Freshening of Subsurface Waters in the Northwest Pacific Subtropical Gyre: Observations and Dynamics

Abstract

Subsurface salinity anomalies propagating between mid- and low latitudes along isopycnal surfaces have been shown to play an important role in modulating ocean and climate variability. In this study, a sustained freshening and southwestward propagation of subsurface salinity anomalies in the northwest Pacific subtropical gyre and its dynamical mechanism are investigated using observations, numerical outputs, and a predictive model. Analyses of the observations show a pronounced subsurface freshening with salinity decreasing about 0.25 PSU near the 24.5-σθ surface in the northwest Pacific subtropical gyre during 2003–11. This freshening is found to be related to the surface forcing of salinity anomalies in the outcrop zone (25°–35°N, 130°–160°E). A predictive model based on the assumption of salinity conservation along the outcrop isopycnals is derived and used to examine this surface-forcing mechanism. The resemblance between the spatial structures of the subsurface salinity derived from the predictive model and from observations and numerical outputs suggests that subsurface salinity anomalies are ventilated over the outcrop zone. A salinity anomaly with an amplitude of about 0.25 PSU generated by the surface forcing is subducted in the outcrop zone and then propagates southwestward, accompanied by potential vorticity anomalies, to the east of Luzon Strait (~15°N) in roughly one year. When the anomalies reach 15°N, they turn and move gradually eastward toward the central Pacific, associated with an eastward countercurrent on the southern subtropical gyre.

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

“…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.…”

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

“…Details of data processing, gridding, and error estimation of MOAA GPV are described comprehensively in Hosoda et al [2008] and Li et al [2012a]. This data set has been widely used in recent studies of interannual and decadal salinity variability in the northwest Pacific Ocean [e.g., Li et al, 2012aLi et al, , 2012bLi et al, , 2012cQiu and Chen, 2012;Kobashi and Xie, 2012;Wang et al, 2013;Yan et al, 2013]. …”

“…The mean subducton rate averaged over the NPSG box is about 48 m yr 21 . According to Yan et al [2013], subsurface salinity anomalies in the NPSG are generated via surface subduction from the outcrop region along the Kuroshio Extension. Subducted salinity anomalies could be advected southwestward along isopycnals, approximately following the mean flow.…”

“…Sugimoto et al [2013] also found that the North Pacific subtropical mode water (STMW) characterized by low potential vorticity freshened markedly in 2009 and 2010, with the salinity being 0.1 psu lower than the mean value from 2003 to 2011. In this study, whether the freshening trend observed by Yan et al [2013] continued, and what caused the great salinity anomalies of the STMW observed by Sugimoto et al [2013] will be discussed.…”

Decadal variations in mixed layer salinity in the Kuroshio Extension recirculation gyre region: influence of precipitation during the warm season

“…In a recent report, using data obtained from Argo float profiles between 2003 and 2011, Sugimoto et al (2013) noted the importance of precipitation during the warm season in determining the salinity of the NPSTMW. Yan et al (2013) reported freshening and southwestward propagation of salinity anomalies in the subsurface layer during the period 2003-2011 in the northwestern part of the subtropical gyre, and concluded that these variations were related to surface forcing in the outcrop area. The long-term behavior of salinity in the KERG, on the other hand, has not yet been established.…”