Northeast Fisheries Science Center Reference Document 04-12
Description of the 2003 oceanographic conditions
on the Northeast continental
shelf
by Cristina Bascuñán, Maureen H. Taylor,
and David G. Mountain
National Marine Fisheries Serv., Woods Hole Lab., 166 Water St., Woods
Hole, MA 02543
Print
publication date September 2004;
web version posted November 26, 2004
Citation: Bascuñán, C.; Taylor, M.H.; Manning, J.P. 2004. Description of the 2003 oceanographic
conditions on the Northeast Continental Shelf. U.S. Dep. Commer., Northeast Fish. Sci. Cent.
Ref. Doc. 04-12; 81 p.
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ABSTRACT
A
summary of hydrographic observations for 11 surveys on the northeast
continental shelf during 2003 is presented. Distributions of CTD
stations, surface and bottom temperature, salinity, and anomalies are
portrayed. The average surface and bottom temperatures and salinities
have been calculated in five geographic regions over the northeast
continental shelf: western Gulf of Maine (GOMW), eastern Gulf of Maine
(GOME), Georges Bank (GB), northern Middle Atlantic Bight (MABN) and
southern Middle Atlantic Bight (MABS). Time series plots from various
shipboard environmental sensors are included if available.
Hydrographic data collected during 2003 were sorted into six 2-month
time bins to provide broad bimonthly coverage used in the averaging method. Review
of the computed areal average temperature and salinity data indicates
that temperatures showed a fairly typical seasonal pattern. Salinities
were found to be similar to the MARMAP reference period with the exception
of surface salinity in the southern MAB.
INTRODUCTION
The Northeast Fisheries Science Center (NEFSC) conducts several different
surveys off the northeast continental shelf each year. Complete coverage
of the shelf (Cape Hatteras to the Gulf of Maine) occurs during the spring
and fall bottom trawl surveys and during some of the Ecosystem Monitoring
cruises. Station coverage on other cruises throughout the year varies.
Temperature and salinity observations from 11 NEFSC surveys conducted
during 2003 are summarized and presented in this report. Cruise operation
summaries are presented for all cruises. Distribution plots of surface
and bottom temperature, salinity, and anomalies are contoured where sufficient
data are available. Areal average temperature and salinity and the corresponding
anomalies also are presented for the five different regions on the shelf
and for 6 time periods throughout the year. Contour maps are presented
chronologically in an atlas form. Environmental data from the SCS (Ship-board
Computing System) are presented as time series figures for each leg of
a cruise. No attempt has been made here to rigorously analyze the data
or discuss in detail individual observations from the cruises.
DATA AND METHODS
Temperature and salinity measurements were obtained with a Seabird
(SBE) model 19 profiling CTD (Profiler), which measures the pressure,
temperature and conductivity of the water twice per second. Two different
methods of deployment were used depending upon the type of work conducted
at a station (See Taylor and Bascuñán, 2000). Whenever a plankton haul
was done, the Profiler was placed above the bongo nets (sensors facing
up), and a double oblique tow was made. Upcast data are used as the
primary data when the Profiler is deployed with bongo nets. The turbulence
generated by the bongo nets during the downcast adversely affects both
the temperature and conductivity data quality. If no plankton haul was
done, the Profiler was deployed vertically (sensors facing down) through
the water column and the downcasts are processed as the primary data. Salinity
samples are taken from the bottom of a vertical profile cast, generally
twice per day, in order to determine a salinity correction or offset
for the data. These samples are analyzed on shore using a Guildline
Autosal Salinometer maintained at the NEFSC Narragansett laboratory.
On two ECOMON cruises, DEL0305 and ARM0301, a fluorometer was used
on the CTD instrument. No special processing of the data was performed
and only the raw fluorometer voltage values are presented.
During the deep-water systematics cruise, DEL0304, hydrographic data
were collected using an Applied Microsystems CTD 12+ that was placed
in a protective tube and attached to the trawl net. These data were
collected as part of an ongoing instrument evaluation conducted by the
Oceanography Branch with the goal of being able to deploy a CTD instrument
from a non-traditional platform (i.e. on fishing trawl nets). There
was very little quality control of these data, other than checking for
water column stability, since it was not possible to take salinity samples. The
project description, cruise notes, and processed data may be downloaded
from: ftp://ftp.wh.whoi.edu/pub/hydro/cruise_rpts/2003/del0304/DEL0304_ctd.html.
All raw Profiler data were processed using the Seabird software: DATCNV,
FILTER, ALIGNCTD, BINAVG, DERIVE, and ASCIIOUT to produce 1 decibar averaged
ASCII files. The data were quality controlled and converted to a standard
80-column ASCII formatted cruise file and were archived in ORACLE tables
and in the NEFSC anonymous FTP account (ftp://ftp.wh.whoi.edu/pub/hydro).
