Northeast Fisheries Science Center Reference Document 03-05
Description of the 2002 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 March 2003;
web version posted April 8, 2003
Citation: Taylor, M.H.; Bascuñán, C.; Manning, J.P. 2003. Description of the 2002 oceanographic conditions on the
Northeast Continental Shelf. Northeast Fish. Sci. Cent. Ref. Doc. 03-05; 100 p.
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Abstract
A summary of hydrographic observations for 12 surveys on the northeast
continental shelf during 2002 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 2002 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 during the majority of the year the northeast continental shelf
experienced warmer temperatures relative to the MARMAP reference period. But
temperatures were only slightly above expected values by November with
the exception of the northern MAB where Gulf Stream rings were located
at the shelf edge. Surface salinity observations in the eastern GOM
and Georges Bank were fresher than the expected conditions during both
the winter and spring although higher than expected salinity values
were observed by fall. The positive salinity anomalies in the GOM
and Georges Bank persisted through the end of the field season. The
pattern of increasing salinity anomalies observed in the GOM regions
may indicate a change in the contribution of slope water flowing into
the GOM through northeast channel. Both surface and bottom salinity
values were relatively saltier during most of the year in the southern
MAB region. The latter observation is most likely associated with
the shoreward movement of the shelf/slope front.
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 12 NEFSC surveys conducted
during 2002 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 (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 calibrate the conductivity data. These samples
are analyzed on shore using a Guildline Autosal Salinometer maintained
at the NEFSC Narragansett Laboratory.
During the deep-water systematic cruise, DEL0206, 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 a trial instrument evaluation that was 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 controlling 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/del0206/DEL0206_ctd.html.
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 (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. 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 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. 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 further explanation of SDV1 and SDV2).
Figures 2 and 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 4-57. 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 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 warmer
surface and bottom temperature conditions during 2002 compared to the
MARMAP reference values. The highest surface and bottom temperature
anomalies (> 3° C) occurred during the January - April time periods
in the southern MAB (see Figure 23). The positive
temperature anomalies in this region gradually subsided over the course
of the year with the only exception occurring during the June ECOMON
survey (ALB0206) when slightly below expected surface temperatures were
observed. The cooler surface temperatures during June may have been
caused by the advection of relatively cooler shelf water from the north
but it is beyond the scope of this report to investigate this in any
detail. By November the northeast continental shelf, with the exception
of the northern MAB, was at expected or only slightly above expected
temperature conditions. Two separate warm-core Gulf Stream rings were
observed in COASTWATCH satellite imagery during October and early November
in the northern MAB region and most likely contributed to the persistence
of the relatively warmer temperature conditions and the increase in the
salinity anomaly. Similarly, the Georges Bank region was most likely
influenced by the passage of the above-mentioned Gulf Stream rings during
the late summer and early fall.
Both regions of the Gulf of Maine and Georges Bank experienced fresher
surface conditions during the first half of the year. However, the salinity
anomaly time series shown in Figure 3 displays a
pattern beginning in early summer of relatively high surface and bottom
salinity values that persisted until the end of the field season in November. It
is possible that the higher salinities resulted from either a decrease
in the overall contribution of Scotian Shelf water to the GOM, or the
inflow of relatively warm, salty Slope water through Northeast Channel
increased during the latter part of 2002.
The Northeast Regional Climate Center (NRCC, Cornell University) compiles
seasonal and yearly summaries of mean air temperature and precipitation
using 108 years of compiled historical data. Seasons and years are assigned
a rank according to their mean air temperatures (1= coolest, 108 = warmest)
and total precipitation (1 = driest, 108= wettest). The northeast region
(Maine to Virginia) ranked the highest,“108”, in average air temperature
and “8” in total precipitation during the winter of 2002. The fall of
2002 ranked “62” in air temperature and “97” in total precipitation. The
above average air temperatures are consistent with the warmer sea surface
temperatures observed throughout much of the year on the northeast continental
shelf that subsided to near expected conditions by late Fall. Further
information about the NRCC and its data products may be obtained at: http://met-www.cit.cornell.edu/climate/Climate_summary.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.
