NEFSC Reference Document 12-16 Description of the 2010 Oceanographic Conditions on the Northeast U.S. Continental Shelf

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Table of Contents

Abstract

Introduction

Data and Methods

Results

References Cited

Appendix

Northeast Fisheries Science Center Reference Document 12-16

Description of the 2010 Oceanographic Conditions on the Northeast U.S. Continental Shelf
by Paula S. Fratantoni, Tamara Holzwarth-Davis, Cristina Bascuñán, and Maureen H. Taylor
National Oceanic Atmospheric Administration, National Marine Fisheries Service, Northeast Fisheries Science Center, 166 Water Street, Woods Hole, MA, 02543 USA

Web version posted September 4, 2012
Citation: Fratantoni PS, Holzwarth-Davis T, Bascuñán C, Taylor MH. 2012. Description of the 2010 Oceanographic Conditions on the Northeast U.S. Continental Shelf. US Dept Commer, Northeast Fish Sci Cent Ref Doc. 12-16; 32 p. Available from: National Marine Fisheries Service, 166 Water Street, Woods Hole, MA 02543-1026, or online at http://www.nefsc.noaa.gov/nefsc/publications/

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 ten surveys spanning the Northeast U.S. Continental Shelf are combined into a descriptive overview of the broad-scale oceanographic conditions that were observed during 2010. Temperature and salinity observations are combined into six 2-month time periods in order to maximize both the spatial coverage of the data and its temporal resolution during the year. Maps of near-surface and near-bottom property distributions are presented for each bi-monthly period and time series of regional average properties are discussed for five geographic regions spanning the shelf: western Gulf of Maine (GOMW), eastern Gulf of Maine (GOME), Georges Bank (GBNK), and northern and southern Middle-Atlantic Bight (MABN and MABS, respectively). Surface conditions along the entire Northeast U.S. Continental Shelf were generally warm and fresh in 2010 relative to the reference period (1977-1987). Warming was largest in the spring and early summer, penetrating to the bottom throughout the region. The annual average air temperature over the Northeastern U.S. was warmer in 2010 than historical averages, with the greatest warming occurring during spring and summer. This may be responsible for the anomalously warm surface ocean temperatures at this time of year and for the enhanced warming that followed at depth in the near shore regions of the Middle Atlantic Bight. Freshening was greatest in the surface waters near the coast in the western Gulf of Maine and southern MAB during periods of maximum spring discharge, suggesting some influence from local fresh water sources in these regions. The surface freshening observed over Georges Bank and throughout the MAB penetrates to the bottom, while bottom waters in the Northeast Channel and deep basins of the GoM were saltier than normal reflective of the slope water influence in these regions.

Introduction

The Northeast Fisheries Science Center (NEFSC) conducts multiple surveys on the Northeast U.S. continental shelf each year in support of its ongoing mission to monitor the shelf ecosystem and assess how its components influence the distribution, abundance, and productivity of living marine resources. In support of this mission, the Oceanography Branch provides conductivity, temperature and depth (CTD) instruments to all NEFSC cruises for the measurement of water column profiles of temperature and salinity. In addition to providing oceanographic context to specific field programs, these data contribute to a growing database of historical measurements that are used to monitor seasonal and interannual variability in the water properties on the northeast continental shelf.

Broad-scale surveys, sampling the shelf from Cape Hatteras, North Carolina, into the Gulf of Maine, are conducted up to six times per year during shelf-wide spring and fall bottom trawl surveys and typically on four dedicated seasonal Ecosystem Monitoring (EcoMon) surveys. Profiles of conductivity, temperature and depth are collected at each station on these shelf-wide surveys. Observations are also collected on other more regionally focused NEFSC surveys, where station coverage varies depending on the objectives of the particular field program. During 2010, hydrographic data were collected on 10 individual NEFSC cruises, amounting to 1,807 profiles of temperature and salinity (Table 1). Here we present an annual summary of these observations, including surface and bottom distributions of temperature and salinity, as well as their anomalies relative to a consistent reference period. In addition, regional average values of temperature and salinity and their anomalies are computed for five different regions of the shelf during six bi-monthly periods. Finally, the volume and properties of the shelf water are examined for the Middle-Atlantic Bight region.

