NOAA Bering Sea Bottom Trawl Survey CTD data in 2010, Forage Distribution and Ocean Conditions (B62)

Metadata:

• Identification_Information
• Data_Quality_Information
• Spatial_Data_Organization_Information
• Spatial_Reference_Information
• Distribution_Information
• Metadata_Reference_Information

Identification_Information:

Citation:

Citation_Information:

Originator: Cokelet, Edward D.

Publication_Date: Unpublished material

Title:

NOAA Bering Sea Bottom Trawl Survey CTD data in 2010, Forage Distribution and Ocean Conditions (B62)

Geospatial_Data_Presentation_Form: digital

Publication_Information:

Publication_Place: Seattle, WA

Publisher: NOAA/PMEL/EcoFOCI

Description:

Abstract:

One goal of BSIERP (Bering Sea Integrated Ecosystem Research Program) Project 62: Forage Distribution and Ocean Conditions was to add CTD measurements to the NOAA/AFSC bottom trawl survey to determine the composition, distribution and abundance of demersal fish, shellfish and epibenthic invertebrates each summer at over 350 sites on the southeastern Bering Sea continental shelf on a 37 x 37 km grid. We added rugged CTDs to the net hauls to obtain gridded data sets of temperature and salinity measurements for 2008 to 2010. The temperature and salinity measurements allowed us to compute the mass density field, its gradients, the mixed layer depth and the geostrophic circulation on a three-dimensional grid covering a large region of the shelf from the Alaska Peninsula to 66°N and from the 30-m to the 200-m depth contours. Maps show the survey age (Figure 3) and the temperature (Figure 5) and salinity (Figure 6) at 5 m and the bottom.

Purpose:

To add CTD measurements to the NOAA/AFSC bottom trawl survey cruises on the eastern Bering Sea continental shelf, 7 June-10 August 2010.

Time_Period_of_Content:

Time_Period_Information:

Range_of_Dates/Times:

Beginning_Date: 20100607

Ending_Date: 20100810

Currentness_Reference: observed

Status:

Progress: Complete

Maintenance_and_Update_Frequency: None planned

Spatial_Domain:

Description_of_Geographic_Extent: Eastern Bering Sea continental shelf

Bounding_Coordinates:

West_Bounding_Coordinate: -178.2

East_Bounding_Coordinate: -158.3

North_Bounding_Coordinate: 65.4

South_Bounding_Coordinate: 54.6

Keywords:

Theme:

Theme_Keyword_Thesaurus: None

Theme_Keyword: Cruise

Theme_Keyword: Date

Theme_Keyword: Cast

Theme_Keyword: Station Name

Theme_Keyword: Latitude

Theme_Keyword: Longitude

Theme_Keyword: Depth

Theme_Keyword: Water Temperature

Theme_Keyword: Salinity

Theme_Keyword: Sigma_t

Theme_Keyword: Dynamic Height Anomaly

Theme:

Theme_Keyword_Thesaurus: Core Variable Map

Theme_Keyword: Time:CTD_DATE_AX

Theme_Keyword: Pressure:P_AX

Theme_Keyword: Latitude:LAT

Theme_Keyword: Longitude:LON

Theme_Keyword: Water Temperature:TEMP

Theme_Keyword: Salinity:SAL

Theme_Keyword: Water Density Anomaly:SIGMA_T

Theme_Keyword: Dynamic Height Anomaly:DYN_HT_ANOM

Theme:

Theme_Keyword_Thesaurus: Variable Map

Theme_Keyword: CTD_DATE_AX:Time:time axis:DAYS since 2010-06-01 00:00:00

Theme_Keyword: P_AX:Pressure:pressure axis:DBAR above atmospheric pressure

Theme_Keyword: LAT:Latitude:GPS Latitude:degree_north

Theme_Keyword: LON:Longitude:GPS Longitude:degree_east

Theme_Keyword: TEMP:Water Temperature:CTD Temperature:degree_Celsius

Theme_Keyword: SAL:Salinity:CTD Salinity:psu

Theme_Keyword: Water Density Anomaly:SIGMA_T:kg/m**3

Theme_Keyword: Dynamic Height Anomaly:DYN_HT_ANOM:dyn m

Theme:

