CONTENTS Introduction Data and Methods Results Discussion References
Northeast Fisheries Science Center Reference Document 03-05
Description of the 2002 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 April 8, 2003Citation: 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.
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 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.
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.
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.
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
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.