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CONTENTS
Introduction
Data and Methods
Results
Discussion
References

Northeast Fisheries Science Center Reference Document 09-12

Description of the 2008 Oceanographic Conditions on the Northeast U.S. Continental Shelf

Maureen H. Taylor, Tamara Holzwarth-Davis, Cristina Bascuñán, and James P. Manning
NOAA Fisheries, Northeast Fisheries Science Center, 166 Water Street, Woods Hole, MA 02543

Web version posted August 14, 2009

Citation: Taylor MH, Holzwarth-Davis T, Bascuñán C, Manning JP. 2009. Description of the 2008 oceanographic conditions on the Northeast U.S. Continental Shelf. US Dept Commer, Northeast Fish Sci Cent Ref Doc. 09-12; 28 p. Available from: National Marine Fisheries Service, 166 Water Street, Woods Hole, MA 02543-1026.

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.

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Abstract

Hydrographic observations from 17 surveys that were conducted on the northeast continental shelf during 2008 are summarized.   Contoured distributions of temperature, salinity, density stratification and anomalies are presented for six two-month time periods if coverage was sufficient.  Average temperature/salinity values were calculated in five geographic regions over the northeast continental shelf:  western Gulf of Maine (GOMW), eastern Gulf of Maine (GOME), Georges Bank (GBNK), northern Mid Atlantic Bight (MABN) and southern Mid Atlantic Bight (MABS). 

Review of the computed regional average temperature data indicate that surface temperatures were predominately warmer than the reference values in all regions.  The eastern Gulf of Maine exhibited slightly colder than expected bottom temperatures for all time periods for which data were available.   Regional average salinities were mostly lower, but the largest (negative) anomalies were observed in the Mid-Atlantic Bight.  There were localized areas of positive salinities anomalies observed during the year, but these tended to be associated with warm core rings or the variable position of the shelf/slope front.  Warmer sea surface temperatures and lower salinities contributed to a higher degree of density stratification along the outer shelf region of the Mid Atlantic Bight during the 2008 summer.  The volume of shelf water in the Mid Atlantic regions was high indicating that the shelf / slope front was located further offshore.  The northeast climate experienced higher than normal precipitation and slightly warmer air temperatures during 2008.  


Introduction

The Northeast Fisheries Science Center (NEFSC) conducts multiple surveys off the northeast U.S. continental shelf each year in support of ecosystem research and management.  Broad-scale sampling of the shelf (Cape Hatteras to the Gulf of Maine) occurs during the spring and fall bottom trawl surveys and ideally during four dedicated Ecosystem Monitoring (EcoMon) surveys.  Coverage on other surveys varies depending on the specific objectives of the field program.   During 2008 the Oceanography Branch supported 17 surveys representing 8 different NEFSC field programs.  Surface and bottom temperature, salinity and anomaly distributions were displayed as plan-view contour plots where sufficient data were available.  Estimates of the density stratification of the upper 50m of the water column were included.  Regional average temperature and salinity and the corresponding anomalies were computed for the five different regions of the shelf shown in Figure 1 and for six time periods throughout the year.  The volume of shelf water observed in the upper 100 meters in the Mid Atlantic Bight regions was calculated.   The regional values are summarized in both tables and time series figures.


Data and Methods

The Oceanography Branch provides CTD (Conductivity, Temperature, and Depth) recording instruments to the NEFSC programs that request operational support (Taylor and Bascuñan, 2000).  Training was provided prior to sailing in CTD deployment, maintenance, data acquisition and preliminary post processing procedures.  All CTD data were processed on shore using Seabird Electronics software to produce 1-decibar averaged ASCII files.  Salinity samples, typically taken twice daily at sea, were analyzed using a Guildline Autosal (salinometer) located at the Narragansett Laboratory.   A salinity offset was applied to instrument data if the mean difference between the reference Autosal readings and the CTD values exceeded +/- 0.01.   The data were quality controlled and converted to a standard 80-column ASCII formatted cruise file.  These files were then loaded into ORACLE database tables and also posted in the NEFSC anonymous FTP account (ftp://ftp.nefsc.noaa.gov/pub/hydro).   Hydrographic cruise reports were prepared for each survey and include cruise notes, a station distribution map, plan-view contour plots of the surface and bottom temperature, salinity, and anomalies (if station coverage was sufficient), and a table of regional average values and anomalies.  CTD profile data can be retrieved from http://www.nefsc.noaa.gov/epd/ocean/MainPage/.

