CE17-01 Internal Waves

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Internal Wave Distortions
RV Celtic Explorer Cruise CE17-01
Chief Scientist: Kevin Sheehan, MI
July 22nd to August 6th 2017

John E. Hughes Clarke
Anand Hiroji
Jose Cordero Ros
Center for Coastal and Ocean Mapping
University of New Hampshire

Overview:

As the seabed relief in the survey area is so subdued, any small distortion of the bathymetry by refraction shows up very clearly. As well as the across track biases due to outdated watermasses, there are pronounced short period oscillations that are due to the passage of internal wave packets on the thermocline. This page illustrates this phenomena by focusing of a single soliton wave packet that was picked up and tracked over 4 survey lines (so about 8 hours). Each time the vessel passed over the wave packet, the oblique transect of the wave crests shows up as periodic oscillation of the outer beam data.

The map below shows the 4 lines that picked up the target.

	Initiation of Soliton Packet This wave appears to have been triggered from the nose of a lateral intrusion along the thermocline. The nose was crossed at 02:26:40 (July 24th, JD205).
	Second View of Packet (Wobble A) This occurred at 04:06:26 on July 24th (JD205) as the vessel was steaming west at ~ 7.5 knots. Close up views of this are shown below.
	Third View of Packet (Wobble B) This occurred at 07:56:44 on July 24th (JD205) as the vessel was steaming back to the east at ~ 8 knots. About 3 hours later as we had to go to the west end of the line and return. For a close up view of the wave packet, the vertical and horizontal scattering and the seabed distortion, see below.
	Final View of Packet (Wobble C) This occurred at 08:38:07 on July 24th (JD205) on the return leg going west again. This was only 41 minutes later as the eastern end of the line is not far away. For a close up view of the wave packet, the vertical and horizontal scattering and the seabed distortion, see below.

Close up views of the Internal Wave A:

The plots below show the topographic distortion (which is ~ _+/- 0.75% of depth)

above: - horizontal slice through water column imagery at 40m below surface.

above: - vertical sections along track through water column, stepping sequentially from port side (closer to observer) to starboard.

still at nadir

Showing the correlation of the across track thermocline undulations to the seabed distortions.

Internal Wave "B"

Note the pronounced curvature of the leading wavefront. Also note that 3/4 of the way back along the wave,
there is another wave traveling to the SW crossing directly over this packet.
Horizontal Section is taken at 40m depth.

Internal Wave "C"

Horizontal Section is taken at 38m depth.

Note that for both the wave packets B and C there appear to be enhanced scatterers in the watermass just over the first group of waves. The animations below show the across swath view and reveal that those scatterers follow the upper surface of that low scattering layer, riding up and down with the waves.


scattering around the leading waves of packet B	scattering around the leading waves of packet C

page composed offshore -- JEHC