Churchill#1 Report

### Investigation of the Bottom Boundary Layer during the Georges Bank GLOBEC Stratification Study

    INVESTIGATORS:

    J. Churchill
    Woods Hole Oceanographic Institution
    Woods Hole, MA
    email: jchurchill@whoi.edu

    A. Williams
    Woods Hole Oceanographic Institution
    Woods Hole, MA
    email: awilliams@whoi.edu

    GRANT PERIOD: October 1993 - June 1995

#### STATEMENT OF OBJECTIVES:

To examine turbulent mixing and its influence on stratification in the bottom boundary layer over the southern flank of Georges Bank.

#### STATEMENT OF WORK:

Our study of stratification and mixing in the bottom boundary layer on Georges Bank began with the deployment of a BASS (Benthic Acoustic Stress Sensor) tripod at station ST1 (40 51'N, 67 33'W) on 3 February 1995. The BASS was equipped with 5 acoustic current meters located from 0.25 to 4.5 m above bottom. These sampled the flow at 2 Hz for 7.5 min. every half hour. Also attached to the BASS were: 8 thermistors, 5 optical backscatterance sensors (to estimate suspended particulate concentration), a pressure sensor, a CTD and Benthos camera and strobe. The BASS was scheduled for an at-sea turn-around with data recovery in mid-April. Unfortunately this was not successful. Only one of the two buoyed recovery lines on the BASS released upon command. Before it could be retrieved, it wrapped around the mooring chain of the SW guard buoy at ST1 and severed on the mooring chain when recovery was attempted. On June 7 a second recovery attempt using the SeaProbe Submarine also failed. A third recovery attempt with the SeaProbe was successful and the tripod was returned to WHOI on 11 July. The tripod sensor data, which extended till mid-June, have been recovered and are now being processed.

To acquired further bottom boundary layer measurements during a time of "mature" stratification, a second BASS tripod was deployed at ST1 on July 12. This was outfitted with the same complement of current meters and thermistors as were attached to the first BASS tripod. It is slated for recovery on 19 August.

Analysis of the tripod measurements, coupled with modelling of bottom mixing, will be carried out over the next year.