Abstract
The 137°E repeat hydrographic section for 50 winters during 1967–2016 has been analyzed to examine interannual to interdecadal variations and long-term changes of salinity and temperature in the surface and intermediate layers of the western North Pacific, with a particular focus on freshening in the subtropical gyre. Rapid freshening on both isobars and isopycnals began in the mid-1990s and persisted for the last 20 years in the upper main thermocline/halocline in the western subtropical gyre. In addition, significant decadal variability of salinity existed in the subtropical mode water (STMW), as previously reported for the shallower layers. An analysis of the 144°E repeat hydrographic section during 1984–2013 supplemented by Argo profiling float data in 2014 and 2015 revealed that the freshening trend and decadal variability observed at 137°E originated in the winter mixed layer in the Kuroshio Extension (KE) region and was transmitted southwestward to 137°E 1–2 years later in association with the subduction and advection of STMW. The mechanism of these changes and variations in the source region was further investigated. In addition to the surface freshwater flux in the KE region pointed out by previous studies, the decadal KE variability in association with the Pacific Decadal Oscillation likely contributes to the decadal salinity variability through water exchange between the subtropics and the subarctic across the KE. Interdecadal change in both the surface freshwater flux and the KE state, however, failed to explain the rapid freshening for the last 20 years.
Highlights
Long-term changes of salinity in the surface and intermediate layers of the world oceans have been investigated on the basis of repeat hydrography, historical data archives, and the Argo profiling float network (e.g., Wong et al 1999; Boyer et al 2005; Hosoda et al 2009; Durack and Wijffels 2010) to detect intensification of salinity contrast between regions/basins suggesting that of the global hydrological cycle (Rhein et al 2013)
When we compare the linear trend of isobaric salinity during 1967–2005 (Fig. 2a in Nakano et al 2007) and that during 1992–2009 (Fig. 4c in Nan et al 2015), both based on the repeat hydrographic section along the 137°E meridian, a freshening trend reaching −0.0015 year−1 existed from the lower main halocline to North Pacific Intermediate Water (NPIW)/Tropical Salinity Minimum (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
When we look at the relation to the Kuroshio Extension (KE) index (Fig. 6d), which was defined by Qiu et al (2014) as the average of four KE-related parameters based on the satellite altimeter measurements, there is a connection that the winter mixed layer S at 144°E decreased during the unstable KE period represented by a negative KE index
Summary
Long-term changes of salinity in the surface and intermediate layers of the world oceans have been investigated on the basis of repeat hydrography, historical data archives, and the Argo profiling float network (e.g., Wong et al 1999; Boyer et al 2005; Hosoda et al 2009; Durack and Wijffels 2010) to detect intensification of salinity contrast between regions/basins suggesting that of the global hydrological cycle (Rhein et al 2013). In the first region at 100–200 dbar and 8–17°N, which corresponds to the upper permanent thermocline in the strong portion of the North Equatorial Current (NEC; e.g., Qiu and Joyce 1992), θ changed interannually and was high (low) during La Niña (El Niño) periods (Fig. 3a; θ anomalies from the low-pass filtered values were negatively correlated with 5-month running mean of the Niño-3 index with a coefficient R = −0.69), as demonstrated by previous analyses (e.g., Shuto 1996) It showed a longterm increase after the mid-1990s, which is consistent with the southward shift of NEC detected by satellite altimeter measurements since 1992 (Qiu and Chen 2012).
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