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Internal waves Waves can propagate between any two fluids of different density; in stratified waters a density gradient exists between the waters either side of the thermocline. Internal waves were observed within the thermocline in the coastal waters off Falmouth, and were present for the entire duration of the time series recorded. These internal waves have a longer period than surface gravity waves because the restoring buoyancy force (i.e. reduced gravity) is small in comparison. A quick back-of-the-envelope calculation was made to try and find the period of the internal wave observed. A plot was produced of depth verses scan number for each time the yo-yo CTP profiler passed the signature of the internal wave within the water column (~15m depth). As the density gradient (thermocline) was shifted up and down with the propagating internal wave during this profiling session, so the plot produced showed part of that wave profile. Each scan took 0.707 seconds to complete and from the plot the wavelength of the internal wave was estimated at 550 scans. This gives an estimated wave period (T) of 338.93 seconds. Using the brunt-vaisala frequency calculation to work out what the natural frequency, based upon the density gradient, of an internal wave propagating within this density gradient should be gives periods of between 489.56 and 356.04 seconds - these match reasonably well with the estimated period concluding that the frequency of the wave that was traced over the depth-scan plot was estimated correctly.
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