Deep-ocean mixing driven by small-scale internal tides

Clément Vic,Maarten C Buijsman,Alberto C Naveira Garabato,J A Mattias Green,Angélique Melet,Gordon R Stephenson,Amy F Waterhouse,Casimir De Lavergne,Zhongxiang Zhao

doi:10.1038/s41467-019-10149-5

Abstract

Turbulent mixing in the ocean is key to regulate the transport of heat, freshwater and biogeochemical tracers, with strong implications for Earth’s climate. In the deep ocean, tides supply much of the mechanical energy required to sustain mixing via the generation of internal waves, known as internal tides, whose fate—the relative importance of their local versus remote breaking into turbulence—remains uncertain. Here, we combine a semi-analytical model of internal tide generation with satellite and in situ measurements to show that from an energetic viewpoint, small-scale internal tides, hitherto overlooked, account for the bulk (>50%) of global internal tide generation, breaking and mixing. Furthermore, we unveil the pronounced geographical variations of their energy proportion, ignored by current parameterisations of mixing in climate-scale models. Based on these results, we propose a physically consistent, observationally supported approach to accurately represent the dissipation of small-scale internal tides and their induced mixing in climate-scale models.

Highlights

Turbulent mixing in the ocean is key to regulate the transport of heat, freshwater and biogeochemical tracers, with strong implications for Earth’s climate
We present an estimate of the modal partitioning of global internal tide generation with a resolution of up to 50 modes, and show that its predictions are consistent with available observations of tidal energy conversion, radiation and mixing
Semi-analytical model of barotropic-to-baroclinic tidal energy conversion based on the assumptions of subcritical topography and small tidal excursion[19,20,21,22]

Summary

Introduction

Turbulent mixing in the ocean is key to regulate the transport of heat, freshwater and biogeochemical tracers, with strong implications for Earth’s climate. Contrary to current views, reflected in parameterisations of tidal mixing, near-field mixing, associated with locally generated high-mode internal tides, dominates tidal mixing on a global scale (>50%) and exhibits a strong geographical variability: the parameter q is widely distributed across values from 0 to 1. These findings have important implications for the representation of deep-ocean mixing and overturning in climatescale ocean models, which we discuss

Methods

Results

Conclusion