Recently in Internal Waves Category

From Pete:

Today was great. Sunny, calm, and beautiful. Right now we're steaming to our last station of the day, the moon is rising (pic below) and the bioluminescence is awesome. Our bow wave is glowing brilliant green and you can see schools of fish swimming by like shooting stars under the wAter. Too low light for good pictures, sorry. Today we saw (to name a few):  several fin whales, a minke, a basking shark right off the stern, lots of shearwaters, blackbacks, and storm petrels, a few gannetts and a northern fulmar.

Becoming one with my thesis project I covered my head with water at Platts bank and ate a euphausiid alive. I swear, I can still feel it squirming in my belly.

Couldn't help playing a bit with one of the airplane photos from the previous entry.  Thepicture below (left) shows a series of vortices trailing off of the upper edge of the island.  The vortices were hard to see when the plane was just above them, but they really jumped out when the moved into the sun glint.  While you can see them in the raw photo, a little work in Matlab highlights the vortices (image on the right).  To get this image, I made a black & white version of the image.  I then created a smoothed version (median smoother with a radius of 5 pixels).  The smoothed version removes the fine scale features we're interested in, leaving only large-scale continuous features like the island and the sun.  I then subtracted the smoothed version from the raw, to create an anomaly.  I then squared the anomaly (to make the large differences pop) and multiplied by the sign of the anomaly (so that +/- were retained).  Finally, I replaced values close to zero (absolute value <5) with nans and then overlayed on to the photo.  So, the vortices highlighted in red are reflecting more light than expected (sea surface angled towards the camera or smoother water), and those in blue are reflecting less light than expected (rougher water?). 

One of the perks of my job is getting to visit cool places.  Of course, Hawaii is my all time favorite (three Ocean Sciences conferences), with Copenhagen (ASLO Meeting) and Aquafredda (Italy, NATO summer institute) also meriting consideration.  Worst business travel experience: Orlando (Ocean Science conference).  My guess is that hell looks a lot like International Dr. in Orlando, with the Orlando-Orange County Civic Center containing the 8th and 9th circles. 

The model I'm using to look at internal wave dynamics was created by Daniel Bourgault and Dan Kelley (see Aestus on Daniel Bourgault's website, or Bourgault and Kelley 2004). It is a laterally averaged non-hydrostatic model. This means two things:

1. In a laterally averaged model, we assume that advection perpendicular to the primary flow is minimally significant. This allows us to reduce the number of dimensions in the model and reduce the time necessary to complete each run (i.e. more experiments in the same amount of time).
2. A Non-hydrostatic model does not hold the hydrostatic assumption as true. This means the system does not have to be in hydrostatic balance, where the pressure gradient is balanced by the buoyancy forces,.  Generally, this makes sense: the deeper you go the denser the water and the greater the pressure. But when you want to investigate the dynamics of water that is being mixed, by internal waves for example, the denser water mass is not always deeper. This makes the math for solving the model more complicated and a non-hydrostatic model is capable of handling that.
   

A big part of running any model is setting up initial conditions that will accurately recreate your system. This is something I'm still fine tuning, and perhaps I'll talk more about that in a later post.

For now, here's what a sample output looks like:

     The methods by which animals take advantage of their environments to increase foraging and locamotive efficiency are sometimes astounding. Albatross (family Diomedeidae) can lock their wings and surf the pressure gradients in front of surface waves for days (see dynamic soaring). Striped bass (Morone saxatilis) are known to sit in one location, swimming in place, facing the current (this, by the way, is called positive rheotaxis, for those of you looking for your $2 word of the day) with their mouths agape, waiting for the current to bring them the food.
    A recent paper by Moore and Lien (2008) documents a pod of pilot whales (Globicephala macrorhynchus) following internal waves through the South China Sea. Moore and Lien suggest that the Globichephala may do this to take advantage of the increased concentrations of prey entrained in the physical mixing generated by the waves. (In fluid dynamics, to be entrained, literally, is to be picked up by and carried with a flow).
 
     While I think the ideas within the paper are very interesting, I'm disappointed that the authors didn't go further. How about some prey sampling in the waves? No nets? If the data was collected with a depth-sounder, there is post-processing software that can be used to estimate plankton and fish abundances. The other papers cited, Ramp et al. (2004) and Lien et al. (2005), describe methods used to investigate the internal waves in the South China Sea. Data collection methods included: ADCP, moored current, temperature, conductivity, and pressure sensors as well as other acoustically gathered data. If the pilot whale were observed by chance during the course of the internal wave research-cruises, Moore and Lien would have the capacity to describe the (probable) presence or absence of prey for the whales that was aggregated by the waves. Such inferences could be made from the available acoustic data.
     Although the authors mention that the shoaling waves bring nutrients and plankton to the surface, there is no mention of plankton in the cited paper by Lien et al. (2005) describing the internal waves in the area.
     Basically, my issue with the paper is that Moore and Lien don't offer any evidence that prey were present in the waves. They merely say the waves are capable of entraining likely prey for the pilot whales. That said, I think the phenomenon is interesting, and I agree with Moore and Lien's conclusion that the issue merits further study.

An introduction to internal waves for scientists and future-scientist alike

What are internal waves?
     Internal waves are similar to surface waves (the kind you see and surf at the beach). The biggest difference is that surface waves exist at the boundary between water and air while internal waves exist at a boundary between two layers of water that differ in density. Think of oil and vinegar in the same container: if you let the container sit for a while the oil settles on the bottom in a state of equilibrium (as oil is the denser of the two liquids). You can clearly see the boundary layer between them. Disturbances in this layer (e.g. a drop of oil pushed above the boundary) result in restoring forces trying to bring things back to equilibrium. The dissipation of these forces result in waves that propagate along the boundary between the two liquids. These disturbance-generated waves are just like the waves you see moving across a pond after you throw a rock in and disturb the pond surface layer. In the ocean heavy, cold, and dense water plays the role of the oil in our above thought experiment, and lighter, warmer, less dense surface waters play the role of the vinegar. The boundary between the two water masses in the ocean is called the pycnocline.
     Most frequently internal waves are tidally generated, but they can also be created by underwater earthquakes, significant freshwater inputs flowing into the ocean, the mixing of the water-column by storms, or flow of water over certain bathymetric features.

Why are internal waves important?
    Internal waves provide energy to ocean ecosystems. They induce mixing, and transport nutrients from depth to the surface where they can be utilized by photosynthetic plankton. Internal waves have also been shown to advect (micro)organisms through the water.
 
What are we studying them for?
     The biota of the ocean are not evenly distributed. In fact there exists a patchiness that is documented throughout the literature which scientists have been trying to understand for quite some time. Areas of high biological activity are often called "hotspots". At a hotspot you might find 40 Eubalaelena glacialis (right whale) feeding on a dense patch of Calinus finmarchicus, or hundreds of seabirds following a pair of Megoptera novaeangliae (humpback whale) diving with a pod of Lagenorhynchus acutus (Atlantic white-sided dolphin) for krill or Ammodytes americanus (sandlance).
      One of the goals of the EMLab is to understand this patchiness. To do this we're looking at internal waves and how they advect euphausiids (krill) as a potential driving mechanism in determining the patchy distribution of hotspots.