SHEBA Upper Ocean CTD and Thermal Microstructure, Western Arctic Ocean

Summary

This data set includes conductivity, temperature, and depth (CTD) measurements along with thermal microstructure data (such as, turbulent thermal variance, turbulent kinetic energy, and vertical temperature gradient) collected from 12 October 1997 to 30 September 1998. The storage, redistribution, and ultimate release of the energy contributed by incoming shortwave radiation has major effects on the thickness and seasonal evolution of sea ice cover and the state of the upper ocean. As part of the Surface Heat Budget of the Arctic (SHEBA) project, this research was designed to (1) investigate upper ocean processes and their role in ice-albedo feedback, and (2) develop and test suitable models or parameterizations for use in large-scale simulations.

Data are in space-delimited ASCII text format and are available via FTP.

Citing These Data

Stanton, T., and B. Shaw. 2006. SHEBA upper ocean CTD and thermal microstructure, Western Arctic Ocean. Boulder, Colorado USA: 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

Investigators

Timothy Stanton
Oceanography Department
Naval Postgraduate School
1833 Dyer Road
Monterey, CA 93943 USA

Bill Shaw
Oceanography Department
Naval Postgraduate School
1833 Dyer Road
Monterey, CA 93943 USA

Technical Contact

Acknowledgements

This research was supported by the National Science Foundation (NSF) Office of Polar Programs (OPP) Arctic Sciences Section (ARC) grants 9701391 and 0084296.

2. Detailed Data Description

Format

Investigators converted the data from MATLAB format to space-delimited ASCII text format.

File and Directory Structure

Spatial Coverage

Southernmost Latitude: 74.6° N
Northernmost Latitude: 80.3° N
Westernmost Longitude: 168° W
Easternmost Longitude: 143° W

Spatial Resolution

Horizontal resolution is 1 km and smaller. Depth resolution is 0.5 m for the CTD data 1 m for the microstructure data.

Temporal Coverage

Investigators collected these data from 12 October 1997 to 30 September 1998.

Temporal Resolution

Data were collected at intervals that ranged from 15 minutes in length to once daily.

Parameter or Variable

This data set includes measurements of conductivity, temperature, depth, salinity, turbulent thermal variance, turbulent kinetic energy, and vertical temperature gradient.

3. Data Access and Tools

Data Access

Data are available for ordering through NCAR.

Volume

The entire data set is approximately 533 MB.

Related Data Collections

4. Data Acquisition and Processing

Theory of Measurements

Incoming shortwave radiation (F_r), which passes through the ice cover and is absorbed in the water, is the key element in interactions between the ice and upper ocean. The storage, redistribution, and ultimate release of this energy to the ice has major effects not only on the thickness and seasonal evolution of the ice cover, but also on the state of the upper ocean. Shortwave energy enters the ocean primarily through leads, areas of thin ice, and melt ponds. A theoretical analysis showed that a large fraction of F_r entering the leads is absorbed beneath the bottom of the ice and does not contribute directly to lateral melting. Instead, much of the energy goes to increasing the oceanic heat flux at the underside of the ice (F_w), producing a positive feedback between decreasing ice thickness and increasing F_w.

Because they lacked data, it was not possible for investigators to determine how much of F_r contributed to lateral melting and how much went to F_w, nor could they conclude anything specific about the residence time of shortwave energy in the water. In subsequent field measurements they discovered the presence of laminar sublayers at the underside of the ice, which inhibit the rate at which heat can be transferred from the water to the ice. The significance of these layers was demonstrated by mid-summer observations in a dynamically active portion of the Marginal Ice Zone, which revealed a substantial buildup of heat with time in the upper 20 m of the water column. The observations showed that solar heating rates in the mixed layer can exceed heat losses due to F_w and lateral melting, even when ice concentration is relatively high (80-90%) and vertical mixing is strong. Summer accumulation of heat in the mixed layer was also found in oceanographic data from AIDJEX (Arctic Ice Dynamics Joint Experiment). Reanalysis of these data (Maykut and McPhee, 1995) indicated that F_w is strongly seasonal, with maximum values reaching 40-60 W*m^-2 in August. Further, it was shown that F_w must have been derived almost entirely from shortwave radiation rather than from diffusion of heat from warmer water below the mixed layer.

Efforts to treat the effects of solar heating in the polar oceans have long been hampered by lack of suitable data. With this problem in mind, SHEBA was carefully designed to provide comprehensive and simultaneous data on the following:

• State of the ice (for example; concentration, thickness, and pond fraction)
• Salinity and shortwave fluxes entering the mixed layer
• Spatial variations in bottom and lateral melting of the ice
• Heat content, density structure, velocity field, and optical properties of the upper ocean
• Rate of heat release from the water to the ice.

Sensor or Instrument Description

The instrumentation on the system consisted of the following:

• A SeaBird 911+ CTD with two independent pumped systems for measuring conductivity and temperature and a single SeaTech transmissometer
• A pair of thermometrics FP07 fast-response thermistors for measuring microscale temperature and temperature gradient.

The FP07s were mounted to extend below the cage of the CTD and are referred to below as the clipon package. The CTD outputs were sampled at 24 Hz and the FP07s were sampled at 188 Hz.

Data Acquisition Methods

Investigators deployed a profiling CTD and thermal microstructure system using an automated winch as part of the upper ocean measurement program of the SHEBA project. The instrument package cycled from the surface to a depth of approximately 150 m at a rate of approximately 30 cm/s at a nominal interval of one cast every 15 minutes. The system was in operation from 12 October 1997 to 30 September 1998. Typically, the system was running continuously for about half of each day, although there were 21 days when no data were obtained. In total, 12350 casts were made for an average of nearly 35 casts per day.

For each cast, the best-performing conductivity/temperature pair and fast-response thermistor records were chosen (based on degree of correlation with casts adjacent in time) and obvious outliers were removed. Salinity and density were calculated using standard procedures. Rates of dissipation of turbulent thermal variance and turbulent kinetic energy were obtained by making two-parameter fits of the Batchelor theoretical spectrum to spectra of the fast-response temperature gradient over 1 m vertical bins.

5. References and Related Publications

Maykut, G. A., and M. G. McPhee. 1995. Solar heating of the Arctic mixed layer. Journal of Geophysical Research 100, 24691- 24703.

6. Document Information

Acronyms and Abbreviations

The following acronyms and abbreviations are used in this document.

Document Creation Date

December 2006

Document URL

http://data.eol.ucar.edu/codiac/dss/id=106.ARCSS151