Theme_Keyword_Thesaurus: None

Theme_Keyword: project_number:B62

Theme_Keyword: data_url:<http://data.eol.ucar.edu/codiac/dss/id=245.B62-014>

Theme_Keyword: archive_url:<http://www.eol.ucar.edu/projects/bsierp>

Theme:

Theme_Keyword_Thesaurus: Dataset Map

Theme_Keyword: id=xxxxx

Place:

Place_Keyword_Thesaurus: Geographic Names Information System

Place_Keyword: Bering Sea

Temporal:

Temporal_Keyword_Thesaurus: None

Temporal_Keyword: 7 June-10 August 2010

Access_Constraints:

Access is restricted. Data policy: <http://www.eol.ucar.edu/projects/best/program_management_plan.pdf>

Use_Constraints:

Use is restricted. Data policy: <http://www.eol.ucar.edu/projects/best/program_management_plan.pdf>

Browse_Graphic:

Browse_Graphic_File_Name: Figure_1.jpg

Browse_Graphic_File_Description:

Figure 1: NXIC CTD with protective polypropylene case components and net bag.

Browse_Graphic_File_Type: JPG

Browse_Graphic:

Browse_Graphic_File_Name: Figure_2.png

Browse_Graphic_File_Description:

Figure 2: Photograph of an NXIC CTD mounted inside a 160-mm diameter polypropylene pipe and attached to the trawl net's headrope.

Browse_Graphic_File_Type: PNG

Browse_Graphic:

Browse_Graphic_File_Name: Figure_3.png

Browse_Graphic_File_Description:

Figure 3: Map of the 2010 bottom trawl survey age in days since inception. The black dots represent the CTD cast sites. The long-term EcoFOCI mooring sites (M2, M4, M5 and M8) and 70-m isobath section are shown in red. Depths are contoured at 30, 50, 100, 200, 500, 1000 and 2000 m.

Browse_Graphic_File_Type: PNG

Browse_Graphic:

Browse_Graphic_File_Name: Figure_4.png

Browse_Graphic_File_Description:

Figure 4: Time series of the underway (black lines) and CTD (red circles) measurements of temperature (top) and salinity (bottom) at 3 m on F/V Aldebaran, summer 2010.

Browse_Graphic_File_Type: PNG

Browse_Graphic:

Browse_Graphic_File_Name: Figure_5.png

Browse_Graphic_File_Description:

Figure 5: Map of the ocean temperature at 5 m depth (top) and the sea bottom (bottom) for the 2010 bottom trawl survey. The black dots represent the CTD cast sites. The long-term EcoFOCI mooring sites (M2, M4, M5 and M8) and 70-m isobath section are shown in red. Depths are contoured at 30, 50, 100, 200, 500, 1000 and 2000 m.

Browse_Graphic_File_Type: PNG

Browse_Graphic:

Browse_Graphic_File_Name: Figure_6.png

Browse_Graphic_File_Description:

Figure 6: Map of the salinity at 5 m depth (top) and the sea bottom (bottom) for the 2010 bottom trawl survey. The black dots represent the CTD cast sites. The long-term EcoFOCI mooring sites (M2, M4, M5 and M8) and 70-m isobath section are shown in red. Depths are contoured at 30, 50, 100, 200, 500, 1000 and 2000 m.

Browse_Graphic_File_Type: PNG

Analytical_Tool:

Analytical_Tool_Description:

The data file is a self-documenting NetCDF file that meets the COARDS NetCDF standards. It was generated by Ferret software.

