Wind-driven western boundary currents, such as the Kuroshio in the western Pacific, transport large amounts of heat, salt and momentum northward, contributing to the ocean overturning circulation and the climate system heat balance. How western boundary currents vary in time and space, and how they lose energy to their surroundings, are long-standing questions. Unlike the Gulf Stream in the Atlantic where topographic features are rare, the Kuroshio frequently interacts with ridges, islands and seamounts. Energy is thus extracted from the large-scale flow through turbulent mixing, particularly in settings with large shear in horizontal velocities. These are known to support a variety of shear instabilities, such as Kelvin-Helmholtz (KH) or Holmboe, whereby small perturbations in the flow amplify, to become important mechanisms for turbulence generation. Although KH shear instability has been widely studied in idealized laboratory experiments and numerical simulations, direct observations in the ocean are rare and mostly confined to estuaries and sills. Consequently, the generation, evolution and decay of KH-like billows in high-Reynolds-number oceanic flows, their modulation by background flow and decay into turbulence are under-observed in the open ocean. The project is a collaboration with Taiwanese scientists, using their available ship-time, to measure high-resolution temperature, salinity and velocity finestructure of primary KH-like billows and evolution in the lee of a seamount in the path of the Kuroshio. Several moorings will also be deployed to gain longer-term spatial assessment of variability in the seamount vicinity. Turbulent dissipation and mixing within the observed KH-like billows will be assessed, to determine their roles in turbulent fluxes of heat, salinity, mass, and nutrients, modification of Kuroshio and local water-masses, and energy dissipation in the Kuroshio, of relevance to the gyre-scale circulation. This project supports two early-career scientists in their transition to observational oceanographers. International collaborations will provide shiptime and access to Kuroshio waters of global scientific importance. Activities within the summer program for UW undergraduate students are included, along with educational visits to K-12 classrooms, and outreach at UW and local science centers.
Shear instabilities with Kelvin-Helmholtz (KH)-like characteristics will be measured in the lee of a seamount in the path of the Kuroshio east of Taiwan. The finescale 3-D (vertical, along- and across-stream) density structure will be resolved to 10-m horizontally and 1-m vertically using towed CTD chain surveys, augmented with shipboard ADCP and echosounder. Such detailed measurements in such high-Reynolds-number oceanic flows are unprecedented, and will elucidate their kinematic structure, dynamic evolution and associated turbulence, to offer guidance for realistic numerical model simulations and laboratory experiments. A moored array of ADCPs and temperature sensors will provide additional time-series profiles of (i) shear and stratification upstream of the seamount, (ii) high temporal and vertical resolution of 3-D velocity and echosounder images on the seamount summit, and (iii) 3-D along- and across-stream time- series profiles of velocity fields in the lee of the seamount. Turbulent dissipation and mixing from multiple estimation methods and direct microstructure measurements will help determine the role of these flow/topography interactions on water-mass transformation and dissipation of a western boundary current. Scientifically, understanding the dissipation of the balanced circulation and the sources of diapycnal mixing in the ocean remain among the most pressing challenges in physical oceanography.
This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.
