Investigating turbulent mixing processes using high-resolution temperature sensors on autonomous ocean gliders

Leadbitter, Philip (2022) Investigating turbulent mixing processes using high-resolution temperature sensors on autonomous ocean gliders. University of East Anglia, Doctoral Thesis, 228pp.

Record type: Thesis (Doctoral)

Abstract

Turbulent processes play a key role in ocean mixing. However, lack of spatial and temporal data collection limits our understanding of these processes, especially in the deep ocean away from boundaries. In this thesis we investigate the suitability of using the Thorpe scale method on high resolution temperature data from buoyancy driven gliders and vertical microstructure profilers to improve provision of estimates of turbulent kinetic dissipation rates.

Using three datasets a robust methodology was developed to estimate dissipation rates from Thorpe length scales using both a vertical microstructure profiler and Seagliders.

Three distinct regions were investigated; open ocean over the Mid-Atlantic Ridge using a vertical microstructure profiler, open ocean in the Subtropical North Atlantic away from boundaries using a Seaglider, and the flank of a submarine ridge in the Faroe-Shetland channel using a Seaglider. All regions showed dissipations ((O)10−11 to (O)10−6 W kg−1 ) and diffusivities ((O)10−6 to (O)10−4 m−2 s −1 ) within expected global ranges, although the latter two showed up to an order of magnitude difference to other studies from the same region.

Over the Mid-Atlantic Ridge, Thorpe scale matched the structure and tidal cycle provided by shear data from the same platform. In contrast, in the Faroe-Shetland channel the method performed poorer, not showing the elevated values of dissipation expected in an internal tide generating region. Additionally, Thorpe scaling showed elevated levels of diapycnal mixing in the Subtropical North Atlantic associated with Thermohaline staircases even though this is non-mechanical process.

The benefits, drawbacks and relevance of Thorpe scaling high resolution temperature are discussed and compared with other finescale parametrisations, leading to suggestions as to where the method is most applicable within the oceans for further study.

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