Exploring near-bottom turbulent mixing across the Challenger Deep based on temperature spectral analysis

Abstract

The hadal Mariana Trench remains poorly understood. In December 2016, an array of high-resolution temperature loggers, attached to the ocean bottom seismometers (OBSs), was deployed from 1665 to 7520 m for two weeks across the Challenger Deep of the Southern Mariana Trench. The temperature variance spectrum reveals that the bottom water is mildly turbulent and it is mainly modulated by the semidiurnal internal tides. At the second deepest observation station (depth of 7015 m), the viscous subrange is resolved in the high-frequency spectrum. Applying the proposed method with Taylor’s frozen field hypothesis and Kraichnan theoretical spectrum analysis, it is revealed that turbulent dissipation rate $\varepsilon$ is $7.8\times 10^{-10}$ $m^2/s^3$ and flow speed U is 8.9 mm/s. Dissipation rates $\varepsilon$ of all stations vary between $5.9\times 10^{-11}$ and $1.4\times 10^{-9}$ $m^2/s^3$, with the northern region of Challenger Deep experiencing stronger energy dissipation than the southern one. The vertical distribution of dissipation rate $\varepsilon$ shows that it decreases with increasing depth from 1000 to 6000 m, but then increases to around 8000 m, which is consistent with previous observations and numerical simulations. The available turbulent mixing data indicates that the energy dissipation is vertically distributed in a distinct multilayer structure in the deep ocean of Challenger Deep, which is proposed to link to the intrusion of water mass in the deep Mariana trench.