Distinguishing ichthyogenic turbulence from geophysical turbulence. Issue 5 (29th May 2015)

• Record Type: Journal Article
• Authors: Pujiana, Kandaga; Moum, James N.; Smyth, William D.; Warner, Sally J.
• Is Part Of: Journal of geophysical research. Volume 120:Issue 5(2015:May)
• Page Start: 3792
• Page End: 3804
• Publication Date: 2015-05-29
• Subjects: Oceanography -- Periodicals; 551.4605
• DOI: 10.1002/2014JC010659
• Languages: English
• ISSNs: 2169-9275

Abstract

<abstract abstract-type="main"> <title>Abstract</title> <p>Measurements of currents and turbulence beneath a geostationary ship in the equatorial Indian Ocean during a period of weak surface forcing revealed unexpectedly strong turbulence beneath the surface mixed layer. Coincident with the turbulence was a marked reduction of the current speeds registered by shipboard Doppler current profilers, and an increase in their variability. At a mooring 1 km away, measurements of turbulence and currents showed no such anomalies. Correlation with the shipboard echo sounder measurements indicate that these nighttime anomalies were associated with fish aggregations beneath the ship. The fish created turbulence by swimming against the strong zonal current in order to remain beneath the ship, and their presence affected the Doppler speed measurements. The principal characteristics of the resultant ichthyogenic turbulence are (i) low wave number roll‐off of shear spectra in the inertial subrange relative to geophysical turbulence, (ii) Thorpe overturning scales that are small compared with the Ozmidov scale, and (iii) low mixing efficiency. These factors extend previous findings by Gregg and Horne (2009) to a very different biophysical regime and support the general conclusion that the biological contribution to mixing the ocean via turbulence is negligible.</p> </abstract>