Fig 1998 Shearmeter Test Mission

Timothy F. Duda
Woods Hole Oceanographic Institution
Woods Hole MA 02543
 
 
 

Dr. J. Ledwell and colleagues ease the first
Shearmeter into the South Atlantic from
the fantail of the RV Seward Johnson,
March 26, 1998.
Brian Guest Photo.
 
 
 

Photo of Shearmeter

Photo of drag elements and weight-release mechanism

Shearmeter home page

The ocean-going prototype shearmeter built by Webb Research Corp. and Seascan Inc. began a 100-day mission in the Atlantic on March 26, 1998 (photo above). It successfully surfaced on schedule and was first detected by ARGOS satellite at 0149 UTC 05 July 1998.

Launch Position: 21-36.79' S, 17-42.19' W.

Surface Position: 21-2.22' S, 16-40.92' W.

While submerged the Shearmeter travelled 123.7 km at heading 59.1. The average speed was 0.014 m/s.

Data were collected at roughly 1670 db pressure in 3.5 deg. C water. Here they are in unprocessed form, as of the indicated date:
 

raw data

Shear as defined in our analysis is scalar. It is the magnitude of velocity difference (m/s) divided by 10 m.

An important test result to date is that hourly-estimated (slightly aliased and attenuated) cross-isothermal velocity has an RMS value of 1.01 mm/s. The mean-squared horizontal currents at the rotors estimated from our calibration curve (other webpage) are 3.21 mm/s. This means that, on average, vertical flow energy available for spinning the instrument is 1/10th the available horizontal flow energy. Another important observation is that shear is variable at time scales of hours, days and weeks and that shear-squared averaged over periods of a day can fluctuate by a factor of 3.

Some processed shear results:
 

raw data
 
raw data
 
 

Below, top panel, a smoothed version of shear to the fourth power is converted to units of energy dissipation using the internal-wave decay formula of Gregg (1989, JGR) which is based on the work of Henyey, Wright and Flatte' (1986, JGR). The shear to the fourth power can also be expressed as diffusivity using the formula of Osborn (1980, JPO), K=dissipation/5N^2, (right axis). The lower panel shows the square of tidal currents in the region computed using a 25-constituent equilibrium tidal model consistent with 8 tidal constituents extracted from the TPXO.3 inverse model (Oregon State University). Peaks appear in dissipation (shear to the fourth power) and in tidal current. The peaks do not align well at this depth, which is 2500 to 3000 m above the bottom, but there is a suggestion of a link between the internal wavefield and the tides.

K and Tides
Additional figures: Design comments:

The Shearmeters have a central 3-inch OD pressure case flanked by 1.5-inch OD upper and lower pressure cases. Internal wiring to sensors, weight-dropper, and the radio antenna passes between the cases. 

This material is based on work supported by the National Science Foundation under Grant No. 9416014 from the Ocean Technology and Inderdisciplinary Coordination section of the Ocean Sciences Division. Any opinions, findings and conclusions or recomendations expressed in this material are those of the author and do not necessarily reflect the views of the National Science Foundation (NSF).  

Links:

WHOI AOPE Dept.
Coastal and Ocean Fluid Dyn. Lab Webpage
Ocean Acoustics Lab Webpage (My formal affiliation.)
Webb Research Corp.Webpage
NSF
 

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