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

Abstract

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 27.1σθ. 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 250m. We estimated the vertical diffusivity coefficient Kρ from the Thorpe scale for all the CTD data. The vertical average of Kρ estimated from all the casts is 60×10−4m2s−1. Overall, Kρ is relatively small in the upper 300m (density range approximately 26.5–26.7σθ), whereas it is relatively large below a depth of 500m (density range of >26.8σθ), with a maximum at the depths of 1100–1700m. The distributions of Kρ 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 Kρ maximum at depths of ∼1100–1700m. We also introduce an empirical relationship between Kρ 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 27.3–27.6σθ, large Kρ values (>60×10−4m2s−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.