For this report, the processed 2008 CTD data were combined and sorted into two-month time bins.  Regional average temperature and salinity were then calculated for the six time periods and for the five regions of the northeast continental shelf.  Both the individual station and the regional temperature and salinity anomalies were computed using derived reference annual cycles that were updated by Mountain et al. (2004).   Regional averages and anomalies were plotted against the calendar mid-date of all observations within each of the six time periods, if there were at least 10 observations.  Regional values were also calculated for each individual cruise.  The density stratification was computed using the density (sigma-t) difference between 50m and the surface value.  If the station bottom depth was less than 50m, the difference was taken between the deepest observation and the surface.  Annual cycles for density stratification (0-50m) were derived for each standard station of the MARMAP data set (Marine Resource Monitoring and Assessment Program, Sherman, 1980).  Stratification Anomalies were then computed for each station using the same method that was used for temperature and salinity (Mountain and Holzwarth, 1990).  An observation is considered to be a “surface” value if it comes from the upper 5m of the water column while “bottom” observations must come within 10m of the station bottom depth.  The anomaly value is the difference between the observed and the expected value for the particular station location and time of year.

The volume of shelf water for the upper 100 meters of the Mid Atlantic Bight regions is included in this report as an additional year-end product that will be included in future hydrographic atlas summaries.  The shelf water volume calculations were made using the method described in Mountain (2004).  Shelf water was defined as water having a salinity value < 34.  The MARMAP hydrographic data were used as the reference for derived shelf water volume anomalies.

The data are presented chronologically in atlas form.  The purpose of this document is to provide a descriptive overview of the hydrographic sampling that was conducted throughout the year and to depict the broad-scale oceanographic conditions that were observed.  The atlas summary is intended to be a foundation for further research into observed hydrographic variability and its influence on the marine populations of the northeast continental shelf.


Results

The listing of NEFSC cruises that took place during 2008 is provided in Table 1.  A total of 1831 CTD casts were collected, processed and archived during the year.  Individual cruise reports can be viewed and downloaded from: http://www.nefsc.noaa.gov/HydroAtlas/.  Station distributions for each two-month time period are shown in Figure 2.  Full shelf coverage was poor during the May-June and November-December time periods.  The spring EcoMon cruise was reduced from 16 to 4 sea-days and resulted in only minimal sampling and gear testing within the northern Mid Atlantic bight region (HB0804).  Regional average surface and bottom temperature and salinity values are summarized in Table 2 and Table 3 and are portrayed in Figure 3 and Figure 4.  Combining all of the hydrographic data from all cruises improved the spatial and temporal coverage within the regions and throughout the year.  In some cases however, a simple average (not an area weighted mean) was determined because coverage within the area remained poor even after combining all observations.  These values are indicated in the tables with a flag value of “1”.   The standard deviations are also listed (see Holzwarth and Mountain, 1990 for further explanation of SDV1 and SDV2).  Tables 1-5 of the Appendix (linked right) contain the regional temperature and salinity values computed for each individual cruise that are also included in the cruise reports.  Regional values computed from less than 10 observations for a given area were not included in Figure 3 and Figure 4.  Distributions of surface and bottom temperature, salinity, density stratification and anomalies are presented in Figures 5–16 (linked right).  Contour levels and color scaling for temperature and salinity are the same for all figures so that it would be easier to see the annual cycles.   The scaling was changed for the water column stability of the upper 50 meters.  No attempt was made to contour the hydrographic distributions for the May-June or November-December time periods.  Shelf water volume and temperature/salinity characteristics for the upper 100 meters of the MAB regions are listed in Table 6 of the Appendix.

Shipboard environmental sensor data from the Albatross IV, Delaware II, and Henry Bigelow’s Scientific Computing System (SCS) were documented and are available for download at: http://www.nefsc.noaa.gov/epd/ocean/MainPage/AlongTrack.html.  The Albatross IV conducted its last survey in the late fall of 2008 and had successfully collected 22,924 hydrographic profiles since 1978.


Discussion

During the January – February time period, relatively low surface and bottom salinities were observed in the two regions of the Mid Atlantic Bight (MAB).  Temperatures were also cooler along the shelf break extending approximately from 39 – 40.5°N.  The shelf water volume anomaly was highest in the southern MAB and suggested that the shelf / slope front was located further offshore than what would be typical for this time period.  A warm core ring was observed in satellite imagery impinging on the shelf in the vicinity of Great South Channel and along southeast Georges Bank and resulted in an area of relatively warm and saltier water.  Conditions in the Gulf of Maine (GOM) during January – February were slightly cooler (~0.5°C) and slightly fresher (~0.2) than the expected conditions.  Density stratification (0-50m) was minimal during this time period, although some evidence of higher stratification was observed at the mouth of the New York Bight area, and was associated with lower salinities.

