Jim Churchill (WHOI)

A principal goal of the drifter and dye studies conducted over Southern Flank in May and June 1999 was to investigate movement of water relative to the tidal mixing front. As discussed elsewhere, the data acquired gave only subtle indications of motion to or from front within the pycnocline.   This was not the case within the surface mixed layer.  Data from two drifter deployments and a surface dye study clearly showed surface mixed layer water moving to, and then across, the tidal mixing front.

The first indication of this came from the initial drifter deployment of Endeavor cruise EN323-4 (May 6-9, 2000), carried out as a "probing study" prior to the dye release experiments. In preparation for this deployment, the tidal mixing front was located by steaming SSE and keeping a close eye on the temperature records from the Endeavor's 1 and 5 m thermistors.  Drifters were deployed on either side of the front.  A surface drifter was set out in the vertically mixed water ~8 km north of the front, and surface and a drogued drifter were deployed in the vertically stratified water ~8 km south of the front (Figure 1a).The 6-m long holey sock of the drogued drifter was set at a mean depth of 19.4 m, so as to follow the flow beneath the pycnocline (which extended to ~12 m).  As revealed by their high tide positions (maximum onbank tidal excursions) the surface and drogued drifters deployed south of the front soon parted company, with the surface drifter moving steadily onbank after the first tidal cycle and the drogued drifter making little subtidal headway in any direction (Figure 1b). The surface drifter ended the deployment in vertically mixed water, as indicated by the CTD profile at its recovery site.  It movement across the tidal mixing front was revealed by the temperature records of the surface thermistors attached to each drifter (Figure 2a).Throughout the first two days of the deployment, surface temperatures at the surface and drogued drifters deployed south of the front were closely matched as both were within the relatively warm surface mixed layer of the stratified region.  Early in the second day of the deployment, the surface drifter apparently crossed the front as it temperature rapidly fell to the temperature recorded at the surface drifter deployed in the mixed region.The movement of this drifter to and across the front was probably the result of wind-driven transport.  During the day before it crossed the front, winds measured at buoy 44001 had a westward component (Figure 2b), which would tend to move surface water onbank through the Coriolis force.  The wind also had a strong northward component during this time, which would tend to directly accelerate surface water onbank.

Data from the second drifter deployment of EN324-4 (May 15-18, 2000) also showed the movement of surface water from the stratified region across the tidal mixing front.  During this deployment, which was conducted in conjunction with a surface dye release, six surface drifters were set out in the stratified water south of the tidal mixing front (Figure 1c).  Three were set out within the dye streak during dye release.  The temperature records from the drifters’ thermistors showed that all drifters were carried to the tidal mixing front within 1.5 days of their deployment (Figure 3a). All temperature records showed a period of rapid decline from the surface temperature within the stratified region to the surface temperature within the vertically mixed region.  Wind forcing likely played a major role in the onbank transport of surface water revealed by the drifter data.  Throughout the deployment, winds measured at buoy 44001 had a strong westward (downwelling favorable) component (Figure 3b).