Eastward salinity anomaly propagation in the intermediate layer of the North Pacific

Abstract

An objective mapping with the data of profiling float array, maintained under the Argo project, revealed eastward propagation of long‐term (>5 years) salinity anomalies in the subsurface and deep neutral density (γ) layers of 27.0–27.6γ along the subarctic front in the North Pacific after 2000. Such propagation was previously inferred from water property variations along a few observation lines and from numerical simulations, mainly for shallow layers. In the western North Pacific, the signs of the anomalies were the same on and below the 27.0γ, whereas in the eastern North Pacific the sign on 27.0γ was opposite to those on 27.4γ. This difference was attributed mainly to slower advection in the deeper layers. These changes were larger than the standard errors inferred from the objective mapping at least. Furthermore, the variation revealed by the float array was similar to decadal changes observed along repeat ship‐based observation lines, and they were also associated with changes in apparent oxygen utilization especially along 165°E. The small salinity changes in the deeper layers inferred from the float array were also detected as decadal differences in highly accurate trans‐basin observations. Furthermore, because the extension of small changes into the subtropical gyre was also captured by the float and ship‐based observations, the influence of the decadal changes on the isopycnal surfaces off the coast of Japan could appear relatively quickly, even in deeper layers (27.0–27.4γ) in the North Pacific.

Decline and Bidecadal Oscillations of Dissolved Oxygen in the Oyashio Region and Their Propagation to the Western North Pacific

“…For the deeper horizon of σ θ = 27.4 kg/m 3 in the E‐ST‐OML, the oscillation was clearly apparent over the zone of 40°N–45°N, and the associated time lags were 3–4 years behind the Oyashio region (Figures d, c, and d). These time lags for O 2 oscillations are similar to those anticipated from the propagation of salinity anomalies over the same isopycnal surfaces (Kouketsu et al, ). This similarity suggests that O 2 oscillations are simply propagated from the Oyashio region with the eastward geostrophic current in the upper OML down to σ θ = 27.4 kg/m 3 as well as on the upper isopycnal surface at σ θ = 26.8 kg/m 3 , although the propagation speed is higher than that of the geostrophic current (Kouketsu et al, ).…”

“…These time lags for O 2 oscillations are similar to those anticipated from the propagation of salinity anomalies over the same isopycnal surfaces (Kouketsu et al, ). This similarity suggests that O 2 oscillations are simply propagated from the Oyashio region with the eastward geostrophic current in the upper OML down to σ θ = 27.4 kg/m 3 as well as on the upper isopycnal surface at σ θ = 26.8 kg/m 3 , although the propagation speed is higher than that of the geostrophic current (Kouketsu et al, ). Oscillations of O 2 in the OML were also found in the southern zone at 32.5°N–37.5°N.…”

“…Kouketsu et al () found that salinity anomalies in the NPIW and OML propagate eastward along the SAF with geostrophic currents. As the geostrophic current over σ θ = 26.9–27.0 kg/m 3 takes approximately 2 years to flow from the Oyashio region to 40°N, 165°E (Kouketsu et al, ), the temporal displacements of 1–2 years between the Oyashio region and the 165°E section we observed in the NPIW at 40°N are consistent. The even longer time lag of 3 years that we found in the subtropics at 30°N, 165°E may be attributable to the additional time taken to travel into the subtropics (Figure a).…”

“…The bidecadal oscillations in dissolved O 2 we identified over the isopycnal surfaces spanning σ θ = 26.8-27.5 kg/m 3 in the Kuroshio-Oyashio confluence region along the 165°E section are thought to have a strong connection with those in the Oyashio region. Kouketsu et al (2017) found that salinity anomalies in the NPIW and OML propagate eastward along the SAF with geostrophic currents. As the geostrophic current over σ θ = 26.9-27.0 kg/m 3 takes approximately 2 years to flow from the Oyashio region to 40°N, 165°E (Kouketsu et al, 2017), the temporal displacements of 1-2 years between the Oyashio region and the 165°E section we observed in the NPIW at 40°N are consistent.…”

Spatial Distribution of Diffusivity Coefficients and the Effects on Water Mass Modification in the North Pacific

“…We used distributions of salinity, potential temperature, and thickness of isopycnal layers estimated by Kouketsu et al (), which are monthly 1° × 1° × 0.05γ gridded data from 2000 to 2015. Monthly geostrophic velocity fields were calculated with approximate stream functions (McDougall & Klocker, ) by using isopycnal surface data and the isobaric data set Grid Point Value of the Monthly Objective Analysis using Argo data (MOAA GPV; Hosoda et al, ).…”