Shallow and Intermediate circulation in the equatorial Atlantic
Final Report Abstract
The project “Shallow and Intermediate Circulation in the Equatorial Atlantic: Moored and Shipboard Observations” focused on the tropical Atlantic and its complex system of zonal current bands, their variability and their relation to climate relevant fluctuations of the upwelling regimes in the eastern tropical Atlantic. The investigation was based on data sets obtained during METEOR cruises, especially those from M62/2, M68/2 and from moored stations which were deployed and retrieved during these cruises. These data sets were supplemented by previously obtained data of the area, requiring an active international cooperation. 1) Our investigation of equatorial zonal currents and associated oxygen distributions showed a mean westward flowing Equatorial Intermediate Current (EIC), consisting of two westward current cores at about 250 and 450 m, below the Equatorial Undercurrent at 35°W and 23°W. The flow below the EUC is characterized by substantial interannual variability and a strong zonally coherent eastward jet was observed at 300 to 350 m depth from May to December 2005. Shipboard hydrographic observations in June – July 2006 (METEOR M68/2) revealed the existence of a high-oxygen tongue in the depth range of the eastward jet prevailing during the preceding year. 2) As regards the South Equatorial Undercurrent (SEUC) in the western to central tropical Atlantic, float trajectories show a well confined SEUC revealing large standing meanders near its western origin. Transports determined from 31 sections across the SEUC increase from 5.6 Sv (1 Sv = 106 m3/s) at 35°W near the western boundary to 10.2 Sv 800 km farther east, supplied out of the deep part of the South Equatorial Current (SEC) that carries on both sides of the SEUC interior water masses westward. 3) Additionally, we studied the upper equatorial Atlantic variability during a warm event in 2002 and a strong cold event in 2005 associated with equatorial Kelvin waves. Basin-wide sea surface height anomalies were used to derive an equatorial Kelvin wave mode and the time evolution of this mode represents the basis for a regression analysis to investigate related oceanic variability. Prior to the cold tongue onset in 2002 (2005), the presence of equatorial Kelvin waves is associated with a flattened (steeper) thermocline slope that might precondition the development of the warm (cold) event.
Publications
- (2008), Oxygen tongues and zonal currents in the equatorial Atlantic, J. Geophys. Res., 113(C04012)
Brandt, P., V. Hormann, B. Bourlès, J. Fischer, F. A. Schott, L. Stramma, and M. Dengler
(See online at https://doi.org/10.1029/2007JC004435) - (2008), The South Equatorial Undercurrent in the western to central tropical Atlantic, Geophys. Res. Lett., 35(L21601)
Fischer, J., V. Hormann, P. Brandt, F. A. Schott, B. Rabe, and A. Funk
(See online at https://doi.org/10.1029/2008GL035753) - (2009), Upper equatorial Atlantic variability during 2002 and 2005 associated with equatorial Kelvin waves, J. Geophys. Res., 114(C03007)
Hormann, V., and P. Brandt
(See online at https://doi.org/10.1029/2008JC005101)