The hydrographic data collected during the spring bottom trawl survey is captured in the March – April time period.  Conditions remained relatively fresh and temperatures ranged between 1-2°C warmer in the MAB regions relative to the MARMAP reference values.  Surface temperatures were also relatively warm in the western GOM.  Observations suggest that bottom temperatures in the eastern GOM were slightly cooler than expected, while both temperature and salinities were near normal on Georges Bank.  Density stratification remained fairly low during the late winter and early spring, although some localized areas of higher stability were observed in the western GOM and off the New Jersey coast.  The reduced sampling during the May-June period impacted our ability to describe the onset and development of seasonal stratification as well as the latitudinal and cross shelf hydrographic gradients for the late spring.

The “cold band” of remnant winter water can be seen in the bottom temperature distribution during July – August (Figure 11) in the northern Mid Atlantic Bight and surface salinities remained fairly low there.  Regional surface temperatures were warm shelf wide.  The co-occurrence of lower salinities and warmer surface temperatures resulted in an area of higher than expected density stratification extending from the mid-outer shelf south of Martha’s Vineyard southwestward to just south of Hudson Shelf Canyon.  Satellite imagery during July and August indicated numerous Gulf Stream meanders, streamers and at least two warm core rings.  This likely contributed to the higher sea surface temperatures along the southern flank of Georges Bank.  Warm core ring and meander activity continued to influence the outer shelf areas during the fall.   The surface salinity anomaly distribution of Figure 15 shows two areas of higher surface salinity values that are associated with warm core rings; one located south of Great South Channel and another at the shelf break east of Delaware Bay.  Regional surface salinities were near expected values and with the exception of the eastern GOM, surface temperatures remained relatively warm during the latter part of 2008.  Density stratification had mostly broken down in the eastern GOM, Georges Bank, and the shelf area south of Block Island.  The highest degree of stratification was observed along the mid - outer shelf from Hudson Shelf Canyon to an outer shelf area east of Delaware Bay and was associated with warmer surface temperatures.  Another area of relatively high stratification was observed over Wilkinson Basin and was likely caused by the lower salinities observed there.

The Northeast Regional Climate Center (http://www.nrcc.cornell.edu) reports that the annual average northeast air temperatures were ~0.4°C warmer than their 1971-2001 baseline period.  Precipitation was 6” above normal, while the states of Maine, Massachusetts, New Hampshire and Connecticut had over 12” above average rainfall.  Out of 114 years of observations, 2008 ranked 111 in annual precipitation (114 being the wettest year).  It is likely that the lower than expected shelf water salinities observed in the MAB during 2008 (Appendix Table 6) were due in part to the above average rainfall in the New England states.  However, the relatively high volume of shelf water observed throughout the year in the northern MAB, and for most of the year in the southern MAB was mainly due to a further offshore position of the shelf / slope front.


References

Holzwarth TJ, Mountain DG. 1990.  Surface and bottom temperature distributions from the Northeast Fisheries Center spring and fall bottom trawl survey program, 1963-1987.  Northeast Fish Cent Ref Doc 90-03.  Available from: NOAA Fisheries, 166 Water St., Woods Hole MA 02543.

Mountain DG.  2004. Variability of the water properties in NAFO Subareas 5 and 6 during the 1990s.  J Northw Atl Fish Sci. 34:103-112.

Mountain DG, Holzwarth TJ. 1990.  Surface and bottom temperature distribution for the Northeast Continental Shelf.  NOAA Technical Memorandum. NMFS-F/NEC-73; 55 p. Available from: NOAA Fisheries, 166 Water St., Woods Hole MA 02543.

Mountain DG, Taylor MH, Bascuñán C.  2004.  Revised procedures for calculating regional average water properties for Northeast Fisheries Science Center cruises.  Northeast Fish Sci Cent Ref Doc. 04-08; 53 p.

Northeast Fisheries Science Center Scientific Computer System (SCS) Alongtrack Data Processing. Available at: http://www.nefsc.noaa.gov/epd/ocean/MainPage/AlongTrack.html (accessed 15 June 2009).

Northeast Regional Climate Center, Cornell University.  Seasonal Climate Summary tables. Available at: http://www.nrcc.cornell.edu/ (accessed 20 June 2009).

Sherman K. 1980.  MARMAP, a fisheries ecosystem study in the NW Atlantic: fluctuations in ichthyoplankton-zooplankton components and their potential for impact on the system.  In: Diemer FP, Vernberg FJ, Mirkes DZ, eds. Advance concepts in ocean measurements for marine biology.  Belle W. Baruch Institute for Marine Biology and Coastal Research, Univ. South Carolina Press, Berkeley; p. 9-37.

Taylor MH, Bascuñán C.  2000.  CTD data collection on Northeast Fisheries Science Center cruises: standard operating procedures.  Northeast Fish Sci Cent Ref Doc. 00-11; 28 p. Available from: NOAA Fisheries, 166 Water St., Woods Hole MA 02543.

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