The Balance of Geostrophic and Ekman Transports in the Tropical Atlantic Ocean

Hydrographic station data from 24°N, 8°N, 8°S and 24°S in the Atlantic Ocean are used to calculate geostrophic transport in eight layers separated by isopycnal surfaces. In the upper ocean, the geostrophic transport is strongly northward across 8°S and strongly southward across 8°N resulting in a geostrophic convergence of ∼25 × 106 m2 s−1 in water of density less than ρ = 26.8. This is equal to the magnitude of the Ekman divergence calculated from observed wind. Similarly, geostrophic divergences of surface layers between 8°N and 24°N and between 8°S and 24°S are balanced by estimated Ekman convergences in those areas. The net upper-ocean transport across each latitude, given by the sum of Ekman transport plus upper ocean geostrophic transport, is ∼10 × 106 m3 s−1 northward. This transport is a component of the large-scale meridional cell which carries surface water and heat northward in both hemispheres of the Atlantic, with a return flow of cold water at depth.