Meridional Overturning Circulation in the Tropical Atlantic - On the Meridional Ageostrophic and Geostrophic Transports, and Water Mass Properties
The Atlantic meridional overturning circulation (AMOC) plays an important role in the global climate. Locally, the AMOC can be decomposed into a wind-driven Ekman transport near the surface and a geostrophic transport in the interior. The meridional Ekman volume transport in the tropics, driven by the strong trade winds is regarded as the primary ageostrophic component of the AMOC. Zonal hydrographic surveys with direct velocity observations in the tropical Atlantic enable detailed inspection of the directly observed Ekman transports. The ageostrophic velocities show an "Ekman spiral"-like structure. Integrating the ageostrophic velocity vertically from the surface to the top of the pycnocline (TTP) yields an Ekman transport, which agrees well with that derived from the in-situ wind stress data. This implies that the TTP is a good approximation for the Ekman depth. Using repeated trans-Atlantic sections at 14.5N (1989, 2013) and at 24.5N (1992, 2015), the AMOC is determined from a box inverse model. Direct comparison between the different realizations at the each latitude shows that the AAIW became warmer and saltier at 14.5N, and that the densest AABW became lighter, while the NADW freshened at both latitudes. The inverse solution shows that the intermediate layer transport at 14.5N was also markedly weaker in 2013 than in 1989, indicating that the AAIW property changes at this latitude may be related to changes in the circulation. At both latitudes, the AMOC was generally weaker during 2013/2015 than during 1989/1992. However, comparison between the inverse solution, the GECCO2, the MOVE and RAPID data suggests that the difference may be explained by the strong variability of the AMOC. Sensitivity experiments of the inverse solution suggest that the overturning structure of the AMOC, as well as the net heat transport across the section are sensitive to the Ekman transport at 14.5N. This emphasizes the importance of the Ekman transport for the AMOC in the tropics.
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