Circulation in the Freshwater Switchyard of the Arctic Ocean: Helicopter CTD Profile Data 2003-2005

Summary

The Freshwater Switchyard of the Arctic Ocean (FSAO) project includes a helicopter based at Alert on northern Ellesmere Island to carry out oceanographic surveys northward across the shelf break in late spring. The "freshwater switchyard" is a region to the north of the eastern Canadian Archipelago and Greenland that is hypothesized as a bifurcation point for freshwater transfer between the Arctic and North Atlantic Oceans.

Measurements were made in April and/or May of 2003, 2004, and 2005 using a Seabird SBE 19plus Seacat internally recording CTD instrument with an SBE 43 dissolved oxygen sensor. Data include depth, pressure, temperature in situ, potential temperature, conductivity, salinity, and density. 2003 and 2004 data also include dissolved oxygen; in 2005 the oxygen sensor failed. Each cast reaches about 500 meters unless the ocean depth at that position is less. Further information and photography from the 2005 survey are posted at the project Web site, The Freshwater Switchyard of the Arctic Ocean. Data are in space-delimited ASCII format, and are available for ordering through NCAR.

The FSAO project is a program to study freshwater circulation (sea ice and upper ocean) in the "freshwater switchyard" between Alert (Ellesmere Island) and the North Pole, through a series of annual springtime hydrographic surveys. These surveys are intended to measure the eastward-flowing boundary current and also determine water property changes that occur in the transition from the deep Arctic Ocean basin to the shallow continental shelf. These data represent the first three years of surveys in 2003-2005; additional surveys are planned through 2007.

Citing These Data

Steele, Michael. 2005. Circulation in the Freshwater Switchyard of the Arctic Ocean: helicopter CTD profile data 2003-2005. Boulder, CO: National Center for Atmospheric Research, ARCSS Data Archive.

Overview Table

Table of Contents

1. Contacts and Acknowledgments
2. Detailed Data Description
3. Data Access and Tools
4. Data Acquisition and Processing
5. References and Related Publications
6. Document Information

1. Contacts and Acknowledgments

Investigator

Michael Steele
Polar Science Center
Applied Physics Lab
University of Washington
1013 NE 40th
Seattle, WA 98105-6698

Technical Contacts

Roger Andersen
Wendy Ermold
Polar Science Center
Applied Physics Lab
University of Washington
1013 NE 40th
Seattle, WA 98105-6698

Acknowledgements

This research was supported by the NSF OPP Arctic System Science (ARCSS) Program grant ARC 0230427.

2. Detailed Data Description

Format

Data are in uncompressed, space-delimited ASCII format. Each 2005 file contains seven columns, and 2003-2004 files have eleven columns. Each file contains one record per depth interval, at various intervals (typically, ~0.04 m), from near surface to about 500 m.

File and Directory Structure

Data are contained in three directories. Each is named as follows:

SwitchyardXXXX_CTD-O2s

where XXXX is the year that data were collected (2003-2005).

Each directory also includes a file ending in "ReadMe.txt" containing information about that year's measurements.

Files include header rows identifying year, time, date, station, cast, and parameters.

File Naming Convention

Each file is named to identify the year of measurement and the location of the station where the cast was made:

2003 file names: SYard2003_CTD-O2_StationX.txt, where X is the station number

2004-2005 file names: SYardYYYY_CTDZ_SSSSSS.txt, where YYYY is the year, Z is the the cast number, and SSSS is the station name.

File Size

Files range in size from 4 KB to 252 KB. Total data volume is approximately 4.6 MB.

Spatial Coverage

Data were taken from measurements at stations in the Arctic Ocean near Ellesmere Island.

Southernmost latitude: 83°38.87' N
Northernmost latitude: 84°39.83' N
Westernmost latitude: 66°14.58' W
Easternmost latitude: 56°00.82' W

Spatial Resolution

Measurements were taken from depths ranging from near the ocean surface to 500 meters below the surface. Depth intervals vary, but are typically ~0.04 m.

Temporal Coverage

Measurements were taken in May 2003, May 2004, and April-May 2005.

Parameter or Variable

Parameter Description

Depth (m)
Pressure (dbar)
Temperature in situ (deg C)
Conductivity (Siemens/meter)
Salinity (psu)
Density (Sigma-theta)
Dissolved Oxygen (ml/liter)
Dissolved Oxygen (mg/liter)
Dissolved Oxygen (% Saturation)
Dissolved Oxygen (Mmol/kg)

Sample Data Record

Error Sources

In 2005, the oxygen sensor failed due to a manufacturing error apparently unrelated to the cold environment in which it was used.

