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Abstract

The Southern Ocean is a huge reservoir of negative heat and substances such as CO₂, and plays a key role in global climate change. Turbulent mixing is thought to be responsible for changes in the physical circulation field, such as the recently discovered shallowing of the meridional circulation in the Southern Ocean (Shimada et al., 2022), in addition to its contribution to the biological system, such as the supply of nutrients. We analyze CTD data collected by the T/V Umitaka Maru along the 110°E longitude of the Southern Ocean over a period of 10 years (from 2011 to 2020) and turbulence data collected by the VMP-5500 in 2016, and investigate the distribution and variability of vertical mixing. We characterize various turbulence parameters by applying the method of Galbraith and Kelly (1996) as well as the method of Gargett and Garner (2008) to the density inversion of the CTD data and deriving the calibration coefficients from the VMP-5500 data. Based on the obtained parameters, we evaluate the distribution and secular variation of turbulent mixing. We find a tendency of high turbulent mixing at high latitudes and a few strong turbulent mixing regions near the seafloor in the mid-ocean ridge and continental slope. As for the secular change, although there is almost no change in the turbulent diffusion coefficient from the deep layer to the seafloor, an increasing trend with a rate of about one order in 10 years is found from the surface mixed layer to the halocline.