Intraseasonal Sea Surface Salinity Variability in the Western Pacific Warm Pool Region

Measurements of Aquarius/SAC-D satellite reveal pronounced intraseasonal variability (ISV; 20-90 day) of sea surface salinity (SSS) in the western tropical Pacific Ocean (PO) with a standard deviation of 0.15-0.20 psu. These variations are not spatially uniform but show distinct regional characteristics. A simulation of the Hybrid Coordinate Ocean Model (HYCOM) well reproduced the observed SSS variations, and a series of additional HYCOM experiments were performed to understand the underlying mechanisms. Surface forcing by atmospheric intraseasonal oscillations, such as the Madden-Julian oscillations (MJOs), is responsible for most of the SSS ISV, while ocean internal variability plays a minor role. Impact of atmospheric forcing is primarily through precipitation and wind stress-driven ocean processes. The relative importance of the two processes shows a spatial variation. They have approximately equal importance in the western equatorial PO west of 155°E and in the South Pacific convergence zone (SPCZ) region. Wind stress effect dominates SSS ISV in the equatorial PO between 155°E-180°, while precipitation effect is larger in the northwestern tropical PO (the Philippine Sea). Effects of wind speed and shortwave radiation through evaporation are relatively small. An evident seasonality in the SSS ISV amplitude is also observed. During boreal summer (winter), SSS ISV is enhanced (weakened) in the northwestern PO and weakened (enhanced) in the southwestern PO. Comparing with the strength of atmospheric forcing, seasonal variation of the ocean state is shown to be more important in causing such seasonality. Especially, the surface mixed layer thickness is a crucial factor determining the amplitude of SSS ISV.