Impact of Aquarius sea surface salinity observations on coupled forecasts for the tropical Indo-Pacific Ocean

This study demonstrates the impact of gridded in situ and Aquarius sea surface salinity (SSS) on coupled forecasts for August 2011 until February 2014. Assimilation of all available subsurface temperature (ASSIM-T_z) is chosen as the baseline and an optimal interpolation of all in situ salinity (ASSIM-T_z-SSS_IS) and Aquarius SSS (ASSIM-T _z-SSS_AQ) are added in separate assimilation experiments. These three are then used to initialize coupled experiments. Including SSS generally improves NINO3 sea surface temperature anomaly validation. For ASSIM-T_z-SSS_IS, correlation is improved after 7 months, but the root mean square error is degraded with respect to ASSIM-T_z after 5 months. On the other hand, assimilating Aquarius gives significant improvement versus ASSIM-T_z for all forecast lead times after 5 months. Analysis of the initialization differences with the baseline indicates that SSS assimilation results in an upwelling Rossby wave near the dateline. In the coupled model, this upwelling signal reflects at the western boundary eventually cooling the NINO3 region. For this period, coupled models tend to erroneously predict NINO3 warming, so SSS assimilation corrects this defect. Aquarius is more efficient at cooling the NINO3 region since it is relatively more salty in the eastern Pacific than in situ SSS which leads to increased mixing and upwelling which in turn sets up enhanced west-to-east SST gradient and intensified Bjerknes coupling. A final experiment that uses subsampled Aquarius at in situ locations infers that high-density spatial sampling of Aquarius is the reason for the superior performance of Aquarius versus in situ SSS. Key Points Assimilation of sea surface salinity (SSS) improves coupled forecasts Aquarius outperforms in situ SSS assimilation SSS assimilation imparts a relative improved upwelling signal