Data and Methods

The Oceanography Branch provides CTD instrumentation and support to all NEFSC programs requesting this service. Training in instrument maintenance and operation, including deployment, data acquisition, recovery and preliminary processing, is provided as needed prior to sailing. On NEFSC surveys, CTD instruments are typically deployed in one of two modes: (1) during a bongo net tow, the CTD instrument is mounted on the conducting wire above the bongo frame and data are collected as a double oblique profile with the ship steaming at approximately 2 knots, (2) during a non-net tow, the CTD is mounted vertically on the wire and the sensors are soaked for one minute at the surface prior to descent. The sensors are not soaked at the surface prior to descent during bongo tows, rendering the upper 30 meters or more of the downcast unreliable. For this reason, the up-cast profile data are routinely processed as the primary data for each station.

In 2010, hydrographic data were collected aboard the NOAA ships Delaware II and HB Bigelow, and the R/V HR Sharp using a combination of Seabird Electronics SBE-19 and SBE-19+ SEACAT profilers and SBE 9/11 CTD units (Table 1). All raw CTD profile data were processed ashore, using standard Seabird Electronics software to produce 1-decibar averaged profiles in ascii-formatted files. Water samples were collected twice daily at sea during vertical casts. Following each cruise, these samples were analyzed using a Guildline AutoSal laboratory salinometer to provide quality control for the CTD salinity data. A salinity offset was applied to instrument data if the mean difference between the reference Autosal readings and the CTD values exceeded +/- 0.01 (a threshold chosen based on the expected instrument accuracy.) Vertical density profiles were examined for inversions due to bad conductivity or temperature readings and/or sensor misalignment. Egregious cases were replaced with a flag value. The processed hydrographic data were loaded into ORACLE database tables and made publically available via anonymous ftp (ftp://ftp.nefsc.noaa.gov/pub/hydro). Cruise reports have been prepared for each survey listed in Table 1 and are available online (http://www.nefsc.noaa.gov/epd/ocean/MainPage). Readers are referred to the individual cruise reports for notes, property maps and aggregate data specific to a particular survey.

Here, we aim to provide a descriptive overview of the hydrographic sampling that was conducted in 2010 and to characterize the broad-scale oceanographic conditions that were observed. In order to maximize both the spatial coverage of the data and its temporal resolution, the processed 2010 CTD data have been sorted into six 2-month time bins. Maps of near-surface and near-bottom temperature and salinity have been produced from profile data falling within each 2-month period. Surface fields include the shallowest observed temperature/salinity at each station that is also in the upper 5 meters of the water column, while bottom maps include the deepest observation at each station that also falls within 10 meters of the reported water depth. In order to examine the spatial and temporal variability over broader areas of the shelf, average values have been computed from the data within five sub-regions spanning the Northeast U.S. Continental Shelf (Figure 1). Regional averages have been computed for the bi-monthly binned fields (Table 1 and Table 2) and for individual cruises (Appendix Tables 1-5).

In order to characterize variability that is not related to seasonal forcing, anomalies have been calculated at each station relative to a standard reference period (1977-1987). During this period the NMFS Marine Resources Monitoring and Prediction (MARMAP) program repeatedly occupied stations spanning the entire Northeast U.S. Shelf so that an annual cycle could be constructed for water properties across all regions of the northeast shelf (Mountain et al., 2004; Mountain and Holzwarth, 1989). The anomalies presented here are defined as the difference between the observed 2010 value at individual stations and the expected value for each location and time of year based on this reference period. Similarly, regional anomalies are the area-weighted average of these anomalies within a given domain. The methods used and an explanation of uncertainties is presented in Holzwarth and Mountain (1990).

Finally, we calculate the temperature, salinity and volume of the shelf water in the Middle-Atlantic Bight during 2010 and relate this to the conditions observed during the MARMAP reference period. Following Mountain (2003), the shelf water mass is defined as water within the upper 100 meters having salinity less than 34. For each survey in 2010, the area of a sub-region was apportioned among its stations by an inverse distance squared weighting. The shelf water volume at a given station is the thickness of the shelf water at the station multiplied by its apportioned area, and the total shelf water volume within the sub region is the sum of these products for all stations within the region. Similarly, the average temperature and salinity was calculated in the shelf water layer at each station and multiplied by the total shelf water volume for that station. The sum of these products over all stations within a given sub-region, divided by the total shelf water volume for the region, determines the volume-weighted average temperature and salinity. Anomalies in the property and volume of the shelf water mass are calculated relative to like variables derived from MARMAP hydrographic data, as described above. Hence, here regional anomalies are computed as the mathematical difference between regional averages, not an average of the anomalies computed for a given sub-region.