Tool_Access_Information:

Tool_Access_Instructions:

<http://www.unidata.ucar.edu/software/netcdf/> <http://ferret.wrc.noaa.gov/Ferret/>

Data_Quality_Information:

Attribute_Accuracy:

Attribute_Accuracy_Report:

The CTD manufacturers specified temperature, conductivity and pressure accuracies of 0.005ºC, 0.009 mS/cm and 0.05% of full scale, respectively. These convert to salinity and pressure accuracies of 0.006 psu (PSS78) at 5ºC and 0.25 dbar (decibar =10,000 Pa ≈ 1 m of water depth) for the 500-m pressure transducers that the CTDs carried. These values are for CTDs factory-calibrated annually in constant temperature and salinity conditions, but the accuracy in a dynamic field environment is undoubtedly less. In addition to the CTDs, one ship, F/V Aldebaran, was equipped with an underway seawater sampling system consisting of a Sea-Bird Electronics SBE 38 digital oceanographic thermometer, an SBE 45 MicroTSG thermosalinograph to measure salinity, and a WETLabs ECO fluorometer with bio-wiper to measure chlorophyll fluorescence. Water was sampled every 60 s from the ship's sea chest, drawing water from approximately 3 m depth. Discrete salinity and chlorophyll samples were taken daily, analyzed in the laboratory after the field season and used to calibrate the salinity and chlorophyll fluorescence sensors. These temperature and salinity measurements were used as an independent check on the CTD measurements. Figure 4 shows time series of temperature and salinity from the underway system and the CTD measurements at 3 m depth for the 2010 bottom trawl survey. The CTD measurements track the near-surface variations very well with correlation coefficients, r2, for temperature and salinity over the three years exceeding 0.99 and 0.98, respectively. The CTDs were not recalibrated against the underway measurements because the turbulent wake of a ship moving in a stratified fluid is a poor calibration environment.

Logical_Consistency_Report:

Quality control was carried out. Plotted CTD profiles were scrutinized for non-physical density overturns. Usually the downcast was chosen to represent each CTD station, but each was compared graphically with its upcast and the latter was chosen if it had smaller spurious density inversions (heavy water above light water) owing to salinity spiking. The data values are within the expected range for these oceanographic variables.

Completeness_Report:

Some CTD casts were lost owing to equipment failure, dead batteries or corrupt data files.

Positional_Accuracy:

Horizontal_Positional_Accuracy:

Horizontal_Positional_Accuracy_Report: GPS positions should be accurate to within 10 m.

Lineage:

Methodology:

Methodology_Type: Field

Methodology_Description:

The CTDs needed to be small and light so as not to affect how the nets fished, be rugged enough to survive hundreds of trips up and down the trawl ramps on the typical 38- to 45-m-long commercial fishing trawlers employed as contract vessels, have long battery life to minimize the chances of waterproof-seal failures when the instruments were opened for battery changes, and sample rapidly to resolve sharp thermoclines. We chose Falmouth Scientific Instruments (FSI) NXIC CTDs that later became Teledyne RD Instruments (RDI) Citadel CTD-NVs (<http://www.rdinstruments.com/citadel.aspx>) when the product line changed hands. Each CTD was 67 cm long, 8 cm in diameter and weighed 3.8 kg in air with a titanium case. It had a rugged thermistor and non-external inductive conductivity sensor (NXIC). The batteries lasted most of the summer field season, requiring only one battery change per instrument. The instruments were deployed at their maximum sampling rate of 15 Hz.
Although the CTDs were designed to be rugged, for extra protection we mounted them in sturdy cases constructed from 160-mm-diameter Asahi/America Proline Pro-150 polypropylene pipe with a 14.6 mm wall thickness (Figure 1). Each case was placed in a mesh bag made from fishing net material and attached to the trawl net's headrope (Figure 2). This configuration was a compromise between adding to the thermal mass and reducing the flow through the conductivity cell of each instrument, and protecting it from damage. The conductance cell faced forward into the flow as the net fished. In a standard net haul, the ship steamed at 1.5 m/s (3 knots) while the net was lowered to the bottom at a vertical speed of ~0.3 m/s, towed with the headrope 2.5 m above the bottom for 30 minutes, and brought up to the surface at ~0.3 m/s.
As the CTD is lowered or towed, water flows through the inductive cell and past the thermistor; therefore no pump is required, thus enhancing battery life. However the residence time of a water parcel within the inductive cell and the time lag between the conductance and temperature measurements vary with flow speed. Temperature effects dominate conductance measurements, and salinity plays a secondary role. It is well known that to determine the changing salinity accurately, one must account for the changing temperature at the instant and location of the conductance measurement. If time constants and lags are not accounted for properly, salinity spikes and inaccuracies result. Some other CTD systems mitigate these problems by pumping water through a small diameter cell at a known, constant rate and fine-tuning time constants and offsets with factory-supplied data processing software. The NXIC CTDs came with no such software; therefore we wrote our own using the Ferret data visualization and analysis computer program (Hankin et al., 1992; Hankin et al., 1991).

