East Sound 1998 Field Season  (June 12-13)
 
          Time series temperature data from the array near the sill and near Donaghay’s and Holiday’s CTD and acoustic arrays showed jumps in the pycnocline depth accompanied by packets of high frequency waves (solitons) (Fig. 1). The formation of solitons near sills is well documented (Farmer and Armi 1999).  However, the solitons that occurred near the sill are not in phase with those observed near the acoustic array.  Those near the array are more likely caused by the constriction of the sound half way along its length. From noon on 12 June through the 13th, the pycnocline descended due to inflow of water from the Frazer River.  Acceleration of the waters through the constriction may have led to the instability of the pycnocline.  Alternatively, if internal waves are present in the sound, they are likely to become unstable at the constriction due to increased contact with sloping boundaries (Thorpe et al. 1996).
Figures: Isotherm displacements in the pycnocline from 12-13 June 1998.  The descent of the pycnocline ( Figure 1, above) was caused by incoming water from the Frasier River.  As a result of the deepening as well as a narrowing of the sound, solitons (Figure 2, below) were generated.  The isotherm displacements indicate a combination of thick solitons (Holloway et al. 1999. JGR 104: 18,333-18,350) and thin solitons.  An expanded view of one of the thick solitons is shown below.  The thermocline descended 1.4 m in 1 minute.  When the thick and thin solitons come in contact with the bottom, they generate along slope flows with shears likely to cause sediment resuspension (Thorpe et al. 1996).
 
       Whether intrusions are likely due to boundary mixing depends on whether internal waves are near frequencies critical for wave breaking.  Critical frequency fc depends on water column stability given by the buoyancy frequency N and bottom slope ß, fc = ßN. Typical values of N in the water column range from 0.01 to 0.08 s-1.  Given that bottom slopes near the lateral boundaries of East Sound range from 0.2 to 0.8, critical frequencies range from 1 to 37 cph.  Many of the high frequency wave packets are in the range critical for breaking.  For example, those in Fig. 2 have a frequency of 14 cph. 

       Besides  their role in generating intrusions, the solitons may also disperse layers or cause formation of new ones.  Energy dissipation rates were up to 10-7 m2s-3 in solitons. During the passage of solitons on 20 June 1998 at ca. 1330 hours, a layer of acoustic scatterers increased from 20 cm to 1 m in vertical extent.  As the wave train initially passed, there were two layers of high fluorescence; by the end there was a third, less concentrated one in the pycnocline.Interaction of Soliton with Thin Layer (click to explore) 

    (click on image to see June 20 results)
 
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last updated: October 28, 1999
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