Schaffer, Janin (2013) Small-scale variability associated with mixing in the Denmark Strait Overflow plume based on horizontally profiling observations. (Master thesis), Christian-Albrechts-Universität Kiel, Kiel, Germany, 109 pp.

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Abstract

The Denmark Strait overflow (DSO) contributes roughly half of the total volume transport of the Nordic overflows. The overflows double their volume by entraining ambient water as they descend into the subpolar North Atlantic and feed the deep branch of the Atlantic Meridional Overturning Circulation. In summer 2012 a multi-platform experiment was carried out in the pathway of the DSO plume on the continental slope of Greenland near 30°W (180 km downstream of Denmark Strait), to observe variability on (sub-)meso- and small-scales associated with entrainment into the DSO plume In this study the focus is set on the observations of small-scale variability by horizontal profiling with an autonomous underwater vehicle (AUV) in the transition layer between the DSO plume and the ambient water. The AUV measurement system provides hydrographic data and microstructure measurements with a horizontal resolution of 6.4 m. Observations show enhanced temperature and salinity variances within distinct segments on wavelength between 13 m and 500 m. These scales reach from the turbulent regime into the internal wave regime. Spectral analysis was applied to the AUV-based temperature data to separate these two regimes. Increased temperature variance is found on wavelength between 21 m and 210 m with a wavenumber-dependence characteristic of turbulence in the inertial-convective subrange. Within the same periods elevated turbulent dissipation of O(10−6)W/kg is observed (compared with O(10−9)W/kg in the ambient water). The variances in temperature and isotherm displacement from the regular AUV-CTD agree with the dissipation rates from microstructure measurements. Both small-scale variability on turbulent scales as well as high dissipation rates imply strong vertical mixing. Two major turbulent events were captured by the AUV. The first was supposedly a stationary feature on the upstream side of a topographic elevation. The second was transient, and associated with processes near the edge of an energetic eddy. Large vertical shear of horizontal velocities and critical Ri numbers from ship-lowered CTD/LADCP profiles support the AUV-inferred strong vertical mixing. At the same time moored current meters indicate advection of ambient water towards the shelf. The observations imply that eddy-driven horizontal advection and vertical mixing go hand in hand in entraining ambient water into the DSO plume.

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