Process_Step:

Process_Description:

Our data processing scheme had several steps. First, with software provided by the manufacturer, we converted each of the CTD's binary data files into an ASCII comma-separated-value (csv) file whose unique name was composed of the instrument's serial number and the date and time of the first sample. Each csv file was a time series of pressure, temperature, conductivity, salinity and battery voltage. Second, we read each csv file and eliminated bad and unwanted values. The CTDs were set up to begin recording when they entered salt water. Data values with conductivities below 5 mS/cm, owing to initial startup transients, were removed at this processing stage. During initial net deployment and final retrieval, the CTDs remained on the surface for a few minutes just behind the moving ship. These measurements in the ship's wake did not represent the undisturbed water column; therefore we removed values for pressures less than 1 dbar. Owing to data handling errors within the CTDs' factory software, especially in the early years of this study, the csv files sometimes had bad sections with blocks of repeated data making it appear that the sampling time had jumped backwards or did not progress. These sequences were detected and removed. The remaining values were linearly interpolated onto an equispaced time axis with an increment of 1/15th second – the instrument's sampling interval – and stored in a NetCDF file (network common data form, Rew et al., 2009). The third step in our processing scheme involved correcting the measured pressures, temperatures and conductivities for sensor lag times. RD Instruments gave the following values for the response times of their sensors: pressure 0.025 s, temperature 0.100 s and conductivity 0.05 s at a flow rate of 1 m/s (Teledyne RD Instruments, 2009). We used the method of Fofonoff et al. (1974) to correct for these lags. The CTD's pressure measurements were noisier than the other variables; therefore we smoothed them first with a 15-point (1-s duration) Hanning window (Press et al., 1986) before computing the three-point centered time derivative and the "true" pressure in (1). For temperature we computed the three-point centered time derivative of the measured temperature and then smoothed it with a 5-point Hanning window before applying (1). For conductivity, we assumed that the more massive conductivity cell's temperature lagged that of the "true" temperature measured with a downstream thermistor. We experimented with lagging the "true" temperature and its 5-point-Hanning-smoothed derivative by various times, used that and the measured conductivity to compute the salinity and checked for salinity spikes. By trial and error, we found that a lag time between the "true" temperature and the conductivity temperature of 0.3 s gave the best results in general, but it did not always entirely eliminate salinity spikes. The fourth step in CTD data processing involved matching the CTD time series with net haul times and positions. The pressure time series was smoothed with a Hanning window whose width in time (51 points) was chosen to be equivalent to 1 dbar at the sampling frequency of 15 Hz and a typical vertical net speed of 0.3 dbar/s. We designated that portion of each net haul a down- or upcast when the down- or upward velocity exceeded 0.08 dbar/s, respectively. Net haul logs provided by the bottom trawl survey scientists gave the time and geographic position when each haul reached and left the sea bottom, and the down- and upcasts were assigned those times and positions. The down- and upcast time series were linearly interpolated onto pressure axes with one-dbar increments, missing values above 3 dbar were assigned their nearest-neighbor values from below, and the results stored as separate NetCDF files. Usually the downcast was chosen to represent each CTD station, but each was compared graphically with its upcast and the latter was chosen if it had smaller spurious density inversions (heavy water above light water) owing to salinity spiking.
Two ships (F/V Aldebaran and either F/V Arcturus, Alaska Knight or Vesteraalen) conducted the bottom trawl survey, sailing parallel north-south tracks in tandem, 37 km apart, repeating the same pattern each year. For each survey year, CTD casts (321 in 2008, 292 in 2009 and 377 in 2010) were gridded onto an equispaced (2/3˚ longitude x 1/3˚ latitude) grid between 178.6667˚W and 157.6667˚W and 54.6667˚N and 66.0000˚N using a Laplacian algorithm with a spline-smoothing parameter of 2 and interpolating no more than 1 grid point away from any data point (Denbo, 1993; Hankin et al., 1992; Hankin et al., 1991). This regular grid approximated the bottom trawl survey grid that was spaced at 37 x 37 km over most of the region, but with finer spacing in areas of enhanced fisheries interest (Lauth, 2011). Some CTD casts were lost owing to equipment failure, dead batteries or corrupt data files such that an interpolated value was not available at every grid point. Unless otherwise mentioned, the results that follow are based upon the gridded data set.

