Intraseasonal sea surface salinity variability in the equatorial Indo-Pacific Ocean induced by Madden-Julian oscillations

Intraseasonal sea surface salinity (SSS) variability in the equatorial Indo-Pacific Ocean is investigated using the Aquarius/SAC-D satellite measurements and Hybrid Coordinate Ocean Model (HYCOM). Large-scale SSS variations at 20-90 day time scales induced by Madden-Julian oscillations (MJOs) are prominent in the central-to-eastern Indian Ocean (IO) and western Pacific Ocean (PO) with a standard deviation of approximate to 0.15 psu. The relationship between SSS anomaly and freshwater flux is nearly in phase in the central-to-eastern IO and out of phase in the western PO during a MJO cycle. A series of HYCOM experiments are performed to explore the causes for SSS variability. In most areas of the equatorial Indo-Pacific Ocean, wind stress-forced ocean dynamical processes act as the main driver of intraseasonal SSS, while precipitation plays a secondary role. In some areas of the western PO and western IO, however, precipitation effect is the leading contributor. In comparison, evaporation effect induced by radiation and wind speed changes is generally much smaller. Besides the external forcing by MJOs, ocean internal variability can also cause considerable intraseasonal SSS changes, explaining 10-20% of the total variance in some regions. Composite analysis for MJO events reveals that the effects of wind stress, precipitation, and evaporation vary at different phases of a MJO cycle. The MJO-induced SSS signature is the result of complicated superimposition and interaction of these effects. The effect of wind stress also varies significantly from event to event. It affects SSS variability primarily through horizontal ocean current advection and to a lesser extent through vertical entrainment.