Ocean salinity changes in the northwest Pacific subtropical gyre: The quasi‐decadal oscillation and the freshening trend

Abstract

AbstractOcean salinity changes play an important role in modulating ocean and climate variability. Analyses of the repeating observations along PN, TK, and 137°E sections reveal that both surface and subsurface salinity in the Northwest Pacific Subtropical Gyre (NPSG) had clear quasi‐decadal oscillation (QDO) of ∼10 year and a sustained freshening trend during 1987–2012. Surface salinity in the NPSG troughed in 1989, 1999, and 2008, and peaked in 1993, and 2003. The peak‐to‐tough range of surface salinity oscillation can reach 0.3 psu. Meanwhile, surface salinity decreased about 0.10 psu from 1987 to 2012 with a freshening trend of −0.0042 psu yr−1. These surface salinity anomalies were subducted into the subsurface layer mainly in the ventilated zone along the Kuroshio Extension, and advected over the NPSG. The QDO of the subsurface salinity maximum (Smax) lagged that of the surface salinity by about 1∼2 years. Both the peak‐to‐tough range of the Smax oscillation (0.15 psu) and its freshening trend (−0.0036 psu yr−1) are smaller than those of the surface salinity. Salinity changes in the NPSG likely began in the mid‐1970s associated with the North Pacific regime shift during 1976/1977. Analyses of mixed layer salinity budget indicated that air‐sea freshwater flux change in the NPSG is the leading factor controlling the surface salinity anomalies, while change of large‐scale ocean circulation (Geostrophic advection) also plays a minor role. Salinity and air‐sea freshwater flux changes in the NPSG are all closely related to the Pacific Decadal Oscillation, indicating the large‐scale ocean‐atmosphere interaction.