Beardsley Report

### U.S. GLOBEC: Seasonal Development of Stratified Water on Georges Bank: Dynamics of Zooplankton and Larval Fish -- Moored Array Component

    INVESTIGATORS:

    Robert C. Beardsley, MS 21
    Woods Hole Oceanographic Institution
    Woods Hole, MA 02543
    rbeardsley@whoi.edu

    Steven J. Lentz, MS 21
    Woods Hole Oceanographic Institution
    Woods Hole, MA 02543
    slentz@whoi.edu

    David Mountain
    National Marine Fisheries Service
    Woods Hole, MA 02543
    dmountain@whsun1.wh.whoi.edu

    Robert A. Weller, MS 29
    Woods Hole Oceanographic Institution
    Woods Hole, MA 02543
    rweller@whoi.edu

    GRANT PERIOD:

    October 1, 1993 to September 30, 1996

### STATEMENT OF OBJECTIVES:

The primary scientific objective of the Georges Bank Stratification Study (GBSS) moored array component is to develop an understanding of the physical processes which control stratification along the southern flank of Georges Bank during spring, particularly those processes at work in the surface and bottom boundary layers. In a very simple sense, the spring-time increase in surface heating and decrease in wind-driven surface mixing lead to the development of a seasonal thermocline and a stratified water column between the tidal mixing front (located roughly along the 60-m isobath) and the shelfbreak front, however, our basic understanding of the physical processes which govern this restratification over Georges Bank is quite poor. Key questions involve the relative role of horizontal processes (especially alongshelf advection of shelf water and cross-shelf advection of water from either frontal region) versus (local) vertical processes, the structure of the surface and bottom boundary layers and their roles in cross-shelf density advection, and the relative importance of tidal processes over the shelf.

To achieve this objective, the GBSS moored array component was designed with the following goals:

- To measure the vertical structure and temporal evolution of stratification (temperature, salinity, and density) on vertical scales of a few meters from surface to bottom on time scales of minutes to months along the southern flank;

- To measure the horizontal variation in vertical stratification in the cross- and along-isobath directions over scales of 15 and 30 km, respectively;

- To estimate the surface momentum, heat, and freshwater fluxes which directly influence stratification at a central site (ST1); and

- To measure the vertical structure and temporal evolution of horizontal currents from surface to bottom with sufficient resolution to identify and quantify vertical shear and transport in the surface boundary layer at ST1.

### STATEMENT OF WORK:

In summer 1994, P. Valentine (USGS) conducted detailed bathymetric and side-scan sonar surveys around the main mooring sites. The GBSS moored array was deployed at sites ST1 and ST2 during late January early February, 1995 on R/V Endeavor 260 (Figure 1).

The instrumentation deployed at the central site ST1 consisted of (a) a surface discus buoy with meteorological sensors to measure wind, incident short and long wave radiation, air temperature and pressure, relative humidity, and precipitation, and eight vector-measuring current meters (VMCMs) with additional sensors to measure water temperature and conductivity down to a depth of 40 m, and (b) a subsurface mooring supporting five VMCMs and additional sensors for temperature and conductivity (Figure 2). The instrumentation deployed at ST2 consisted of a single surface buoy supporting two VMCMs, a vector-averaging current meter (VACM) and addition sensors for temperature and conductivity. The combined array at ST1, ST2 and the Long-term Moored Array site SF (Figure 1) should provide a good description of the along- and cross-shelf variation in stratification and circulation over this region of the southern flank. The GBSS moored array at ST1 and ST2 is scheduled for recovery in late August, 1995.

The Bottom Boundary Layer component (Williams, Churchill, and Butman) deployed an Bottom Acoustic Sensor system (BASS) tripod at ST1 during EN260. In addition, CTD sections have been made along two standard cross-shelf transects, one along the ST1-ST2 mooring line and the other through the Long-term Moored Array SF site, on all GBSS cruises.

### SUMMARY OF KEY FINDINGS:

The bottom topography at the ST1, ST2, and SF sites is relatively smooth, with the sand/gravel surface showing traces of scallop dredging. A relatively abrupt transition between the smooth topography over the deeper mid- and outer shelf and rougher inner shelf with large sand/gravel ridges occurs near the 50-m isobath.

To date, the moored array has remained on station. Several strong storms with winds in excess of 18 m/s have occurred, and two of the three solar panels mounted on the discus have been broken off.

A subsample of the full suite of meteorological measurements is transmitted daily via ARGOS to WHOI. This data showed that one long wave sensor failed March 1, 1995, and the IMET and VAWR wind sensors failed May 16 and June 3, 1995, respectively. The long wave sensor was successfully replaced on June 11, 1995 but replacement of the mechanical component of the IMET wind sensor at that time did not solve the problem. Fortunately, based on data received prior to June, the wind at ST1 is relatively well correlated with the wind measured at the eastern Georges Bank NDBO environmental buoy 41011, located about 86 km to the east, so that the wind at ST1 can be estimated with some success for the rest of the field experiment.

The ST1 moored meteorological data in unedited form can be made available upon request. Post calibration of the moored instrumentation should be completed early in 1996 and the basic editing of the moored array data should be completed in fall, 1996. Basic editing of the hydrographic data collected during GBSS cruises should be completed by fall, 1995.

In addition, this component has supported the thesis research of two WHOI/MIT Joint Program students. Ari Epstein completed his Ph.D. thesis entitled "Physical Processes and Zooplankton Distribution in the Great South Channel: Observational and Numerical Studies" in May, 1995, and William Williams will complete his M.S. thesis in July, 1995.