Impacts of Argo salinity in NCEP Global Ocean Data Assimilation System: The tropical Indian Ocean

Abstract

Salinity profiles collected by the International Argo Project (International Argo Project data are available at http://argo.jcommops.org) since 2000 provide us an unprecedented opportunity to study impacts of salinity data on the quality of ocean analysis, which has been hampered by a lack of salinity observations historically. The operational Global Ocean Data Assimilation System (GODAS) developed at the National Centers for Environmental Prediction (NCEP) assimilates temperature and synthetic salinity profiles that were constructed from temperature and a local T‐S climatology. In this study, we assess impacts of replacing synthetic salinity by Argo salinity on the quality of the GODAS ocean analysis with a focus on the tropical Indian Ocean. The study was based on two global ocean analyses for 2001–2006 with (NCEP_Argo) and without (NCEP_Std) inclusion of Argo salinity. The quality of the ocean analyses was estimated by comparing them with various independent observations such as the surface current data from drifters, the salinity data from the Triangle Trans‐Ocean Buoy Network moorings, and the sea surface height (SSH) data from satellite altimeters. We found that by assimilating Argo salinity, the biases in the salinity analysis were reduced by 0.6 practical salinity units (psu) in the eastern tropical Indian Ocean and by 1 psu in the Bay of Bengal. Associated with these salinity changes, the zonal current increased by 30–40 cm s−1 toward the east in the central equatorial Indian Ocean during the winter seasons. When verified against drifter currents, the biases of the annually averaged zonal current in the tropical Indian Ocean were reduced by 5–10 cm s−1, and the root‐mean‐square error of surface zonal current was reduced by 2–5 cm s−1. The SSH biases were reduced by 3 cm in the tropical Indian Ocean, the Bay of Bengal, and the Arabian Sea. These results suggest that the Argo salinity plays a critical role in improving salinity analysis, which in turn contributed to improved surface current and sea surface height analyses.