Non-tidal sheared turbulent mixing observed in the Halmahera sea

The Indonesian Throughflow (ITF) plays an important role in Earth’s climate system, with the turbulent mixing in the Indonesian seas contributing to global heat sequestration and water mass transformation. Existing studies of the turbulent mixing inside the Indonesian seas have focused primarily on tidal mixing. In this study, we show, using moored current meter profiles in the Halmahera Sea, that tidal currents play a secondary role in the sheared instability. Instead, continuous-spectrum super-tidal Internal Gravity Waves (IGWs) are found to dominate the vertical shear throughout the water column, accounting for over 80% of the total shear below 500 m. In the upper layer above 220 m, the subtidal Indonesian Throughflow, the IGWs, and the tidal currents contribute equally to the total vertical shear. Comparisons with observations suggest that the turbulence parameterization of the Gregg-Henyey-Polzin (GHP) scheme matches the observed dissipation in the upper 300 m well, when an effective Coriolis parameter at 5°N representing averaged equatorial waveguide effects should be used. The GHP scheme failed poorly below 300 m due to the deviation of the IGWs from the Garrett-Munk spectrum. In comparison, the MacKinnon-Gregg (MG) turbulence parameterization performs well below 300 m but fails between 100–300 m where subtidal ITF shear and stratification are strong and unaccounted for. Overall, the vertical diffusivity is two to three orders of magnitude higher than in open oceans, underlining the importance of ITF- and IGWs-driven turbulent mixing.