Causes for intraseasonal sea surface salinity variability in the western tropical Pacific Ocean and its seasonality

Abstract

Pronounced intraseasonal variability (ISV; 20-90 day) of sea surface salinity (SSS) with a standard deviation of 0.12-0.20 psu is detected in the western tropical Pacific Ocean (PO) from measurements of Aquarius/SAC-D satellite. These variations are not spatially uniform but show distinct regional features. The Hybrid Coordinate Ocean Model (HYCOM) well simulated the observed SSS variations, and a suite of parallel experiments were performed to understand the underlying physical processes. Surface forcing by atmospheric intraseasonal oscillations which are dominated by the Madden-Julian oscillation (MJO) is largely responsible for producing the SSS ISV, while ocean internal variability plays a secondary role. Impact of atmospheric forcing is primarily through precipitation and wind stress-driven oceanic processes. Their relative importance shows spatial variations. They have approximately equal importance in the western equatorial PO west of 155°E and the southwestern tropical PO. Wind stress effect dominates SSS ISV in the equatorial PO east of 155°E, while precipitation effect is larger in the northwestern tropical PO. In comparison, the effect of evaporation induced by wind speed change is smaller. The SSS ISV also shows evident seasonality in some areas, particularly in the far western equatorial basin and southwestern tropical PO. 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, especially the mixed layer depth, is generally more important in causing such seasonality.