Process_Date: Unknown

Process_Contact:

Contact_Information:

Contact_Person_Primary:

Contact_Person: Edward D. Cokelet

Contact_Organization: NOAA/PMEL

Contact_Address:

Address_Type: mailing

Address: 7600 Sand Point Way NE

City: Seattle

State_or_Province: WA

Postal_Code: 98105

Country: USA

Contact_Voice_Telephone: 206-526-6820

Contact_Electronic_Mail_Address: Edward.D.Cokelet@noaa.gov

Spatial_Data_Organization_Information:

Indirect_Spatial_Reference:

Time series of CTD position, water temperature, salinity, density anomaly (sigma_t) and dynamic height anomaly. The data file is a self-documenting NetCDF file. See <http://www.unidata.ucar.edu/software/netcdf/>

Direct_Spatial_Reference_Method: Point

Spatial_Reference_Information:

Horizontal_Coordinate_System_Definition:

Geographic:

Latitude_Resolution: 0.0001

Longitude_Resolution: 0.0001

Geographic_Coordinate_Units: Decimal degrees

Geodetic_Model:

Horizontal_Datum_Name: World Geodetic System of 1984

Ellipsoid_Name: World Geodetic System of 1984

Semi-major_Axis: 6378137

Denominator_of_Flattening_Ratio: 298.25722210088

Vertical_Coordinate_System_Definition:

Depth_System_Definition:

Depth_Datum_Name: Local surface

Depth_Resolution: 1

Depth_Distance_Units: meters

Depth_Encoding_Method: Implicit coordinate

Distribution_Information:

Distributor:

Contact_Information:

Contact_Organization_Primary:

Contact_Organization: National Center for Atmospheric Research

Contact_Person: Earth Observing Laboratory

Contact_Address:

Address_Type: mailing and physical

Address: 3450 Mitchell Lane

City: Boulder

State_or_Province: CO

Postal_Code: 80301

Country: USA

Contact_Voice_Telephone: 303-497-8154

Contact_Electronic_Mail_Address: codiac@ucar.edu

Contact_Electronic_Mail_Address: stott@ucar.edu

Distribution_Liability:

No warranty expressed or implied is made regarding the accuracy or utility of the data, nor shall the act of distribution constitute any such warranty. This disclaimer applies both to individual use of the data and aggregate use with other data. It is strongly recommended that careful attention be paid to the contents of the metadata file associated with the data. The NCAR and NPRB shall not be held liable for improper or incorrect use of the data described and/or contained herein.

Standard_Order_Process:

Digital_Form:

Digital_Transfer_Information:

Format_Name: NetCDF

File_Decompression_Technique: No compression applied

Digital_Transfer_Option:

Online_Option:

Computer_Contact_Information:

Network_Address:

Network_Resource_Name: <http://beringsea.eol.ucar.edu>

Fees: None

Metadata_Reference_Information:

Metadata_Date: 20130925

Metadata_Contact:

Contact_Information:

Contact_Organization_Primary:

Contact_Organization: National Center for Atmospheric Research

Contact_Person: Earth Observing Laboratory

Contact_Address:

Address_Type: mailing and physical

Address: 3450 Mitchell Lane

City: Boulder

State_or_Province: CO

Postal_Code: 80301

Country: USA

Contact_Voice_Telephone: 303-497-8154

Contact_Electronic_Mail_Address: codiac@ucar.edu

Contact_Electronic_Mail_Address: stott@ucar.edu

Metadata_Standard_Name:

FGDC Biological Data Profile of the Content Standard for Digital Geospatial Metadata

Metadata_Standard_Version: FGDC-STD-001.1-1999