Distribution of vertical diffusivity in the Bussol' Strait: A mixing hot spot in the North Pacific

Strong vertical mixing in the Kuril Straits is believed to be an important contribution to the ventilation of the intermediate layer and water mass transformation in the North Pacific, especially for density greater than 27.1 sigma(theta). Furthermore, a recent modeling study has suggested that the vertical profile of vertical diffusivity is a key factor in the determination of thermohaline circulation in the North Pacific. Here we report the distribution of vertical diffusivity in the Bussol Strait, the main conduit of water exchange and a possible central site of strong mixing in the Kuril Straits. Our analysis is based on a set of highly densed CTD observations, with a total of 127 casts across the strait in 2001. Vigorous density inversions occurred in the strait with the largest vertical displacement being over 250 m. We estimated the vertical diffusivity coefficient K-rho from the Thorpe scale for all the CTD data. The vertical average of K-rho estimated from all the casts is 60 x 10(-4) m(2) s(-1). Overall, K-rho is relatively small in the upper 300 m (density range approximately 26.5-26.7 sigma(theta)), whereas it is relatively large below a depth of 500 m (density range of >26.8 sigma(theta)), with a maximum at the depths of 1100-1700 m. The distributions of K-rho and the amplitude of the diurnal tidal current are similar, suggesting that the mixing is caused by the strong diurnal tidal current. The amplification of the diurnal (tidal) current over slopes near the bottom causes the K-rho maximum at depths of similar to 1100-1700 m. We also introduce an empirical relationship between K-rho and the amplitude of the diurnal tidal current. The vertical diffusivity is one order of magnitude larger at the spring tide than at the neap tide, suggesting that there is extremely large variability of tidal mixing with the fortnightly modulation. In the intermediate layer at densities of 27.3-27.6 sigma(theta), large K-rho values (> 60 x 10(-4) m(2) s(-1)) corresponds well to the colder and less-saline water mass characterized in the Bussol' Strait, confirming that water mass transformation occurs locally in the strait through strong diapycnal mixing. (C) 2013 Elsevier Ltd. All rights reserved.