Three-dimensional inner-shelf circulation downstream of Otago peninsula, New Zealand

A topographic tidal residual eddy is generated behind the Otago peninsula on the South Island of New Zealand. In Spring, 2018, two moorings with vertical arrays of temperature sensors and current profilers were deployed on the inner shelf, at the shoreward-side of the anti-clockwise rotating eddy. Although the barotropic dynamics of the residual eddy have been previously quantified, three-dimensional, stratified characteristics remain poorly understood. Over the observation period, the vertical thermal stratification at 15-m depth reached a 4 degree top-to-bottom temperature difference but with high temporal variability. Water column vertical structure evolved from supporting a near-surface pycnocline, to a deep surface mixed layer. Additionally, high-frequency internal waves were abundant during these periods. A three-dimensional regional numerical model recreates the tidal residual eddy with ~20 km in diameter. Simulations with variable stratification are used to diagnose and contrast the seasonal momentum balance of the eddy and infer its role as a waveguide for high-frequency coastal internal waves.