Mixing in the Philippine Sea: Geography variability and parameterization

Abstract

The Philippine Sea Mixing Experiment (PhiMix) was carried out in 2021 concurrently from two research vessels, obtaining microstructure measurements and fine structure-based estimates of turbulence and diffusivity covering nearly the whole Philippine basin for the first time. The inhomogeneous spatial distribution and weak magnitudes of turbulent mixing in the Philippine Sea basin were revealed, according to the upper-ocean turbulence measurements and full-depth CTD datasets. The averaged diffusivities in the Philippine Basin were ∼10−2 m2s−1 in the surface mixed layer and ∼10−5.5 m2s−1 in the ocean interior. Below the thermocline, the diffusivities generally increased and reached 10−4 m2s−1 above rough topography. Centered on the Palau Ridge, a significant east-west asymmetry was noted, with stronger mixing on the east side of the ridge. Additionally, the diapycnal diffusivity's spatial variability coincided with that inferred from tidal mixing between 500 and 3000 m. Typical dissipation rate and diapycnal diffusivity profiles in the Philippine Sea differed from the global averaged profiles, which form the basis of most existing mixing parameterizations. Here we proposed an improved parameterization scheme based on these new direct microstructure observations in the Philippine Sea. The new mixing parameterization scheme (PhiMixPar-PMP) performed well for the Northwest Pacific Ocean in ocean simulations. This regional study on mixing properties in the Philippine Sea can fill a knowledge gap in the big picture of global mixing geography and improve turbulence parameterizations in the ocean models.