CONTENTS Introduction Changes in the Reference Annual Cycles Changes in the Calculation of Anomalies Changes in the Calculation of Regional Average Values Data Tables References
Northeast Fisheries Science Center Reference Document 04-08
David G. Mountain, Maureen H. Taylor and Cristina Bascuñán
Revised Procedures for Calculating Regional Average Water Properties for Northeast Fisheries Science Center Cruises
National Marine Fisheries Serv., Woods Hole Lab., 166 Water St., Woods Hole, MA 02543
Web version posted July 28, 2004Citation: Mountain, D.G.; Taylor, M.H.; Bascuñán, C. 2004. Revised procedures for calculating regional average water properties for Northeast Fisheries Science Center cruises. Northeast Fish. Sci. Cent. Ref. Doc. 04-08; 53 p.
Information Quality Act Compliance: In accordance with section 515 of Public Law 106-554, the Northeast Fisheries Science Center completed both technical and policy reviews for this report. These predissemination reviews are on file at the NEFSC Editorial Office.
Temperature and salinity measurements are made routinely on most Northeast Fisheries Science Center (NEFSC) survey and research cruises. The area of coverage extends from Cape Hatteras northeastward to the Gulf of Maine, and from near the coast to just seaward of the shelf break. Since 1991 a report of the water properties has been issued almost every year (e.g., Taylor and Bascuñán, 2003). This report includes regional average values of surface and bottom water temperature and salinity, and of the anomalies of temperature and salinity for five regions of the shelf (Figure 1). The anomalies are determined relative to reference annual cycles derived from data collected during the NEFSC Marine Resources Monitoring and Assessment Program (MARMAP) program. Holzwarth and Mountain (1992) present similar regional values for surface and bottom temperature as measured on the NEFSC spring and fall bottom trawl survey cruises from 1963 to 1990.
The methods used to determine the MARMAP annual cycles are described by Mountain and Holzwarth (1989). The methods used to calculate the anomalies, the regional average values, and the uncertainty in the regional average values are described by Holzwarth and Mountain (1992). This report documents changes that have been made to the reference annual cycles and to the methods used to calculate the regional average values. The revised cycles and methods will be used in the annual summary of water properties beginning with the report for 2003 (Bascuñán and Taylor, 2004). All previously reported regional average values have been recalculated using the revised annual cycles and methods, and are included in this report. In most cases differences between the original and the revised regional values are small (< 0.1°C or < 0.1 PSU).
Changes in the Reference Annual Cycles
The NEFSC MARMAP program (1977-1987) made observations of plankton and water properties at approximately 160 standard station locations 4-8 times per year. Temperature and salinity measurements were made using water bottles with reversing thermometers. The water samples collected were analyzed for salinity by a laboratory salinometer. During the last year of the program an electronic conductivity/temperature/depth (CTD) instrument was used.
Because the MARMAP measurements were made at fixed locations over a 10 year period, characteristic annual cycles of the surface and bottom temperature and salinity could be determined for each of the locations. The annual cycles were derived by sequentially fitting up to three harmonic cycles (periods of one, one-half, and one-third year) to the observations at a location (Mountain and Holzwarth, 1989). For each harmonic a significance test was performed to determine if the fit was statistically significant. In some cases no significant harmonic cycle was found and the annual cycle consisted only of the mean of the observations.
In 2002 an ORACLE database was established for NEFSC’s hydrographic data. As part of that effort the historic hydrographic data was reviewed for quality before it was entered into the database. This quality control process resulted in some MARMAP data either being changed or deleted from the data set. Because of these changes the annual cycles based on the MARMAP data were recalculated using the data in the ORACLE database. In determining the bottom annual cycles, observations had to be no more than 10m or 10% of the water depth (which ever was greater) from the bottom to be included in the calculations as representing a bottom value. Generally, only minor changes resulted in the annual cycles. In a few cases no significant annual cycle could be determined where previously one had been, and in a few other cases a cycle was determined where previously one had not been.
Changes in the Calculation of Anomalies
To calculate the anomaly for an observation, the MARMAP stations closest to the observation location are determined. The expected value of the property (i.e., surface or bottom temperature or salinity) at each of these MARMAP stations is determined from the appropriate annual cycle for the calendar day on which the observation was made. The MARMAP values are averaged by a weighting inversely proportional to the square of the distance each is from the observation location. The anomaly is the difference between the observed value and the resulting averaged MARMAP value.
For calculating bottom anomalies, only MARMAP stations with bottom depths similar to the bottom depth of the observation station are included. The criterion used is that the depth difference has to be smaller than 25m or 25% of the bottom depth of the observation station, whichever is greater.
When calculating salinity anomalies, MARMAP stations that do not have a significant annual cycle are included by using the annual mean value for that station (i.e., the C1 value in Table 1). For calculating temperature anomalies, MARMAP stations without an annual cycle previously had not been included. Under the new procedures MARMAP stations without an annual cycle are included in calculating temperature anomalies by using the mean value for that station. The only MARMAP stations that do not have an annual cycle for bottom temperature are in the deep Gulf of Maine or seaward of the shelf break (>100m depth). Therefore, this change in the procedure for calculating bottom temperature anomalies only affects observations in those areas.
