Low-pass Filter

A low-pass filter is used to remove tidal and higher-frequency fluctuations from the time-series data that sometimes mask smaller fluctuations in the time-series plots that are driven by winds and the density field. The filter, called PL33 (Flagg and others 1976; Beardsley and others, 1985), operates on hourly data values. The digital filter replaces each point with a weighted average of the 33 points on either side of the central point (figure 20). The filtering reduces the total length of the time series by 66 hours (33 hours on each end). The filter transfers signals at unreduced amplitude that have periods longer than about 50 hours (figure 21). The half-amplitude point of the filter is at 33 hours and the half power point is at 38 hours. The filter removes more than 99% of the amplitude at the semidiurnal tidal periods and more than 90% of the amplitude at the diurnal tidal periods (table). The low-passed data are subsampled every 6 hours.

Vector-Measuring Current Meter (VMCM)

VMCM's record data on 1/4" cassette tapes using Sea Data recorders. After the VMCM's were recovered, the data were read from the cassette into a file on a personal computer, and then translated into WHOI Carp format, using programs Seadata and PCARPHP (Danforth, 1990). Decoding and calibration were performed using the Buoy Group Data Processing System. Until September 1999, data were edited, truncated, averaged, and filtered using the Buoy System, but since then VMCM data have been written to NetCDF files, and these procedures have been conducted in the WHOI-USGS system. Since February 2001, data were also decoded and calibrated using the WHOI-USGS system.

SEACAT and MicroCAT

SEACAT and MicroCAT data are stored internally. After recovery, SEASOFT programs (Sea-Bird Electronics, Inc.) were used to read the data into a file on a personal computer, convert to calibrated oceanographic units, calculate salinity and density, and write the data to ASCII flat files. ASCII files were translated to Buoy Format or NetCDF, and the data were edited, truncated, averaged, and filtered using the Buoy System (until September 1999) or the WHOI- USGS system.

Acoustic Doppler Current Profiler (ADCP)

The ADCP observations were processed using USGS software (available at
http://woodshole.er.usgs.gov/operations/stg/pubs/ADCPtools/) and elements of the WHOI- USGS Oceanographic Data Processing system. The ADCP's were normally configured to record data in beam coordinates (rather than earth coordinates). Upon recovery, the ADCP data were transferred to a personal computer using a PCMCIA flash memory card. These data were converted to NetCDF format using software available for the ADCP Toolbox (above). Matlab routines were used to check for data quality, flag bad values, convert to earth coordinates using a 4-beam or 3-beam solution, truncate the data at the beginning and end of the deployment, and discard bins that were always beyond the water surface. Some near-surface bins were not discarded even though the side-beam reflection at times of low tide renders these data invalid, so near-surface ADCP data must be interpreted with care. On occasion, the ADCP skips an ensemble record because the data are poor. Data collected since 2000 have blank placeholders for the missing ensemble records. The end result of processing is an EPIC-compatible data file.

Data Logging Current Meter (DLCM) Tripods

When DLCM data were recorded on Sea Data cassettes, the data were read from the cassette into a file on a personal computer, and then translated into WHOI Carp format, using programs Seadata and PCARPHP (Danforth, 1990). When DLCM data were recorded on a Tattletale hard disk, the Tattletale was attached to a personal computer and the data copied into a file on the computer's hard disk, and then translated into WHOI Carp format using a C language program called SEADAT. Carp format DLCM data from both sources were processed using a Fortran program called NEWDDISC that translates into calibrated oceanographic units and derives current speed and direction, PSDEV, and salinity. The WHOI Buoy Group routine NSINP was then used to translate the data into Buoy Group format, and the data were edited, truncated, averaged, and filtered using the Buoy System.

MIDAS Tripods

MIDAS data were recorded on a Tattletale hard disk and then copied to a personal computer's hard disk after recovery. Until February 1998, a C language program was used to translate the data to calibrated oceanographic units, rotate the velocity to produce east, north, and up components, and calculate pressure standard deviation and velocity variances and covariances from the high-frequency measurements. The result was an ASCII flat file that was translated into the WHOI Buoy Group format using routine NSINP, and the data were edited, truncated, averaged, and filtered using the Buoy System. Since February 1998, the WHOI- USGS system has been used to decode and calibrate the data, compute secondary variables, and perform all further processing.

Initial processing of BASS current meter data is based on the assumption that the speed of sound is constant at 1500 m/s. BASS data from 1991 to 1998 were corrected using a time series of sound speed that was calculated from measured pressure, temperature, and salinity using the UNESCO algorithm (Fofonoff and Millard, 1983). In 1998, it became apparent that the sound speed corrections were smaller than the uncertainty caused by the imprecision of the compass and tilt sensors, so sound speed corrections were discontinued.

Transmissometer

Transmissometer data were processed along with the other data from SEACAT and tripod systems. Beam attenuation coefficients (units of m^-1) were computed from the light transmission observations as -4(ln(T/100)), where T is percent light transmission over a beam length of 0.25 m. The beam attenuation coefficient is linearly proportional to the concentration of suspended material in the water if the particles are of uniform size and composition (Moody and others, 1987). However, the size of the particles in the water changes with time, especially during resuspension events, and thus the beam attenuation measurements must be interpreted with care.

Wind Stress

Wind stress was calculated from wind speed and direction using the formulas of Large and Pond (1981).

Sediment Trap Samples

After recovery of the time-series trap, the trap bottles are cleaned and photographed. The contents are then sieved using a 1000-micron polyethylene screen in order to remove filamentous organic matter, such as seaweed, which would complicate the splitting process. Samples are split on a 4-way splitter described by Honjo (1978). The split designated for determination of mass is allowed to settle in a refrigerator for 3-5 days, the overlying clear sea water (and sodium azide) is measured for salinity, siphoned off, and the wet residue is subsequently freeze dried. The mass is corrected for salt content using the weight lost on drying and the measured salinity of the overlying water. The grams collected per m² per day are calculated from the measured weight, the time of exposure under the funnel, and the cross sectional area of the trap (0.5 m² ).