ACSEX
LOCO
MARE
PROCS
	Research

 

Processes at Continental Slopes (PROCS)
The Generation of Strong Turbulence in the Near-bed Region Contributors: Phil Hosegood, Hans van Haren
In order to maintain the Meriodional Overturning Circulation (MOC) which represents the large scale (‘thermohaline’) oceanic circulation, a level of mixing of water masses is required that is observed to be an order of magnitude too small in the oceans interior; instead it is believed that enhanced mixing occurs at the oceans’ boundaries. During PROCS, the processes that may facilitate such enhanced mixing were studied as well as their impact on sediment transport and the benthic biological community.  We have found evidence of two processes that generate turbulence near the sea-bed, the first related to the asymmetric response of the bottom boundary layer to a variable flow along the continental slope, and the second due to the response of the deep thermocline to atmospheric forcing.
An initial inspection of data obtained from moorings deployed at the sea-bed during PROCS revealed spike-like reductions in current speed when sampled by mechanical rotor and vane type current meters at a rate of once per minute. Concurrent measurements by acoustic current meters on the same mooring exhibited higher noise levels than those periods during which no spikes were observed in the records of the mechanical current meters. Given the isolated nature of the spikes and their duration of only one data point (one minute), it was thought that they represented an instrumental error; this was rejected however due to a directional dependence which required the mean current to be flowing primarily to the north-east along the continental slope for the spikes to occur.		Fig.2: The asymmetric response of the bottom boundary layer under contrasting long-slope flows and the consequences for turbulent mixing in the near-bed region.
Instead, it was found that the change in mean current direction between different heights on the moorings was consistent with Ekman dynamics which, through the influence of friction and rotation, causes the current direction to rotate anticlockwise towards the seabed.  Over the slope in the Faeroe-Shetland Channel this results in upwelling at the seabed during periods when the mean long-slope velocity component is directed equatorward (towards the south-west), potentially resulting in enhanced stratification and suppressed turbulence (Fig. 2). In contrast, during periods when the long-slope velocity is poleward (towards the north-east), static instabilities may occur as lighter water is advected beneath heavier water lower down the slope.  The static instabilities then promote the growth of turbulence in the near-bed region which cause the spikes in the current meters due to their inability to resolve such short time-scale fluctuations in the current direction. The intermittent nature of the spikes is explained by the bursting phenomenon from a turbulent boundary layer.
The turbulence generated by the above process is related to a downslope flow near the bed. At particular times during PROCS however, spikes were found when the flow was directed upslope, requiring an alternative explanation for the apparent generation of turbulence.  The periods concerned, marked by strong temperature gradients observed at the sea bed, have been explained as ‘solibores’ (Fig. 3), which are intense internal wave trains which display the properties of both turbulent internal bores and nonlinear wave trains. Our observations are unusual because of the great depth (near the bed) at which these solibores occur and the origin of their forcing, which appears to be atmospheric rather than tidal, considering the 4 days periodicity. The mixing rates, measured as diapycnal diffusivity, Kρ, associated with the solibores reach 10-2 m2 s-1	Fig.3: a) Temperature and b) cross-slope (positive upslope),  c) long-slope, d) vertical velocity components during the passage of a large solibore during which large sediment fluxes occurred.
However, as the rapid passage of such strong solibore occurred only once in 14 days, the average Kρ < 10-4 m2 s-1, the value supposedly required to maintain the global thermohaline circulation.  In contrast, the solibores appear to dominate the resuspension of sediment at the sea bed, with daily averaged fluxes measured at 10m above the bottom two orders of magnitude larger than background levels. The solibores are forced due to a sudden depression of the thermocline caused by the passage of a nonlinear large-scale wave possibly generated by atmospheric storms.  When the resulting hydraulic jump (bore) propagates obliquely up the slope it overturns due to kinematic instabilities when the particle velocities exceed the speed of the bore itself. _______________