Northeast Fisheries Science Center Reference Document 02-08
Description of the 2001 Oceanographic Conditions on the Northeast Continental ShelfMaureen H. Taylor, Cristina Bascuñán, and James P. Manning
National Marine Fisheries Serv., Woods Hole Lab., 166 Water St., Woods Hole, MA 02543
Web version posted May 10, 2002Citation: 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.
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 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.
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.
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 Table 2 and Table 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 4-54. 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.
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
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.