Home > Publications > Characterizing turbulent overturns in CTD-data

Hans van Haren and Louis Gostiaux

Characterizing turbulent overturns in CTD-data

Dynamics of Atmospheres and Oceans, 66, 58–76 (2014)

We are concerned with the shape of overturns due to irreversible effects of turbulent mixing through internal wave breaking in the ocean. Vertical (z) overturn displacements (d) are computed from ship-borne SeaBird-911 CTD-data using the well-established method of re-ordering unstable portions in vertical density profiles. When displayed as function of z, the displacements d(z) reveal a characteristic zig-zag shape.

Here, we primarily investigate the particular slope (z/d) of this zig-zag signature assigning the displacements to the end-point depths. Using model-overturns we show that this slope equals ½ for a solid body rotation, while a more sophisticated Rankine vortex overturn model, here employed in the vertical, has slopes slightly >½ in the interior and >1 along the sides. In the case of a near-homogeneous layer, displacement points fill a parallelogram with side-edges having a slope of 1. The models are used to interpret overturn shapes in NE-Atlantic Ocean data from moderately deep, turbulent waters above Rockall Bank (off Ireland) and from deep, weakly-stratified waters above Mount Josephine (off Portugal). These are compared with salinity-compensated intrusion data in the Canary Basin Mediterranean outflow. Dynamically, most overturns are found to resemble the Rankine vortex model overturn and very few a solid body rotation. Additionally, the usefulness and un-usefulness of upcast-CTD-data is discussed for overturn characterization.

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