Northeast Fisheries Science Center Reference Document 04-08
Revised Procedures for Calculating
Regional Average Water Properties
for Northeast Fisheries Science Center Cruises
by David G. Mountain, Maureen
H. Taylor and Cristina
Bascuñán
National Marine Fisheries Serv., Woods Hole Lab., 166 Water St., Woods
Hole, MA 02543
Print
publication date April 2004;
web version posted July 28, 2004
Citation: 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.
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Introduction
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.
Data Tables
Annual Cycles:
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)
i=1
where ri = 2*π (i/365) and the C’s,
A’s and B’s are the harmonic coefficients.
Table 1, Table 2, Table 3, and Table 4 contain the coefficients for surface temperature, bottom
temperature, surface salinity and bottom salinity, respectively. The
columns in the tables are:
MARMAP standard station number
Latitude
Longitude
Bottom depth
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
23 MARMAP
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.
References
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.
