The potential role of thermohaline–shear instability in turbulence production in the Bering Sea and the subarctic North Pacific

Abstract

A recent linear stability analysis and a numerical simulation suggest that diffusive convection (DC), a regime of the double-diffusive convection, plays a potential role in onset of thermohaline–shear instability, implying that DC could contribute to turbulence production in the oceans. However, an existence of such a thermohaline–shear instability has not been examined in real oceans. We examine if this newly proposed instability mechanism exists in the subarctic North Pacific by analyzing our fine- and micro-scale turbulence measurement data. Vertical inversions were cautiously detected in seawater density profiles and used as a proxy for instability events for gradient Richardson number larger than the critical value of 1/4. We found that a portion of inversions were associated with active DC. Such DC-related inversions exhibited elevated levels of turbulent kinetic energy dissipation rate even for gradient Richardson number largely exceeding 1/4. Our estimate suggested that the thermohaline–shear instability contributes to roughly only 10% of the dissipation of turbulent kinetic energy in the diffusively convective layer in our observation site.