ST2 Mooring Documentation & Results

(J.Manning contribution to Alessi et al 's WHOI-11-2001 data report)

2. Stratification Study Moored Array

    2.3  ST2 (40 57.35N  67 37.59W    in 69 meter water depth)

The National Marine Fisheries Service (NMFS) deployed the ST2 mooring.  This consisted of a single surface buoy supporting two VMCMs, a near-bottom Vector Averaging Current Meter (VACM with added conductivity and light transmission sensors), four SeaCats, three Brancker TPODs, and one VEMCO Minilog temperature recorder (Figure 10 and Table 10). The VMCMs , VACM, and the SeaCats were set to record every 7.5 minutes, the TPODs every 30 minutes, and the Minilog every 60 minutes. This mooring used an elastic (6 bungy cords) member to maintain tension on the near-bottom element of the mooring.  Its position and water depth listed in Table 1 were taken at the time of the anchor drop.

After surviving several months of winter/spring conditions, the upper portion of the mooring was evidently severed  by a passing trawler on July 27th, a few weeks prior to the scheduled recovery. The upper portion of the mooring drifted for two months until a fisherman recovered it off the Hudson Canyon approximately 250 miles away. Consequently, all data was recovered except for one SeaCat.  The lower portion of the mooring was acoustically released as scheduled in mid-August from the ENDEAVOR.

4.  Data Processing

    4.2 ST2

       4.2.1 SEACATs

        The records from the three Model 16 SEACATS from ST2 were processed with standard Seabird processing programs: scterm, scsoft, and binavg. The function of these routines is to download the data from the instrument, convert the data to engineering units, and bin the data, respectively. Since the second step was conducted with both the Seabird's pre-deployment (Dec.94) and post-deployment (Sep.95) calibration coefficients and the sensor drift was insignificant, the post-deployment calibration coefficients were used. Since each scan (every 7.5 minutes) was requested in the final binavg routine, no averaging was done. This raw data was then subjected to two locally generated (NMFS) MATLAB routines "avg2mtl.m" and "runave.m" to reformat the raw data and obtain hourly average data, respectively. The reformatting step generates a fractional yearday vector assuming equally spaced samples initiated at 0500 (GMT) on Jan. 26, 1995 (yd=25.2083) when the instrument was set to begin recording data. The averaging is simply an 8-point mean of the raw data without any weighting with the first hour centered at 2100 (GMT) on February 2, 1995 after the mooring was deployed.
    4.2.2 Brancker TPODS
        The records from the three XL-105 temperature probes were run through the standard Brancker xl.exe routine to download the data to ascii files. Again the difference in WHOI pre-deployment and post-deployment calibration of the instruments was insignificant. These ascii files were subjected to a NMFS Matlab conversion routine "tp2ml.m" and the hourly data was produced by simply extracting every other point of the 30 minute samples.
        The record from the one Minilog-TX/TR temperature probes was run through the standard VEMCO minilog.exe routine (Version 1.13) to download the data to an ascii file. Factory generated pre-deployment calibration coefficients were used with this data. This ascii file was subjected to a Matlab conversion routine "ml2mtl.m". Since the sampling rate on this instrument was set to one per hour, an extra step to generate "hourly" data was not needed. After hand-editing a single spike, the hourly Minilog record was compared to the hourly Brancker TPOD record from the same depth.  The overall mean of the Minilog differed from the Brancker by less than 0.1 degree and corresponded surprising well (Figure XX) considering the low cost of the Minilog.
    4.2.4 VMCM/VTCM
        After the standard WHOI processing steps (conducted by Fran Hotchkiss, USGS), records from the ST2 current meters were exported from "buoy format" to ascii format using the WHOI "batchlis" routine and then converted to column data using a NMFS "bl2ml.f" routine which adds a fractional yearday vector (in GLOBEC yrday0_gmt convention) for ease of processing in later steps. These 7.5 minute records were hourly averaged with the NMFS "runave.m" Matlab routine similar to the Seacat data as described above.
6. Results of GBSS Moored Array Time Series

  6.2 ST2

        The seven temperature records from ST2 indicate very little thermal stratification until late May. The temperature remained isothermal until mid-may typically less than 6.5 degrees with a minimum in late-March of near 4.5 degrees C (Figure XXa). The later part of the thermal stratification record was evidently influenced by the influx of slope water at depth in mid-May (Figure XXb).  Another look at the evolution of water properties as recorded on the Seacats is provided in a time series plot of temperature (Figure AA)   and salinity (Figure BB)   The Brancher TPods also provided a series of temperature records (Figure CC). The biggest signal in the four salinity records is a week-long influx of slope-like water at this same time in mid-May (Figure YY). The three current records (15, 35, and 62 meters) recorded several wind-driven events during the first half of the deployment including four westward events in March, one eastward event in early April, and a westward event in early May (see progressive vector figures like Figure ZZ). While the monthly mean flow (Feb-Jul) was affected by these events, the magnitude is still within the low-frequency standard deviations as reported by Butman and Beardsley (1987) for station "A" (1975-79, 85m) on the southern flank.  There was a general increase in both the monthly mean magnitude and vertical shear.   The overall statistics for ST2 records is presented in tabular form (Table XX).