Latitudinal dependence of diapycnal diffusivity in the thermocline observed using a microstructure profiler

Abstract

The un‐tethered free‐fall microstructure profiler “TurboMAP‐D” collected 53 velocity microstructure profiles in the interior Pacific Ocean. The data between ∼1000 m and ∼1600 m depth was used to infer diapycnal diffusivity in the main thermocline. We found diapycnal diffusivity at 20°–30°N was roughly proportional to the logarithm of locally available energy density of the semidiurnal internal tide and reached more than 10−4 m2 s−1 near the Izu‐Ogasawara Ridge. In contrast, diapycnal diffusivity was found to remain at ∼10−5 m2 s−1 equatorward of 20°N and poleward of 30°N, irrespective of the amount of locally available semidiurnal internal tide energy. The observed mixing hotspots were a factor of 2–3 less intense than predicted based on fine‐scale parameterizations, although further microstructure measurements are needed to better quantify them. The present study has confirmed the existence of a latitudinal dependence of diapycnal diffusivity which must be taken into account in global circulation models.