Research Presentations

Abstract: Mid-depth, time-mean circulation in the western North Pacifc Ocean (28°—45°N, 140°—165°E) is investigated using drift information from the profling floats deployed in the Kuroshio Extension System Study (KESS) and the International Argo programs. A well-defined, cyclonic recirculation gyre (RG) is found to exist north of the Kuroshio Extension jet, confined zonally between the Japan Trench (~145°E) and the Shatsky Rise (~156°E), and bordered to the north by the Subarctic Boundary along ~40°N. This northern RG, which is simulated favorably in the eddy-resolving OFES hindcast run model, has a maximum volume transport at 26.4 Sv across 159°E and its presence persists on the interannual and longer time scales. An examination of the time-mean x-momentum balance from the OFES hindcast run output reveals that horizontal convergence of Reynolds stresses works to accelerate both the eastward-flowing Kuroshio Extension jet and a westward mean flow north of the meandering jet. The fact that the northern RG is eddy-driven is further confirmed by examining the turbulent Sverdrup balance, in which convergent eddy potential vorticity fluxes are found to induce the cyclonic RG across the background potential vorticity gradient field. For the strength of the simulated northern RG, we find the eddy dissipation effect to be important as well.

Abstract: Fine horizontal-scale ADCP/CTD surveys across the first meander trough of the Kuroshio Extension were made during the Kuroshio Extension System Study (KESS) in 2004, while the current was in its stable meander state. Stream coordinate analysis revealed a mean core surface velocity of 1.7-1.8 m/s and cross-stream velocities of the order of 0.1 m/s. Rossby numbers greater than one were found consistently north of the core. The passage of a frontal wave was associated with southward cross-frontal flux. An array of Current and Pressure sensor-equipped Inverted Echo Sounders (CPIES) provided a longer time series of the full water-column jet structure, albeit with coarser horizontal resolution. The six-month time series during the stable state showed that entering the mean trough, core velocities rotated clockwise with depth, and cross-stream fluxes were southward implying subpolar-to-subtropics down-welling. These fluxes are important for the formation of mode and intermediate waters. Variability suggested an event-driven process, but peak southward flows did not always coincide with the passage of frontal waves. North and south of the core, strong recirculations exist with surface and deep currents aligned.

Abstract: The mooring and float observations collected during the Kuroshio Extension System Study (KESS) show that the formation of Subtropical Mode Water (STMW) is a highly variable process in both space and time. Global high-resolution numerical simulations confirm that the strength of atmospheric air-sea heat fluxes are the dominant factor controlling the volume of STMW formed for a given year, but the formation is dominated by episodic events with time scales of days that can be directly related to wind storms frequently coming from the northwest. One-dimensional mixed-layer models also suggest that enhanced vertical diffusivity values in the upper ocean are necessary during the summer and fall to describe accurately the evolution of the mixed layer depth and top of the STMW layer. Our observations elucidate the causes of this enhanced mixing and the impacts it has on the evolution of the STMW.

Abstract: Observations from the Kuroshio Extension System Study (KESS) demonstrate the existence of recirculation gyres flanking the Kuroshio Extension. The KESS program had its observational stage over a 2-year period from June 2004 - June 2006 during which an array spanning the jet axis consisting of current meter moorings and inverted echo sounders equipped with near-bottom pressure and current sensors was deployed, additionally profiling floats were released. In the KESS observations we have found evidence for barotropic recirculation gyres both to the south and for the first time north of the jet. Direct evidence comes from the moored current meters, which show an eastward flow under the surface jet flanked by a barotropic westward flow in the abyssal ocean to the north and south of the jet. The existence of a pair of recirculation gyres is also suggested by maps of the echo sounder and float data. Additional evidence for the recirculation gyres comes from the analysis of an eddy-resolving model simulation of the region. Estimates of the gyre transports are computed and the dynamics of the recirculation gyres are explored.

Abstract: AVISO sea level anomaly (SLA) products are compared with in situ sea surface height (SSH) anomaly measurements from an array of 43 pressure-recording inverted echo sounders (PIESs) in the Kuroshio Extension during 2004−2006. PIESs measure bottom pressure and round-trip acoustic echo time from the sea floor to the sea surface, which are used to estimate, respectively, mass-loading and steric height variations in the SSH anomaly. Good correlations (>0.9) are found between satellite and PIES measurements with the best agreement near the jet, where the largest SSH variations exist. The mass-loading component affects the correlation and regression coefficients slightly and improves them overall by ~5%. Comparisons agree best with the AVISO up-to-date product, merging all available satellite measurements. Satellite-derived absolute dynamic topography (ADT) maps are also compared with PIES-derived ADT maps. Several mean dynamic topography (MDT) maps, added to SLA products to obtain satellite-derived ADT maps, are tested. Comparisons reveal that Rio05, the most recent AVISO MDT utilizing GRACE data, is the best MDT in this region. It is significantly improved over Rio03, a previous AVISO MDT without GRACE contributions.

Abstract: Forty-eight profiling floats have been deployed in the Kuroshio Extension region since May 2004 as part of the Kuroshio Extension System Study (KESS) project. By combining the float temperature-salinity measurements with satellite altimetry data, this study investigates the role played by mesoscale eddies in controlling the property changes in North Pacific Subtropical Mode Water (STMW). Following a 3-yr period of low eddy activity in 2002-04, the KE transitioned to a high eddy kinetic energy state in 2005. This transition is the result of delayed oceanic response to the 2002 shift in the basin-scale surface wind forcing in connection with the Pacific decadal oscillation. By transporting northern-origin, high potential vorticity KE water into the recirculation gyre, the enhanced eddy activity affects STMW in two ways: first, it hinders the formation of deep winter mixed layer (hence the source for STMW) by modifying the upper ocean stratification and, secondly, it provides a direct high-PV source to mix with the surrounding low-PV STMW. The eddies's influence upon STMW is observed to be both significant in magnitude and efficient in time. Compared to 2004, the PV signal in the core of STMW was reduced by half in 2005 and this weakening of STMW's intensity occurred within a period of less than 7 months.

Abstract: We present new results from the Kuroshio Extension System Study (KESS), a large-scale observational program of the Kuroshio Extension addressing the processes that govern the jet's variability and the relation between the jet and its recirculation gyres, together with results from a theoretical study of eddy-mean flow interactions in an unstable, quasi-geostrophic jet. We show that in an idealized model of a boundary-forced jet in a parameter regime relevant to the Kuroshio Extension and the Gulf Stream, eddy fluxes play important roles in both stabilizing the jet as it evolves downstream, and driving recirculations through the mechanism of an up-gradient PV flux that occurs downstream of jet stabilization. We also find properties of the eddy-driven time-mean circulation can be predicted given the stability properties of the upstream jet that was the source of the eddy variability. By comparing the model's observable predictions with KESS results, we test the relevance of these theoretical findings to actual oceanic western boundary current jets and address the role of eddy-mean flow interactions in the Kuroshio system.