On the meridional ageostrophic transport in the tropical Atlantic

The meridional Ekman volume, heat, and salt transport across two trans-Atlantic sections near 14.5 degrees N and 11 degrees S were estimated using in situ observations, wind products, and model data. A meridional ageostrophic velocity was obtained as the difference between the directly measured total velocity and the geostrophic velocity derived from observations. Interpreting the section mean ageostrophy to be the result of an Ekman balance, the meridional Ekman transport of 6.2 +/- 2.3 Sv northward at 14.5 degrees N and 11.7 +/- 2.1 Sv southward at 11 degrees S is estimated. The integration uses the top of the pycnocline as an approximation for the Ekman depth, which is on average about 20m deeper than the mixed layer depth. The Ekman transport estimated based on the velocity observations agrees well with the predictions from in situ wind stress data of 6.7 +/- 3.5 Sv at 14.5 degrees N and 13.6 +/- 3.3 Sv at 11 degrees S. The meridional Ekman heat and salt fluxes calculated from sea surface temperature and salinity data or from highresolution temperature and salinity profile data differ only marginally. The errors in the Ekman heat and salt flux calculation were dominated by the uncertainty of the Ekman volume transport estimates.