Characteristics and mechanisms of intraseasonal oscillation of mixed layer salinity in the western formation region of North Pacific Tropical Water

Based on a time series of seawater salinity profile data collected by moored buoys, in conjunction with Soil Moisture Active Passive (SMAP) sea surface salinity (SSS) data from June 2015 to May 2022, as well as additional observational data, this study investigates the characteristics and mechanisms of intraseasonal oscillation of mixed layer salinity (MLS) in the western formation region of North Pacific Tropical Water (NPTW). The analysis reveals significant intraseasonal oscillations of MLS with two dominant periods of approximately 32 days and 16 days. Furthermore, the intraseasonal variability in MLS exhibits noticeable seasonality, being stronger during winter and spring and weaker during summer and autumn. Our analysis further reveals that these pronounced intraseasonal MLS oscillations are primarily attributed to evaporation minus precipitation (E-P), which is closely associated with Madden-Julian Oscillation (MJO) events. The MJO exerts dominant influence on MLS variations in the western formation region of NPTW. During the MJO active phase, the study area is influenced by cyclonic wind anomalies. The enhanced convection and increased precipitation, lead to a significant decrease in MLS. Similarly, during the MJO suppressed phase, the region is controlled by the anomalous anticyclonic winds, resulting in weakened convection and reduced precipitation, which causes an increase in salinity in the western formation region of NPTW. In addition to the dominant influence of MJO-induced changes in freshwater flux, the oceanic advection processes play a secondary role in the intraseasonal variability of MLS.