Shelf Seas Baroclinic Energy Loss: Pycnocline Mixing and Bottom Boundary Layer Dissipation

Mark E. Inall,Tom P. Rippeth,Matthew Toberman,Jeff A. Polton,J. A. Mattias Green,Matthew R. Palmer

doi:10.1029/2020jc016528

Abstract

AbstractObservations of turbulent kinetic energy dissipation rate () from a range of historical shelf seas data sets are viewed from the perspective of their forcing and dissipation mechanisms: barotropic to baroclinic tidal energy conversion, and pycnocline and bottom boundary layer (BBL) dissipation. The observations are placed in their geographical context using a high resolution numerical model (NEMO AMM60) in order to compute relevant maps of the forcing (conversion). We analyze, in total, 18 shear microstructure surveys undertaken over a 17 year period from 1996 to 2013 on the North West European shelf, consisting of 3,717 vertical profiles of shear microstructure: 2,013 from free falling profilers and 1,704 from underwater gliders. A robust positive relationship is found between model‐derived barotropic to baroclinic conversion, and observed pycnocline integrated . A fitted power law relationship of approximately one‐third is found, giving a simple new parameterization. We discuss reasons for this apparent power law and where the “missing” dissipation may be occurring. We conclude that internal wave related dissipation in the bottom boundary layer provides a robust explanation and is consistent with a commonly used fine‐scale pycnocline dissipation parameterization.

Highlights

Continental shelf seas occupy ∼ 7% of the global ocean surface area yet are disproportionately influential in the Earth’s system as a critical interface linking the marine, atmospheric and terrestrial components (Rippeth, 2005)
In this article we examine the way in which internal tides interact with the sea bed of the shelf seas, and show that it is friction at the sea bed, rather wave breaking, that takes most of the energy from internal tide waves
Within regions of seasonal stratification and linked to the presence of the steep shelf break the internal tide has been shown to make a larger contribution to diapycnal mixing than the barotropic tide (Rippeth et al, 2005)

Summary

Introduction

Continental shelf seas occupy ∼ 7% of the global ocean surface area yet are disproportionately influential in the Earth’s system as a critical interface linking the marine, atmospheric and terrestrial components (Rippeth, 2005). ∼ 70% of tidal energy dissipation (Munk & Wunsch, 1998) They play a significant role in the global cycling of carbon by the oceans (Sharples et al, 2019), estimated to account for between 10 and 30% of total marine primary production, and as a consequence a significant proportion of carbon burial (Bauer et al, 2013). In contrast to the deep ocean, the seasonal shelf sea pycnocline is observed to exist predominantly in a state of marginal stability with respect to a fine scale (of order several meters) Richardson Number (Ri ∼ 1)

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