U.S. Geological Survey Data Series 74, Version 3.0
Long-Term Oceanographic Observations in Massachusetts Bay, 1989-2006
The data collected at LT-A and LT-B are stored in numerous NetCDF data files. A graphical catalog, a list of data files by variable, a selection of data files that provides a concatenated time-series of variables, and tables of data files for each variable and year (see Digital Data Files), are provided in this report to facilitate identifying a data set of interest.
A graphical catalog illustrates where and when observations of a particular variable were made at LT-A and LT-B for each year of observation. In this catalog, colored lines indicate the availability of data at the three nominal measurement depths (table 1) for each parameter. A fourth line indicates the ADCP measurements throughout the water column. The first part of the name of the file containing the data is superimposed on the data line. The first four digits of the file name are the mooring number, and the letters, where present, indicate instrument type (See table 8 for the conventions used to name the data files). Cruises made to deploy, recover, or otherwise service the moorings are indicated by a dashed vertical line and labeled with the Woods Hole Science Center Field Activity Number (YYNNN). The first two digits in the Field Activity Number (YY) are the last digits in the calendar year of the cruise and the last three digits (NNN) are a sequential number. The full names of all data files containing ADCP, attenuation, current, pressure, salinity, and temperature data are listed in tables 9 and 10 (below). These file names are needed to download the digital data (Digital Data Files).
The data collected at LT-A and LT-B are stored in numerous individual NetCDF data files, at least one file for each instrument deployed. In some cases, several measurements of the same variable were made at about the same depth and time by different instruments. For example, temperature was often measured on the bottom tripod at LT-A by both the current meter system and the ADCP, and sometimes by other instruments. For the analysis and plots presented in the following sections of this report, a subset of these data files was selected to provide a time-series for each variable at each depth over the entire study period. A set of data records was selected to provide a time series of hour-averaged data at the nominal measurement depths of 4 to 10 m, 22 m (10 m above bottom), and 31 m (1 m above bottom) at LT-A, and at 12 m (10 m above bottom) and 21 m (1 m above bottom) at LT-B. The data in this concatenated series are from slightly different water depths and locations. The files selected for the concatenated series are identified in the figures showing the data available for LT-A and LT-B by an asterisk (*) and in the list of data files by a '1' in the concatenate column. Where multiple measurements were made, the file with the longest data record was selected. For temperature, data recorded by SEACAT, VMCM, tripod data logger (BASS, etc.), ADCP, and oxygen sensors were selected, in that order. The concatenation of the individual data files was carried out with a MATLAB m-file that is described and included with the concatenated data.
Data included in this report were collected at LT-A from December 1989 to February 2006 and at LT- B from October 1997 to February 2004. At LT-A, measurements were made at three depths, nominally 5 m, 22 m (10 m above bottom), and 31 m (1 m above bottom). Beginning in 1996, ACDP current measurements were made from 2 to 24 m in 2-m bins. At LT-B, measurements were made at two depths, nominally 12 m (10 m above bottom) and 21 m (1 m above bottom), and ADCP current measurements were made from 2 to 16 m in 2-m bins. Changes in the parameters measured and instruments used during the 17-year program reflect programmatic and logistical changes. Variations in measurement depths are a result of slightly different water depths at deployment and variations in the mooring design. The ADCPs measured currents throughout the water column in 2-m bins. The deepest bin (table 11) was typically 2 to 4 m above the head of the ADCP. At LT-A the head was located about 3 m above bottom; at LT-B the head was about 1.5 m above bottom.
The data presented in the following sections are reported at nominal depths from the surface, with the assumption that the water was 32 m deep at LT-A (see below) and 22 m deep at LT-B. The measurements at the nominal 5-m depth were obtained from a surface mooring with the instrument suspended beneath a surface-following buoy. The instrument’s elevation above the bottom varied with the tide. The subsurface instruments at LT-A and LT-B were deployed 10 m above bottom on a subsurface mooring. The actual depth of the observations is a few meters different from the nominal depth depending on the actual water depth where the mooring was deployed.