Results

Table 1 provides a listing of the NEFSC cruises that collected hydrographic data in 2010. In total, 1807 profiles of temperature and salinity were collected, processed and archived during the year. Combining the hydrographic data from multiple cruises into bi-monthly bins improves the spatial coverage compared with that of individual surveys, enabling us to examine the spatial and temporal patterns in hydrography over the region. Nonetheless, there are still significant gaps in several of the bi-monthly distribution maps shown in Figure 2, particularly in the Middle Atlantic Bight during July/August and in the Gulf of Maine during January/February and September/October. These gaps result in part from a misalignment between the bi-monthly periods and the longer bottom trawl surveys that work from south to north along the shelf. For instance, the September/October period encompass all but the final third of the fall ground fish survey, when sampling was focused in the Gulf of Maine. In some cases poor weather slowed sampling operations and shortened surveys. The August EcoMon survey worked from north to south, opposite normal operations, to improve temporal coverage in the Gulf of Maine. However, several days were lost to poor weather and an emergency evacuation, leaving large areas of the Middle Atlantic Bight unsampled. Heavy weather during the February EcoMon survey prevented sampling in the eastern Gulf of Maine. These large gaps in station coverage preclude the calculation of a representative regional average surface/bottom temperature and salinity value during July/August in the southern MAB and during January/February in the eastern Gulf of Maine (Table 2 and Table 3, Figure 3 and Figure 4). While station coverage is better during all other periods, not all of the average properties reported are true area-weighted averages representative of the entire region. Those cases are flagged in Table 2 and Table 3, and the reader should keep this in mind when interpreting results.

Overall, surface conditions along the entire Northeast U.S. Continental Shelf were warm and fresh in 2010 relative to the reference period, except in the southern MAB where temperatures were near reference values (Figure 3 and Figure 4). Freshening was greatest in the surface waters of the western Gulf of Maine and southern MAB, suggesting some influence from local fresh water sources in these regions (Figure 4). Bottom waters were uniformly warm throughout the year in the Gulf of Maine and on Georges Bank (Figure 3). Weaker warming was observed in the northern MAB and bottom temperatures were near reference values in the southern MAB. Freshening was observed near the bottom on Georges Bank and in the Middle Atlantic Bight. Bottom waters in the western Gulf of Maine were fresher than normal early in the year, becoming saltier than normal later in the year. Surprisingly, bottom waters in the eastern Gulf of Maine were not significantly different than the reference salinity at any point in the year despite the fact that deep inflow to the Gulf of Maine was warmer and saltier in 2010, close to the upper limit of the historical range. Compared with the reference period, the shelf water in the MAB was up to 0.5 units fresher than was observed during the MARMAP period (shelf water is defined by salinities less than 34; Figure 5). The shelf water temperature fluctuated between warm and cold anomalies during 2010 but the volume was high relative to the reference period throughout the year. This suggests that the shelf/slope front was consistently located offshore of its position during the MARMAP period (Figure 5).

Details related to the temporal trends in Figure 3 and Figure 4 are explored in surface and bottom property distribution maps (Figures 6-11). Maps of surface temperature reveal the seasonal cycle of warming and cooling over the region, with warmest temperatures observed at the surface during late summer (Figure 6-11a). Even though regional averages indicate warming over most of the region relative to the MARMAP reference period, the details of this warming varies from region to region. Surface temperature anomaly maps in the Gulf of Maine tend to be uniformly warm, with the exception of January/February. By contrast, maps in the MAB are less uniform, characterized by a mixture of warmer and colder anomalies (Figures 6-11b). Maps of surface salinity show the seasonal influence of freshwater discharge in the nearshore regions, with the freshest waters appearing very near the coast in the eastern Gulf of Maine and MAB in March/April followed by more wide-spread freshening over these regions in May/June (Figure 7a and Figure 8a). While overall surface waters were fresher than normal in 2010, the largest anomalies were observed near the coast in the vicinity of major freshwater sources (e.g. western Gulf of Maine, Hudson River, Delaware and Chesapeake Bays) during periods of maximum spring discharge (March/April and May/June; Figure 7b and Figure 8b).

