After much oscillation, I decided the first slice of my research existence I wanted to discuss would be waves. My motivation for this decision was that when I started this post (a while ago at this point), we had one of the largest wave events experienced by our coast in years. Registering this from my office chair as a buoy reading (http://www.ndbc.noaa.gov/) is one thing but in person they were undeniably the largest waves I have ever seen.
It’s hard to get perspective as this is from the top of Yaquina head (a good distance above the ocean) but take my word that those are big fellows. Like massive earth-shacking mobile mountains. Lake watery whales smashing into shore, roaring and flinging foam at us like salty missiles. From the beach the view looked a bit like I should be running the other direction (something I promptly did).
The beach was literally a wall of white water as far as one could see. During these photos the significant wave height at Bouy 46089 was around 38 feet.
All of us on the coast see waves on a daily basis but, like most things in life, never stop to think about what they are, what causes them, and how do they effect the world we live in. Before we jump into this thought, I should sharpen the focus of our question as waves are one of the most common things in the universe. The light hitting your eyeball as you read this past is a wave as is the sound coming from your headphones. Even matter in small enough sizes behaves as a wave as “Dr. Quantum” would be happy to explain to you:
As I am not nearly as awesome as “Dr Quantum” I will be focusing on water surface gravity waves, a microscopic topic in the vast spectrum of wave phenomenon. This said it is fairly awesome to consider the fact that what we see on the ocean is in many ways a metaphor for some of the more complex and mind blowing physics out there. For example the interference pattern from the double slit experiment in the video above is something that we (as coastal engineers) would have to consider. Previous Malouf scholar Annika O’Dea (scholar http://seagrant.oregonstate.edu/education/sea-grant-scholars/meet-scholars/annika-odea-2013) was working on this problem in the context of offshore wave energy converters causing a pattern of high and low wave energy which could potentially effect the shoreline.
So shifting back to the question at hand, the first obvious question would be “what is a surface gravity wave?” The first part of the name is wave. I like to think of waves as a transfer of energy, generally accomplished through an oscillation of “something” (medium would be the more scientific name). In our case the “something” is water. It’s important to note that despite appearances, what is moving not water but energy. The graphic below shows the path of water particles (circular in deep water and elliptical for shallow water).
This is why if you see a seagull on the surface of the ocean, it doesn’t get “pushed” in the direction of wave propagation, but instead appears to do little circles on the surface.
An important caveat to this is that this concept is only true for linear waves and breaks down for nonlinear conditions (within the surf zone would be an example). Linear wave theory results in waves that are sinusoidal. Not so true in the surfzone.
The details of what the “linear assumption” means is a bit mathy and probably beyond what anyone wants to know, but it’s generally quite accurate for normal ocean conditions. I would say remarkably to magically accurate considering what a drastic assumption it is. The realm beyond this is known as non-linear waves.
The second component of the name is “gravity.” This refers to the restoring force for this particular type of wave. All waves require that the medium has an equilibrium position that is being disturbed. The restoring force is what is trying to return the disturbance to the equilibrium position. If we consider a perfectly calm ocean, it would be perfectly flat and in equilibrium. If something disturbs this (say wind for swell or an earthquake for a tsunami) then the surface is moved upward or downward. Gravity tries to restore the surface to the flat condition that would be perfect balance. Other restoring forces could be the capillary force for very small waves (say when wind is just starting to blow over the surface and create ripples). The restoring force can also be much more complex. For example, with Rosby waves it is the variability of the Coriolis force and the requirement for conservation of absolute vorticity. This article has a fairly good description of this concept for atmospheric Rosby Waves (there is a corresponding phenomenon within the ocean).
The final part of the name is surface. Surface refers to the fact that that the ocean waves are at the surface of the medium. This may seem obvious but the ocean is actually full of internal waves.
Internal Waves within the South China Sea (NASA’s Shuttle- June 1983)
Waves can form at any interface between two layers. For the surface this is an interface between water and air. For internal waves it is generally between layers of different density.
video credit: Office of Naval Research, NSF, Sixth Man Productions, Edgeworx.
These waves can be larger than sky scrapers and contain massive energy that is critical to the earth’s climate.
So now we have pretty much only defined waves and I’m probably over my word limit so that may be all for the time being. Next up: How do Surface Gravity Waves effect the world we live in?