Using collocated altimetry sea surface height anomalies (SSHA) and Argo profiles within detected eddies, we investigated structures of temperature, salinity, potential density, geostrophic current, mixed layer depth (MLD), potential vorticity (PV), and buoyancy frequency (N) in the Kuroshio Extension (KE) region under the influences of oceanic eddies. We identified 54,302 oceanic eddies (snapshots) in the KE region during the period of 1999–2013. The composite analysis showed that changes in physical parameters modulated by the climatological composite eddies (hereinafter referred to as composite eddies) were mainly confined in the upper 800 m. At the eddy core, the maximum cooling in the composite cyclonic eddy (CE) reaches 2.00°C at ∼360 m, with maximum salinity change of −0.13 psu at ∼260 m and maximum potential density change of +0.27 kg/m3 at ∼310 m. In contrast, the maximum warming in the composite anticyclonic eddy (AE) reaches 1.78°C at ∼410 m of the eddy core, with maximum salinity change of 0.12 psu at ∼260 m and maximum potential density change of −0.22 kg/m3 at ∼410 m. There were obvious anticlockwise and clockwise geostrophic current anomalies associated with the composite CE and AE, respectively. The seasonal mean eddy‐modulated MLD anomaly had significant seasonal variations. In addition, they could modulate opposite PV changes, the magnitude of which varied with depth.
Analysis of the 62-yr hindcast outputs from an eddy-resolving ocean general circulation model reveals a prominent decadal variability in the upper-layer (0-745 m) Luzon Strait transport (LST), a key component of the South China Sea throughflow. This variability is in phase with the basin-scale wind stress anomalies associated with the Pacific decadal oscillation (PDO). A composite analysis shows that during the positive phase of the PDO, the Aleutian low and its related positive wind stress curl anomalies intrude southward, reducing the trade winds and enhancing the westerly wind anomalies in the tropical North Pacific. In response, the North Equatorial Current bifurcation shifts northward, resulting in a weaker Kuroshio east of Luzon and consequently a stronger South China Sea throughflow in the upper 745 m.
Previous studies of the exchanges of the subtropical‐tropical salinity in the North Pacific are still not conclusive. Some studies suggest that surface salinity anomalies formed in the northeastern subtropics can subduct and penetrate to the subsurface of the northwestern Pacific, whereas others demonstrate that subsurface salinity anomalies in the northwestern Pacific cannot be traced back to those of the eastern subtropics. This study examined the variability of subsurface salinity in the northwestern Pacific, with focus on its propagation characteristic and forcing mechanism in its outcrop region. It is found that salinity anomalies on 24.5–25.4 kg m−3 isopycnals exhibited a pronounced freshening trend in the northwestern Pacific during 2003–2012. This freshening can be traced back to the surface salinity anomalies in the outcrop zone of the northwestern subtropical gyre, rather than that of northeastern subtropics. The representative transit time from its outcrop region to the eastern Luzon Strait is about 1–3 years. The surface salinity anomalies in the outcrop zone are mainly controlled by air‐sea freshwater flux change, although the contribution of horizontal advection is also important, in particular for year‐to‐year variation. Compared to the air‐sea freshwater flux and horizontal advection, the contribution of mixed‐layer depth to the surface salinity anomalies is smaller and can be neglected in the outcrop zone of the northwestern Pacific.
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