Maps of near bottom salinity and salinity anomaly suggest that the freshening that is observed at the surface penetrates to the bottom on Georges Bank and throughout the MAB (Figures 4 and 6-11). However, the same is not true in the Gulf of Maine where slope water dominates the lower layer properties. Near-bottom temperature and salinity anomalies in Northeast Channel and in the deep basins of the GoM suggest that the slope water was warmer and saltier from March through June during 2010 (Figure 7b and Figure 8b).

Maps of near-bottom temperature show the seasonal formation of the cold pool in the Middle Atlantic Bight, with coldest temperatures observed during the May/June period (Figure 8a). The accompanying maps of near-bottom temperature anomaly suggest that temperatures in the cold pool were colder than normal, particularly in the vicinity of the temperature minimum and along the shelf to the south (Figure 8b). By September/October the cold pool has begun to weaken and warm, becoming less coherent in the central MAB (Figure 10a). Localized cold anomalies scattered along the axis of the temperature minimum (Figure 10b) are indicative of interannual variations in the erosion of this feature. By November/December, the cold pool is completely eroded and near-bottom shelf temperatures in the southern MAB are warmer than normal (Figure 11a and Figure 11b).

The erosion of the cold pool in late summer is preceded by warming of bottom waters near the coast in September/October (Figure 10a). This is consistent with historical observations in this region, which indicate that by late summer the effects of seasonal heating extend all the way to the ocean bottom in the near-shore regions (Castelao et al., 2010). The anomaly fields shown in Figure 10b suggest that waters near the bottom were up to 2 degrees warmer in late-summer 2010 than during the reference period.

Based on the climate summaries compiled by the Northeast Regional Climate Center (http://www.nrcc.cornell.edu), the air temperature across the Northeastern U.S. was warmer than the long term mean throughout the year except December (Figure 12). The annual cycle of heating and cooling in 2010 was aligned with the long-term annual cycle but warming was enhanced during spring (up to 5°F warmer in March and April), remaining warm through late summer (September). This timing is coincident with the largest surface temperature anomalies on the Northeast U.S. Shelf (Figure 3). Overall, the annual mean air temperature across the Northeastern U.S. was almost 2°F warmer than long-term annual mean values.

According to the Northeast Regional Climate Center records, the annual mean precipitation over the Northeastern U.S. was near normal, although several months recorded significant departures from the long-term average (referenced to 1971-2000). Precipitation was almost 1.5" above normal in March and nearly 2.5" above normal in October 2010. The increase in March may account in part for the fresh surface conditions observed in the March/April time period, particularly in regions where freshwater discharge has a significant influence (e.g. western Gulf of Maine and southern MAB; Figure 4). However, a more thorough analysis would be needed to discriminate between local freshening due to precipitation and river run-off and freshening due to advection from remote sources. Anomaly maps suggest that a saltier variety of near-bottom waters may have been entering the eastern Gulf of Maine through Northeast Channel and along the southwestern Scotian Shelf during May/June (Figure 8b). These saltier source waters mix vertically with surface waters within the Gulf of Maine, including the near surface and inshore waters where freshening due to increased precipitation will be greatest.

References Cited

Castelao R, Glenn S, Schofield O. 2010. Temperature, salinity, and density variability in the central Middle Atlantic Bight. Journal of Geophysical Research, 115, C10005, doi:10.1029/2009JC006082.

Holzwarth TJ, Mountain DG. 1990. Surface and Bottom Temperature Distributions from the Northeast Fisheries Center Spring and Fall Bottom Trawl Survey Program, 1963-1987. NEFSC Reference Document 90-03, 62 pp.

Mountain DG, Holzwarth TJ. 1989. Surface and Bottom Temperature Distribution for the Northeast Continental Shelf. NOAA Technical Memorandum NMFS-F/NEC-73, 55 pp. Available from: NOAA Fisheries, 166 Water Street, Woods Hole, MA 02543.

Mountain DG. 2003. Variability in the properties of Shelf Water in the Middle Atlantic Bight, 1977-1999. Journal of Geophysical Research, 108, 3014, doi:10.1029/2001JC001044, (14-)1-11.

Mountain DG, Taylor MH, Bascuñán C. 2004. Revised procedures for calculating regional average water properties for Northeast Fisheries Science Center cruises. NEFSC Reference Document 04-08, 53pp.

Northeast Regional Climate Center. Seasonal Climate Summary Tables [Internet]. Cornell University. [cited 2011 March 31]. Available from: http://www.nrcc.cornell.edu/.