Internal Waves, so you thought they were invisible, did you?

[Capt. Mike Maurice]

Wind waves are created by wind disturbing the surface of the water(perturbation). In the open ocean we generally consider all waves to be wind generated. However, there is another form called “internal waves” which occur only under very specific conditions. With one warning: wind waves(swell, wind chop) encountering a counter current will be enhanced or in the case of a following current: decreased. All such wind generated waves are not the subject of this discussion.

William Van Dorn wrote “Oceanography and Seamanship” in two editions, the latest in 1993. On pages 156-158 he briefly mentions “internal waves”, but specifically in regard to “dead water”, where a vessel is brought to or nearly to a standstill. There is an interesting graph describing the phenomenon. In the intervening years much research has been conducted into internal waves typically using synthetic aperture radar. https://internalwaveatlas.com/

The interalwaveatlas website has a magnificent collection of radar and visual pictures of internal waves arranged in two atlases( 2002, 2004), with pictures from all over the world.

First you need to know what the essential characteristics of “internal waves” are.
1. They occur near land, inward of the continental shelf, and in restricted waters like the Strait of Georgia in British Columbia or offshore of the Columbia River Entrance.
2. They are most likely to occur in calm summer months, Jun., July, Aug. with August being the most likely (in the Northern Hemisphere).
3. They require a discontinuity(such as: fresh water overlying salt water).
4. They move at 0.5 to about 1.0 meter per second( about 1-2 knots) in comparison to wind waves which generally propagate at 10-50 knots.
5. Their surface amplitudes(heights) range from a few inches to as much as 30 feet or so.

We have all seen them, but were most likely oblivious to the difference between them and wind waves, which generally surround us. If all this still seems “like Greek”, go study the pictures in the atlas. Take a look at the pictures for the British Columbia Coast, one picture is worth a thousand words. https://internalwaveatlas.com/Atlas2...shColombia.pdf

Oceanography of the British Columbia Coast by Richard Thomson(1981) has some of the best early description of internal waves.

If you read thru the various research papers one of the things you may note, as I did, words to the effect: this issue is not well understood. And I note, that is a vast understatement.


https://www.goodreads.com/book/show/...and_Seamanship
https://www.goodreads.com/book/show/...columbia-coast