Station distributions and horizontal contour plots of the surface and
bottom temperature, salinity, and temperature anomaly were prepared for
each survey if coverage was sufficient. In addition, all the hydrographic
data were combined and sorted into 2-month time bins. Areal average
temperatures and salinities were then calculated for the six time periods
and for the five regions of the northeast continental shelf shown in
Figure 1a: western and eastern Gulf of Maine (GOMW,
GOME), Georges Bank (GB), and the northern and southern Middle Atlantic
Bight (MABN, MABS). Station
distributions for each time period are shown in Figure
1b. Anomalies
for the temperature and salinity observations were determined relative
to reference values, using the method described by Holzwarth and Mountain
(1990) as modified by Mountain et al. (2004). The areal averaging was
also done using the method described in Holzwarth and Mountain (1990)
as modified by Mountain et al. (2004). The areal averages and anomalies
were plotted against the calendar day mid-date of all observations within
each of the six time periods. Areal averages and anomalies were also
calculated by cruise and are listed in Tables C1 and C2 of Appendix
C.
RESULTS
The NEFSC cruises that are included in this report are listed in Table
1. A summary of each cruise is described in Appendix
A and includes
information on the type of cruise, its objectives, dates, the number
of hydrographic stations, type(s) of instruments used, salinity calibration
value, and notes pertaining to instrument performance. No salinity
correction was applied to the cruise data if the mean salinity offset
was less than +/- 0.01 psu.
Table 2 lists
the surface and bottom areal average temperatures and temperature anomalies
that were calculated for each of the five regions. Table
3 lists the surface and bottom areal average salinity and salinity
anomalies for the same five regions. For most cruises, the areal averages
and anomalies could not be calculated for all regions due to limited
station
coverage. Combining all the hydrographic data from all NEFSC programs
and ships provided a better chance of adequate spatial and temporal coverage
within the regions of the northeast continental shelf. When there was
insufficient spatial coverage (see Holzwarth and Mountain, 1990), a simple
average (not an areal weighted mean) was determined for the observations
in the region; these values are indicated in Tables 2 and 3 by an asterisk. The
standard deviations are also listed. SDV1 indicates how well the calculated
anomaly represents the true regional average anomaly. SDV2 is an indicator
of how closely the areal average matches the anomaly at any particular
location within that region (see Holzwarth and Mountain, 1990 for further
explanation of SDV1 and SDV2).
Figures 2 - 3 present
the time series of surface and bottom average temperature/salinity and
temperature/salinity anomaly
for each region. Cruises
having less than 10 observations were not included in the time series
figures. We were not able to resolve small-scale, localized events because
of the regional averaging method used in this report. Station positions
and distributions of surface and bottom temperature, salinity, and anomalies
for the different cruises are presented in Figures 4
- 44. Contour distribution
figures were not prepared for some of the cruises because of poor station
coverage. In addition, contour levels were selected to highlight variability
within a cruise and therefore the contour levels used may vary between
cruises. Contour distributions have been routinely produced for the
scallop survey although the station coverage for this survey does not
provide sufficient spatial coverage to allow one to produce realistic
broad-scale hydrographic distributions of the MAB and Georges Bank regions. Environmental
time series plots from shipboard sensors (SCS data) are included in Appendix
B. Further information about this data may be obtained at: http://www.wh.whoi.edu/~jmanning/foi/alongtrack.html.
DISCUSSION
The temperature anomaly time series (Figure 2) indicate that much of
the northeast continental shelf experienced colder surface conditions
for much of the first half of 2003 compared to the reference values. An
exception to this occurred in the southern MAB with surface and bottom
temperatures at the beginning of the year being slightly warmer than
the reference. Salinities in the Gulf of Maine and Georges Bank were
similar to the reference period. In the southern MAB surface salinity
anomalies were almost 1 ppt fresher than they were in 2002.
REFERENCES
Holzwarth, T.J. and D. Mountain. 1990. Surface and bottom temperature
distributions from the Northeast Fisheries Center spring and fall bottom
trawl survey program, 1963-1987. Woods Hole, MA: Northeast Fisheries
Center Reference Document 90-03. Available from: National Marine Fisheries
Service, 166 Water St., Woods Hole, MA 02543.
Manning, J.P. (2001). NEFSC Scientific Computer System (SCS) Alongtrack
Data Processing. http://www.wh.whoi.edu/~jmanning/foi/alongtrack.html (10 Dec 2001).
Mountain, D.G., M.H. Taylor and C. Bascuñán. 2004. Revised Procedures
for Calculating Regional Average Water Properties for Northeast Fisheries
Science Center Cruises. Woods Hole, MA: Northeast Fisheries Center Reference
Document 04-08. Available from: National Marine Fisheries Service,
166 Water St., Woods Hole, MA 02543.
Taylor, M. H. and Bascuñán, C. 2000. CTD Data Collection on Northeast
Fisheries Science Center Cruises: Standard Operating Procedures. Northeast
Fisheries Science Center Reference Doc. 00-11; 28 p. Available
from: National Marine Fisheries Service, 166 Water St., Woods Hole, MA
02543.
