meridional currents along 23°W and 10°W. Data from moored ADCPs and fixed-depth current meters, a satellite-derived velocity product, and a global ocean reanalysis were additionally used to evaluate the mean ADCP+D meridional velocity. The dominant circulation features in the long-term mean ADCP+D meridional velocity in the upper 100 m are the tropical cells (TCs) located approximately between 5°S and 5°N, with near-surface poleward flow and subsurface equatorward flow that is stronger and shallower in the northern cell compared to the southern cell. The thickness of the surface limb of the TCs decreases and the northern cell is found to shift further south of the equator from the central to eastern tropical Atlantic. Analysis of two-season means estimated from the ship-based ADCP, near-surface drift, and moored velocity data, as well as the simulated fields, indicates that the maximum poleward velocity in the surface limb of the TCs intensifies during December–May along 23°W largely due to seasonal compensation between the geostrophic and ageostrophic (or wind-driven) components of the meridional velocity, whereas the maximum equatorward flow in the subsurface limb of the northern cell intensifies during June–November along both 23°W and 10°W due to the seasonality of the geostrophic meridional velocity.
KeywordsTropical Atlantic Meridional currents Tropical cells
This research was carried out in part under the auspices of the Cooperative Institute for Marine and Atmospheric Studies (CIMAS), a Cooperative Institute of the University of Miami and the National Oceanic and Atmospheric Administration (NOAA), cooperative agreement # NA10OAR4320143. Additional support was provided by NOAA’s Climate Program Office, NOAA’s Atlantic Oceanographic and Meteorological Laboratory, the Global Drifter Program NOAA NA10OAR4320156, the Deutsche Bundesministerium für Bildung und Forschung, project RACE (03F0651B), and the Deutsche Forschungsgemeinschaft through SFB754. The authors thank the PIRATA program which makes PIRATA mooring and shipboard data sets freely available to the scientific community. The authors also thank Adam Houk, Andreas Funk, and Arne Körtzinger for help obtaining and processing additional moored and shipboard data sets. Output from the GLORYS2V1 reanalysis product was obtained through a collaborative agreement with Mercator Océan. The GLORYS reanalysis project received support from INSU-CNRS, Mercator Océan, Groupe Mission Mercator Coriolis and the European Community’s Seventh Framework Programme FP7/2007–2013 under grant agreement n°218812 (MyOcean). Nicolas Ferry and Laurent Parent are thanked for their helpful comments concerning GLORYS2V1 output. This study used velocities derived from Argo float surface trajectories of the YoMaHa’07 data set (Lebedev et al. 2007) which are provided by APDRC/IPRC. The OSCAR data were obtained from JPL Physical Oceanography DAAC and developed by Earth and Space Research. NCEP Reanalysis data are provided by the NOAA-CIRES Climate Diagnostics Center, Boulder, Colorado, USA, at their web site http://www.cdc.noaa.gov/. Comments from Gregory Foltz, Silvia Garzoli, Christopher Meinen, Paul Freitag, and two anonymous reviewers led to significant improvements in the manuscript.
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