The depth of a measurement from the sea surface was determined using the water depth at the station and the position of the sensor on the subsurface mooring or on the bottom tripod. In the NetCDF files, water depth is a Global Attribute of the data file and a sensor depth is assigned to each variable. The water depth where a tripod or subsurface mooring was deployed was typically determined using the ship’s fathometer when the system was deployed. In many cases the Coast Guard vessels had the capability to correct these measurements for the phase of the tide. If a fathometer measurement was not available, a water depth was estimated from a navigation chart or taken from a previous deployment. Given the drift of the ship, the roughness of the sea floor, and the uncertainty of the tidal corrections, these depths are estimated to be accurate to a few meters. The water depths measured in the field were entered into the mooring log and as the global water depth in NetCDF data files. For sensors on bottom tripods, the sensor depth was computed as the water depth minus the measured height of the sensor above the bottom (measured prior to deployment). For instruments on subsurface moorings, the sensor depth was computed as the water depth minus the height of the instrument above bottom, as determined by the measured length of the mooring components (see figures in Field Program). For sensors on surface moorings, the sensor depth was the depth of the sensor below the surface, as determined from the measured length of the mooring components.
In most deployments of the tripods at LT-A, pressure measurements, which provide a more accurate measure of water depth than the fathometer measurements, were made. For these deployments, the water depth at the tripod was computed by averaging the bottom pressure over the duration of the deployment and converting to depth by correcting for atmospheric pressure (water depth (m) = pressure sensor height above bottom (m) + (average pressure (mb)/100) – 10.13 m). The average water depth for all tripod deployments at LT-A was 31.8 m (List of average pressure); based on this average, a nominal water depth of 32 m at LT-A has been used throughout this report. The difference between depth estimated from bottom pressure and depth estimated from fathometer or other measurements was a few meters. The best estimate of water depth is from the pressure observations; the global water depth and sensor depths in the NetCDF files have been adjusted for the pressure-determined water depth.
The ADCP processing program also determines water depth based on a water-surface-tracking algorithm. This water depth is used as the global water depth, in the sensor depth, and in the depth variable for the ADCP bins. The depth determined by average bottom pressure was about 1 m deeper than that determined by the ADCP algorithm (List of average pressure) (N=23); the cause of this small difference has not been determined. However, the global water depth in the ADCP NetCDF files differs slightly from the global water depth in NetCDF files for other measurements on the same tripod.
Nine subsurface moorings had a pressure measurement at about 13 m below the surface (List of average pressure, sheet 2). Water depth was computed as the average pressure over the deployment plus the height of the sensor above the bottom as determined by the length of the mooring components. For these moorings, the nominal water depth was about 34 m. To date, the global water depth and sensor depths in the NetCDF files for these moorings have not been adjusted to the pressure-determined water depth.
The instruments at LT-A and LT-B were recovered and redeployed several times each year, typically in February, June, and September; these instrument exchanges resulted in gaps in the time-series data a few hours long. Instrument malfunctions, biological or mechanical fouling, deployment delays, and accidental disruption of the instrumentation resulted in longer gaps in the data. In some instances, data are missing for an entire 4-month deployment period.
The bottom tripods tipped over during 5 of the 85 deployments (moorings 389, 407, 428, 717, and 775). Two tripods tipped over during large storms—tripod 389 in October 1991, during the second largest storm of the period 1990-2006 (Butman and others, 2008c), and tripod 717 in December 2003, during the third largest storm. Tripods 407, 428, and 775 tipped over during non-storm times, possibly as a result of entanglement with fishing gear. Unfortunately, tripod 407 tipped over prior to the December 1992 storm, the largest storm of the period 1990-2006. Because the tripod and current sensors were no longer upright, the current data collected after the tripod was turned on its side are not useful; however, the temperature, salinity, and pressure data are valid, although the sensors were at a slightly different depth.
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