3. Data Access and Tools

Data Access

Data are available via FTP.

Volume

Total data volume is approximately 4.6 MB.

Related Data Collections

• North Pole Environmental Observatory Aerial CTD Survey

4. Data Acquisition and Processing

Sensor or Instrument Description

The instrument was a Seabird SBE 19plus Seacat internally recording CTD panel with an SBE 43 Dissolved Oxygen Sensor.

Data Acquisition Methods

The Freshwater Switchyard of the Arctic Ocean project includes a helicopter based at Alert on northern Ellesmere Island to carry out oceanographic surveys northward across the shelf break in late spring. These measurements were made using a very lightweight winch at positions selected from the air for ease of access to the water. Each cast reaches about 500 meters unless the ocean depth at that position is less.

2003-2004 Notes: In spring Arctic conditions with cold air temperatures, a frequent problem has been seawater freezing in the instrument tubes the instant it enters the water and not dissipating until at a substantial depth, even after a long period with the instrument soaking in the mixed layer. We found that introducing a sample of warm seawater orally through a tygon tube to the instrument inlet just before it goes in the water clears up these problems in almost all cases.

Processing followed the recently-modified (June 2003) SEASOFT recipe to minimize salinity spiking with certain constants determined by empirical trial. Conductivity and temperature were low-pass filtered with a time constant of 0.5 seconds, pressure filtered with a time constant of 2.0 seconds, and temperature was advanced relative to pressure by 0.5 seconds. The down-casts show the best resolution and were selected in each case except for Station 7. That profile showed a frozen top of the down-cast, despite the warm seawater trick, and the top of that up-cast was grafted on the rest of its down-cast to get smooth, representative profiles.

2005 Notes: Station Bravo was first visited at the end of the first flying day, and this cast suffered both from ice in the plumbing contaminating the top of the downcast and the instrument hitting the bottom a bit shallower than expected at 292 meters. Taking the opportunity to re-visit the site two days later, a good quality but shallow profile was obtained. The two Bravo profiles, where both are of good quality, line up very nicely, so the profile Bravo is a combination of downcasts from the second visit above 216 meters and from the first visit below.

Processing Steps

Processing followed the recently-modified (June 2003) SEASOFT recipe with certain constants determined by empirical trial. Conductivity and temperature were low pass filtered with a time constant of 0.5 seconds, pressure filtered with a time constant of 2.0 seconds, and temperature was advanced relative to pressure by 0.5 seconds. Varying the temperature advance showed the nominal 0.5 seconds did the best job of minimizing salinity spiking.

Following the SEASOFT recipe, the optimum advancement of SBE-43 dissolved oxygen relative to pressure was empirically found to be 7.0 seconds. The resulting oxygen profiles were extremely clean. However, the SBE-43 instrument is quite new, and seems to be particularly sensitive to damage from very cold air temperatures. Post trip exam found slight damage to the instrument membrane, likely due to freezing. Even though the survey lasted only three days and ten casts, a small drift was found in the Mixed Layer oxygen concentration, which should be close to 100% saturation. This was dealt with by iterating the calibrated value Oxygen Signal Slope (Soc) until saturation was reached at a range of mixed layer depths. Empirically, a depth of 30 meters seems to best represent this characteristic of the mixed layer over our ten profiles, and the optimized value of Soc for that cast and that depth was used to re-calculate the oxygen profiles reported in this data set. For special case Station 7, the long oxygen advance required grafting up-cast oxygen atop the down-cast at an offset level from the graft boundary for the hydrographic parameters. We are working with Seabird, Inc. to improve our procedures for using the SBE-43 in very cold air temperatures.

5. References and Related Publications

• The Freshwater Switchyard of the Arctic Ocean

6. Document Information

Glossary and Acronyms

Please see the EOSDIS Glossary of Terms for a general list of terms.

List of Acronyms

The following acronyms are used in this document:

ARCSS: Arctic System Science
ASCII: American Standard Code for Information Interchange
CTD: Conductivity, Temperature, Depth
FSAO: Freshwater Switchyard of the Arctic Ocean
FTP: File Transfer Protocol
NPEO: North Pole Environmental Observatory
NSF: National Science Foundation
NCAR: National Center for Atmospheric Research
OPP: Office of Polar Programs
URL: Uniform Resource Locator

Document Creation Date

03 May 2006

Document URL

http://data.eol.ucar.edu/codiac/dss/id=106.ARCSS129