Role of submesoscale processes in the isopycnal mixing associated with subthermocline eddies in the Philippine Sea

Abstract

Submesoscale processes are very typical in the ocean interior with a front formed by mesoscale eddies, and contribute to ocean mixing. However, the role of submesoscale processes in the isopycnal mixing associated with ocean eddies has never been explored from observations. Based on hourly velocity and temperature data from the mooring at 8.5°N, 130°E between 26 September 2015 and 22 December 2016, this study provides the horizontal eddy diffusivity using a mixing-length framework and a parameterization related to the Rhines scale, and estimates the contribution of submesoscale processes to isopycnal mixing for the first time. The subthermocline eddies in the Philippine Sea exist between 200 m and 800 m depth, have characteristic speeds of 0.1–0.5 m/s with a typical period of 40–100 days, and can cause the temperature fluctuation of ∼0.5 °C. The horizontal diffusivity due to subthermocline eddies is about 4000–11,000 m2/s, and has a significant maximum near ∼400 m depth but the horizontal diffusivity associated with mixing length gradually increases below ∼700 m. The submesoscale processes can change the current velocity and temperature by ∼5 cm/s and ∼0.1 °C, respectively. The submesoscale processes in the ocean interior can increase the isopycnal mixing associated with subthermocline eddies by about 6–7%. The profile of horizontal eddy diffusivity derived in this study is useful for parameterization of ocean eddies and submesoscale processes in the Philippine Sea in coarse-resolution ocean models.