Surface salinity advection in the tropical oceans compared with atmospheric freshwater forcing: A trial balance

• Published in: Journal of Geophysical Research. C. Oceans 2002-12, Vol.107 (C12), p.SRF 15-1-SRF 15-11
• Main Authors: Johnson, Eric S., Lagerloef, Gary S. E., Gunn, John T., Bonjean, Fabrice
• Format: Article
• Language: English
• Subjects: climatology; Marine; observations; salinity budget; surface currents; surface forcing; Tropics
• ISSN: 0148-0227; 2169-9275; 2156-2202; 2169-9291
• DOI: 10.1029/2001JC001122

Climatological surface salinities and satellite‐derived surface currents are combined to estimate the horizontal and vertical time mean and seasonal salinity divergence in the tropical ocean's surface layer. The mean salinity divergence has magnitude and spatial patterns matching major features of an estimate of evaporation minus precipitation averaged from several atmospheric general circulation model reanalyses and satellite‐derived products (spatial correlation of 0.63). Thus a substantial portion of the atmospheric freshwater flux appears to be balanced by ocean transports in a relatively thin (32.5 m) surface layer. Large regional discrepancies remain, however, and their magnitude is greater than the scatter among the various atmospheric forcing products used in this study. The present salinity divergence calculation is imprecise because the mean salinity gradients have been weakened by large‐scale smoothing of sparse and unevenly sampled data. Further, other investigators show that higher‐frequency ocean processes (e.g., vertical and horizontal mixing of salinity) are significant at least locally. Nevertheless, several areas of consistent disagreement between mean flow salinity divergence and atmospheric forcing, such as excess evaporation in subtropical gyres, are suggestive of errors in the forcing. In particular, the shortfall of salinity convergence balancing high precipitation in the western Pacific is roughly consistent with regional surface flux corrections required to bring ocean general circulation model integrations into long‐term balance in this region. Thus this shortfall may reflect either overlarge precipitation estimates or a fundamental misunderstanding of how the ocean disperses this flux. Ultimately, further ocean physics must be addressed before firm conclusions can be drawn about the fraction of atmospheric freshwater input unaccounted for by the near‐surface mean flow and seasonal salinity divergence. High‐resolution space‐based salinity measurements will be necessary to provide the temporal and spatial coverage necessary to develop ocean salinity transport calculations into useful constraints on ocean processes, surface hydrological forcing, and model simulations.