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

Abstract

Ocean 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 20.0042 psu yr 21 . 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 (S max ) lagged that of the surface salinity by about 1$2 years. Both the peakto-tough range of the S max oscillation (0.15 psu) and its freshening trend (20.0036 psu yr 21) 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.

Long-term change and variation of salinity in the western North Pacific subtropical gyre revealed by 50-year long observations along 137°E

“…5b). S on σ θ = 25.0 kg m −3 exhibited a similar variation with much larger decadal amplitude and led that on 25.5 kg m −3 by about 1 year, reaching pronounced maxima around 1969, 1982, 2006), as previously reported for the shallower layers including NPTW Nan et al 2015). The 25.5 and 25.0 kg m −3 isopycnals correspond to the denser and lighter varieties of the subtropical mode water (STMW; Masuzawa 1969) formed in the eastern and western parts, respectively, of its formation region that extends zonally south of the Kuroshio and the Kuroshio Extension (KE) and north of 28°N (Suga and Hanawa 1990;.…”

“…2a in Nakano et al 2007) and is a little deeper than that during 1992-2009 located between the sea surface and 600 dbar (Fig. 4c in Nan et al 2015).…”

“…The strong freshening trend for the last 20 years existing in the subsurface layers related to STMW and L-CMW at 137°E suggests that freshening originated in the late winter mixed layer in the KE region where these water masses are formed (e.g., Oka and Qiu 2012), as inferred by Yan et al (2013) and Nan et al (2015). We therefore analyzed the 144°E section in winter (mostly February) from 1984 through 2013.…”

“…When we compare the linear trend of isobaric salinity during 1967 in Nakano et al 2007) and that during 1992during -2009 in Nan et al 2015), both based on the repeat hydrographic section along the 137°E meridian, a freshening trend reaching −0.0015 (on the Practical Salinity Scale 1978) year −1 existed from the lower main halocline to NPIW/TSM during the former period, while the freshening trend was several times larger in magnitude and was located from the upper main halocline to the sea surface during the latter, more recent period. Such a difference suggests significant interdecadal variability and raises the following questions.…”

Long-term variation in the salinity of the Southern Yellow Sea Cold Water Mass, 1976–2006

“…The decreasing salinity trends of the Kuroshio and South China Sea water caused lower-salinity water to be brought to the East China Sea, which then reduced the salinity of the source area of the YSWC, thus further explaining the long-term salinity variation in the SYSCWM. The salinity trend of the Kuroshio water was consistent with the results of Nan et al (2015), where a freshening trend in surface salinity of −0.0042 per year was found, with the change in the air-sea freshwater flux as the leading factor.…”