Fine‐ and microstructure observations in the Urup Strait, Kuril Islands, during August 2006

Abstract

For the first time, we performed continuous observations of turbulent mixing upon the sill of the Urup Strait, Kuril Islands, for a period of 1 day during August of 2006. Time series of velocity profiles, water properties, and isopycnal depths show variability with a period of about 1 day; however, the water mass variability is not explained by simple water exchange between the Sea of Okhotsk and the North Pacific. Empirical orthogonal function (EOF) analysis of isopycnal coordinates extracted variability presumed to represent tidal flows as major EOFs. In profiles of the residual flow after subtracting the mean flow and major EOFs, we detected phase propagations consistent with internal gravity waves, with an approximate period of 9.1–11.4 h. The turbulent energy dissipation rate ɛ and vertical diffusivity K ρ ranged from 1 × 10 −9 to 3 × 10 −8 W kg −1 and from 3 × 10 −5 to 2 × 10 −3 m 2 s −1 , respectively. Occurrences of high dissipation rates generally corresponded to either strong shear or weak stratification. A significant contribution of shear S to the dissipation rate ɛ was indicated with a relationship of ɛ ∼ S 1.4 – S 1.8 , probably related to propagating internal waves. A strong mixing event was observed within the water column, with extremely weak stratification and a Richardson number below the critical value of 0.25, when the flow direction changed from up‐sill to down‐sill.