Changes in the Calculation of Regional Average Values
The regional average values are determined by interpolating the observed values to a fine scale grid, with each grid point representing a known area. Previously the separation between grid points was 0.2 degrees latitude and 0.25 degrees longitude. In the revised procedures the separation has been reduced to 0.1 degree in latitude and 0.1 degree in longitude to provide a more accurate representation of an area. In addition the computer code used to determine if a station was within the boundaries of a region has been modified to correct for an error that in a few, specific situations identified stations close to but south of a regional boundary as being within the region.
The MARMAP annual cycles are calculated as the sum of three harmonics. For calendar day ‘d’, the value of the parameter ‘V’ is calculated by:
V(d) = 3 Σ Ci + Ai * cos(ri * d) + Bi * cos(ri * d)
where ri = 2*π (i/365) and the C’s, A’s and B’s are the harmonic coefficients.
MARMAP standard station number
The harmonic coefficients C1, A1, B1, C2, A2, B2, C3, A3, B3
The standard deviation of the original data values from the fitted curve
Regional Property Values:
For each cruise in the NEFSC hydrographic data base Table 5, Table 6, Table 7, Table 8, and Table 9 contain the revised area average surface and bottom temperature and temperature anomaly values for the five regions in Figure 1. Table 10, Table 11, Table 12, Table 13, and Table 14 contain the same information for salinity. In these tables the columns are:
Cruise name Region Code () Year Calendar day Decimal-Year (year + calendar day/365) For the surface: Number of stations in the region Value Anomaly SDV1 SDV2 Flagg For the bottom: Number of stations in the region Value Anomaly SDV1 SDV2 Flagg Cruise Purpose Code
The Region Codes are:
1 MAB South
2 MAB North
3 Georges Bank
4 Gulf of Maine West
5 Gulf of Maine East
SDV1 indicates the uncertainty in the regional average anomaly value associated with the uncertainties in the MARMAP annual cycles (See Holzwarth and Mountain, 1990). SDV2 is the standard deviation of the anomaly values for the stations within the region. A Flagg value of 0 indicates that there were enough observations to calculate an area weighted value for the region. A Flagg value of 1 indicates that the spatial distribution of the observations was not sufficient to calculate an area average for the region and instead a simple average of the observations was calculated.
The cruise purpose codes are:
01 BLUEFISH SURVEY
02 LOBSTER SURVEY
03 COD SPAWNING SURVEY
10 NMFS NEFSC BOTTOM TRAWL SURVEY
16 NMFS ACOUSTICS SURVEY
20 GLOBEC BROADSCALE SURVEY
21 GLOBEC PROCESS STUDY
22 ECOSYSTEM MONITORING
24 LARVAL HERRING SURVEY
25 12-MILE DUMPSITE
27 WARM CORE RING CRUISE
50 NMFS NEFSC CLAM AND QUAHOG SURVEY
60 NMFS NEFSC SEA SCALLOP SURVEY
70 NMFS NEFSC GEAR COMPARISON
80 FOOD CHAIN DYNAMICS FEEDING ECOLOGY SURVEY
81 COASTAL OCEAN PROGRAM
82 APEX PREDATOR SURVEY
91 BENTHIC HABITAT STUDY
90 MARINE MAMMAL SURVEY
93 MISCELLANEOUS NON-RANDOM RESOURCE INVESTIGATION CRUISE
Regional average surface and bottom temperature for the spring and fall bottom trawl surveys from 1963 to 1990 are listed in Table 15 and Table 16. The columns are the same as those for Tables 5-14 except that cruise name column is omitted.
Bascuñán, C. and M.H. Taylor 2004. Description of the 2003 oceanographic conditions on the northeast contental shelf. Northeast Fisheries Science Center Reference Doc. 04-xx; xx p. Available from: National Marine Fisheries Service, 166 Water St., Woods Hole, MA 02543.
Holzwarth, T.J. and D.G. Mountain 1990. Surface and bottom temperaure distributions from the Northeast Fisheries Center spring and fall bottom trawl survey program, 1963-1987, with addemdum for 1988-1990. Northeast Fisheries Science Center Reference Doc. 90-03; 77 p. Available from: National Marine Fisheries Service, 166 Water St., Woods Hole, MA 02543.
Mountain, D.G. and T.J. Holzwarth. 1989. Surface and bottom temperature distribution for the northeast continental shelf. NOAA Tech. Mem. NMFS-F/NEC-73; 32 p.
Taylor, M.H. and C. Bascuñán. 2001. Description of the 2000 oceanographic conditions on the northeast contental shelf. Northeast Fisheries Science Center Reference Doc. 01-01; 93 p. Available from: National Marine Fisheries Service, 166 Water St., Woods Hole, MA 02543.