CONTENTS Abstract Introduction Data and Methods Results Discussion References
Northeast Fisheries Science Center Reference Document 04-12
Cristina Bascuñán, Maureen H. Taylor, and David G. Mountain
Description of the 2003 oceanographic conditions
on the Northeast continental shelf
National Marine Fisheries Serv., Woods Hole Lab., 166 Water St., Woods Hole, MA 02543
Web version posted November 26, 2004Citation: 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.
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.
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).
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 - 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.