Rapid Eddy-Induced Modification of Subtropical Mode Water during the Kuroshio Extension System Study

Abstract

From 2004From to 2006 an observational array of current-and pressure-recording inverted echo sounders (CPIES) were deployed as part of the Kuroshio Extension (KEx) System Study (KESS). KESS observed a transition from a weakly meandering (''stable'') to strongly meandering (''unstable'') state (Qiu and Chen). As the KEx made this transition, potential vorticity (PV) observed within the southern recirculation gyre (SRG) rapidly increased from January to July 2005. In this study, the authors diagnose eddy PV fluxes (EPVFs) in isentropic coordinates within the subtropical mode water (STMW) layer from the CPIES data to determine the role of mesoscale eddies in this rapid increase of PV.The rapid increase in PV within the SRG coincided with enhanced cross-front EPVFs and eddy PV flux convergence upstream of a mean trough in the KEx path and adjacent to the SRG. The enhanced cross-front EPVFs were the result of the formation of a cold-core ring (CCR) and the interaction of the jet with a preexisting CCR. Eddy diffusivities are diagnosed for the unstable regime with values that range from 100 to 2000 m 2 s 21. The high eddy diffusivities during the unstable regime reflect the nature of mesoscale CCR formation and CCR-jet interaction as efficient mechanisms for stirring and mixing high PV waters from the north side of the KEx into the low PV waters of the SRG where STMW resides. This mechanism for crossfrontal exchange can explain observed increases in the STMW PV in the SRG over the 16 months of KESS observations.

Effects of Eddies on the Formation and Subduction of North Pacific Subtropical Mode Water Based on Argo Observations

“…The diffusion coefficient κ l is assumed to be vertically constant in the ML. Recent observational studies have provided different results concerning the magnitude of the lateral eddy diffusion coefficient κ l in the Northwestern Pacific Ocean (e.g., Abernathey & Marshall, 2013; Bishop & Watts, 2014; Forget et al., 2015; Groeskamp et al., 2020; Liu et al., 2012). The different coefficients indicate that the magnitude of lateral eddy diffusivity varies with the canonical value on the order of 10 3 m 2 s −1 in the Northwestern Pacific Ocean (125°E−180°, 20°N–40°N).…”

“…The sensitivity of eddy-induced subduction to an uncertain parameter, the lateral eddy diffusion coefficient κ l , is quantified in the impact of choice of finite magnitude on the subduction picture according to Equation 11. Here, we restricted the constant diffusion coefficient κ l to the lower (κ l = 1,000 m 2 s −1 ) and upper (κ l = 3,000 m 2 s −1 ) limits as found in previous studies (e.g., Abernathey & Marshall, 2013;Bishop & Watts, 2014;Forget et al, 2015;. In addition, the spatially varying eddy diffusion coefficient κ l at the surface ML, estimated from Groeskamp et al (2020), is also used to calculate the eddy-induced subduction in the Northwestern Pacific Ocean (125°E−180°, 20°N-40°N).…”

Anatomical study of the three-dimensional structure of a supercharged cold eddy generated in the Kuroshio Extension

“…They found that the enhanced eddies affect STMW by altering the stratification in the upper ocean, which impedes the formation of a deep winter mixed layer (hence, the source for STMW), and by providing a high-potential vorticity source that mixes with the surrounding low-potential vorticity STMW. Bishop and Watts (2014) pointed out that cold-core ring formation was the mechanism driving the cross-frontal exchange of high-potential vorticity water from the northern side of the Kuroshio to the south. Xu et al (2016) deployed 17 daily sampled Argo floats to study the effect of anticyclonic eddies on STMW south of the KE.…”

Time Variations in Ocean Bottom Pressure from a Few Hours to Many Years: In Situ Data, Numerical Models, and GRACE Satellite Gravimetry

“…Transport variability of the Antarctic Circumpolar Current (ACC)—the largest ocean current system on the globe—has been monitored in Drake Passage already 40 years ago (Nowlin et al, ) and continues until present days also with the help of in situ OBP recorders (Meredith et al, ). The dynamics of the Kuroshio extension off the Japanese coast—a region characterized by high eddy kinetic energy due to the confluence of warm, salty subtropical and cold, fresh subpolar waters—were recently studied in detail with an array of current meters and pressure recording inverted echo sounders (Bishop & Watts, ). And finally, the Atlantic Meridional Overturning Circulation—responsible for poleward heat transport and the associated rather mild climate in western Europe—is monitored routinely by the RAPID array at 26°N with a series of in situ OBP sensors aligned along the gradient of the continental shelf (McCarthy et al, ).…”