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

Abstract

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.

Mechanisms of Long‐Term Variability and Recent Trend of Salinity Along 137°E

“…To investigate how the observed salinity variability in the western North Pacific is caused, we analyzed the salinity field of an eddy‐resolving OGCM. First, we confirmed that the OGCM well reproduces observed features along 137°E, such as freshening trends since the 1990s and meridionally coherent interannual to decadal variability reported by Oka et al (). Isopycnal analysis of the simulated field suggests that recent salinity trends and meridionally coherent interannual to decadal variability at σ θ = 25.4 kg m −3 can be traced back over the eastern North Pacific around 30°N and 160°W.…”

Long‐Term Thermohaline Variations in the North Pacific Subtropical Gyre From a Repeat Hydrographic Section Along 165°E

“…We further investigate thermohaline changes and variations on the σ θ coordinate to remove the effect of the vertical movement of isopycnal surfaces. There are marked positive θ – S trends exceeding a 5% significance level in the salinity maximum from the sea surface to the layers around σ θ = 24.5 kg m −3 , and significant negative trends in the main thermocline/halocline at σ θ = 25.0–26.5 kg m −3 (Figure ), similar to those seen in terms of the pressure coordinate (Figure ) and along the 137°E section (Oka et al, ). Note that the main thermocline/halocline in the northern boundary of the subtropical gyre where marked negative θ and S trends are detected on the pressure coordinate (region 6 in Figure ) also shows negative θ and S trends on the σ θ coordinate, implying that the trends mainly reflect density‐compensated θ – S changes.…”

“…The maximum negative salinity trend along 165°E is −0.0079 year −1 located around 16°N, 280 dbar. The freshening is also observed at the 137°E section (Nakano et al, 2005; Oka et al, ), at almost the same rate (−0.0064 year −1 around 22°N, 420 dbar) as that in the 165°E section. The spatially homogenous freshening in the western subtropical gyre is consistent with measurements from a zonal section along 24°N by Ren and Riser ().…”

“…Isopycnal surfaces of σ θ = 23.5–24.5 kg m −3 with marked positive θ – S trends correspond to NPTW and those of σ θ = 25.0–26.5 kg m −3 with negative θ – S trends correspond to SMTW and North Pacific Central Mode Water (CMW; Nakamura, ; Oka et al, ; Suga et al, ), which is formed in the deep winter mixed layer north of the KE. A maximum freshening rate in the interior of the subtropical gyre on the σ θ = 25.5 kg m −3 surface is, for example, estimated as −0.0066 year −1 at 22°N and this rate is almost the same as the results from the 137°E section (−0.0079 year −1 around 21°N; Oka et al, ). There are no significant signals below the σ θ = 26.5 kg m ‐3 surface, which is the densest layer to outcrop and ventilate in the North Pacific during winter.…”

“…The maximum salinification rate exceeds 0.005 year −1 . Long‐term salinification of NPTW is also observed along the 137°E section (Oka et al, ). Since the vertical gradient of S is small in the salinity maximum area, the influence of isopycnal heaving (changes in the depth of isopycnal surfaces) on the salinification would be small, and the NPTW salinification reflects the salinification in the subtropical gyre associated with intensification of freshwater circulation at global scale (Durack & Wijffels, ; Hosoda et al, ).…”