Northeast Fisheries Science Center Reference Document 02-08
Description
of the 2001 Oceanographic Conditions
on the Northeast Continental Shelf
by Maureen H. Taylor, Cristina Bascuñán, and James
P. Manning
National Marine Fisheries Serv., Woods Hole Lab., 166 Water St., Woods Hole, MA 02543
Print
publication date May 2002;
web version posted May 10, 2002
Citation: Taylor, M.H.; Bascuñán, C.; Manning, J.P. 2002. Description of the 2001 oceanographic conditions on the
Northeast Continental Shelf. Northeast Fish. Sci. Cent. Ref. Doc. 02-08; 101 p.
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Abstract
A summary of hydrographic observations for 15 surveys on the northeast
continental shelf during 2001 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 2001 were sorted
into six 2-month time bins to provide the best spatial coverage used
in the averaging method. Review of the computed areal average temperature
and salinity data indicate that much of the northeast continental shelf
experienced both warmer and fresher conditions during the year relative
to the MARMAP reference period. Exceptions to this trend were observed
during the spring in the eastern Gulf of Maine which exhibited colder
surface temperatures and during the fall in the southern Mid-Atlantic
Bight where higher than expected salinities were observed. The latter
event is believed to be associated with the shoreward movement of the
shelf/slope front in that region.
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 15 NEFSC surveys conducted
during 2001 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. The
data are presented chronologically in atlas form. Environmental data
from the SCS system (Shipboard Computing System) are presented as time
series figures for each leg of a cruise. No attempt has been made
here to 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 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 calibrate the conductivity
data. These samples are analyzed on shore with a Guildline Autosal
Salinometer.
All raw Profiler data were processed using the Seabird manufactured
software: DATCNV, FILTER, ALIGNCTD, BINAVG, DERIVE, and ASCIIOUT to
produce 1 decibar averaged ASCII files. The data were edited, cleaned,
and converted to a standard 80-column ASCII formatted cruise file and
were archived in ORACLE tables and in the NEFSC anonymous FTP account
(whsun2:/ftp/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. The areal averaging was done using the method described in
Holzwarth and Mountain (1990). 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 4 and 5 of Appendix
C.
Results
The NEFSC cruises for which data are presented in this report are
listed in Table 1. A summary of each cruise
is listed 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. The January Ecosystem Monitoring cruise
was cancelled due to an extended repair period for the Albatross IV
and this resulted in no hydrographic data collected in the Gulf of
Maine during January-February 2001. 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. In
some cases however, 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 explanation of SDV1 and SDV2).
Figure 2 and Figure 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 4-56 . Contour distribution figures were not prepared for some
of the cruises because of poor station coverage. In addition, contour
levels are not always consistent for a variable within a cruise. 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 hydrographic distributions. 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 majority of the northeast continental shelf experienced warmer
and fresher conditions during the year 2001 compared to the MARMAP
reference values. However, the eastern Gulf of Maine experienced colder
surface temperatures, but near-normal bottom temperatures, during the
spring. During July – August, a relatively greater amount of low salinity
Scotian Shelf water was observed in the eastern Gulf of Maine and on
Georges Bank (see figure 38). This coincided with colder bottom temperatures
in both of these regions. It is possible that the presence of the
lower salinity water created a relatively high degree of density stratification
which inhibited vertical mixing of the warmer surface waters with the
cooler and saltier bottom water. Both regions of the Mid-Atlantic
Bight showed a pattern of increasing (positive) temperature anomalies
that subsided somewhat in November.
The salinity anomaly time series for both regions of the Gulf of Maine
indicate that the fresher conditions persisted during most of the year. The
lower salinities in the eastern Gulf of Maine were previously noted
during the fall of 2000 (Taylor and Bascuñán 2001). However, the
mid-November 2001 areal values are consistent with near-normal salinity
conditions in the Gulf of Maine regions. Examination of the salinity
data from Georges Bank reveals that the Bank experienced a gradual
freshening during much of the year but the salinity increased during
the September-October time period. The increase in salinity and the
positive salinity anomalies observed in the southern Mid-Atlantic Bight
during the fall may be associated with shoreward movement of the shelf/slope
front or the passage of Gulf Stream rings.
The Northeast Regional Climate Center (NRCC, Cornell University) compiles
seasonal and yearly summaries of mean air temperature and precipitation
using 107 years of compiled historical data. Seasons and years are assigned
a rank according to their mean air temperatures (1= coolest, 107 = warmest)
and total precipitation (1 = driest, 107= wettest). The northeast region
(Maine to Virginia) ranked "97" in average air temperature
and "5" in total precipitation. The above average air temperatures
are consistent with the warmer sea surface temperatures observed throughout
the year and over much of the northeast continental shelf. Further information
about the NRCC and its data products may be obtained at: http://met-www.cit.cornell.edu/nrcc_home.html
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: Information Services Section,
NMFS/Northeast Fisheries Science Center, 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).
Northeast Regional Climate Center, Cornell University. Seasonal Climate
Summary Tables. http://met-www.cit.cornell.edu/nrcc_home.html (13
Feb 2002).
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.
Taylor, M. H. and Bascuñán, C. 2001. Description of the 2000 Oceanographic
Conditions on the Northeast Continental Shelf. Northeast Fisheries
Science Center Reference Doc. 01-01; 93 p. Available from: National
Marine Fisheries Service, 166 Water St., Woods Hole, MA 02543.
