Generation of Bulk Shear Spikes in Shallow Stratified Tidal Seas

Abstract

Recent finescale observations of shear and stratification in temperate shelf sea thermoclines show that they are of marginal stability, suggesting that episodes of enhanced shear could potentially lead to shear instability and diapcynal mixing. The bulk shear between the upper and lower boundary layers in seasonally stratified shelf seas shows remarkable variability on tidal, inertial, and synoptic time scales that has yet to be explained. In this paper observations from the seasonally stratified northern North Sea are presented for a time when the water column has a distinct two-layer structure. Bulk shear estimates, based on ADCP measurements, show a bulk shear vector that rotates in a clockwise direction at the local inertial period, with episodes of bulk shear spikes that have an approximately twice daily period, and occur in bursts that last for several days. To explain this observation, a simple two-layer model based on layer averaging of the one-dimensional momentum equation is developed, forced at the surface by wind stress and damped by (tidally dominated) sea bed friction. The two layers are then linked through an interfacial stress term. The model reproduces the observations, showing that the bulk shear spikes are a result of the alignment of the wind stress, tidal bed stress, and (clockwise rotating) bulk shear vectors. Velocity microstructure measurements are then used to confirm enhanced levels of mixing during a period of bulk shear spikes. A numerical study demonstrates the sensitivity of the spike generation mechanism to the local tidal conditions and the phasing and duration of wind events.