Peter Hacker (University of Hawaii, Honolulu, Hawaii, USA); Vasco Müller (Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research, Bremen, Germany)

Satellite sea level anomaly (SLA) and sea surface salinity (SSS) data are used to characterize and quantify the contribution of mesoscale eddies to the ocean transport of salt. Given relatively large errors in satellite SSS retrievals, we evaluate the eddy transport of salt using two methods. The first method is based on the so-called eddy composite analysis. Because of the averaging over a large number of eddies in a given geographic area, composite eddies result in quite small standard errors, producing robust estimates of the associated transport of salt. The second method estimates the eddy transport of salt in a traditional way by computing pointwise covariances between eddy-induced velocity and SSS fluctuations. Comparing between the two methods, we find that the estimates of the eddy salt transport agree very well globally, emphasizing the physical mechanism responsible for the eddy transport of salt (eddy advection) and validating the assumption that the eddy transport is mainly due to large mesoscale eddies, observed by satellite altimetry.

Our analysis confirms that the eddy transport of salt (or, equivalently, freshwater) is an essential component of the marine hydrological cycle. The regions of major eddy transport of salt identified in our study occur in the tropical belt, across the equatorward limbs of the subtropical gyres, and across the Antarctic Circumpolar Current (ACC). The eddy salt transport is poleward across the ACC with the largest transport taking place in the Indian Ocean sector. The eddy salt transport is divergent in the subtropical gyres (eddies pump salt out of the gyres) and convergent in the tropics. The eddy salt transport in the sub-polar gyres is substantially smaller than the eddy salt transport in the tropics and subtropics. We also note that the zonal component of the eddy salt transport is quite significant, particularly over the western and eastern boundaries of the gyres and in the near-equatorial belt, where strong zonal gradients of SSS exist. Overall, our study demonstrates that the possibility to characterize and quantify the eddy transport of salt in the ocean surface layer can rely on the combined use of satellite observations of SLA and SSS.