Charismatic Mega Turbulence

I’ve been waiting to write this post for a long time. The problem was, up until now I just didn’t know of a good example of charismatic mega turbulence. Normally, the turbulent eddies that mix the world’s oceans are really small, ranging in size from 10′s of millimeters to at most 100′s of meters. When you compare these eddies to the size of the ocean, which is on average 3 km deep and  thousands of kilometers wide, turbulence occurs on relatively tiny scales. So even though these turbulent vortices are super cute, they certainly can’t be classified under the mega category.

But now, giant turbulence your time has finally come! *insert mad cackling and background lighting strikes here*

In the Strait of Georgia, which lies between mainland Canada and Vancouver Island, super fast tidal currents flow over sills and bumps. Deep, cold, dense water is forced up and over theses ridges, which then plummets back downward into slower moving water below. The plunging water is arrested by the sluggish water below and has no other choice but to come up in the form of huge blobs of water known as boils!

Beware the boils! This one is caused by two streams meeting in the lee of an island.

She’s going from suck to blow! This boil may look like a eddy that’s about to suck you down, but it’s really a giant blob of water that has risen to the surface. [source: http://www.eos.ubc.ca/~rich/research.html]

Over 70 meters in diameter, you can’t call these boils anything else but charismatic mega turbulence. The boils are so large their cold water signature can be detected from space using satellite thermal imagery! And they form incredibly fast, at a rate of ~60 m/s. In human terms, that means a single boil grows to the size of two olympic sized swimming pools in under a minute. Not even Michael Phelps can escape that shit.

[source: Figure 4 from Marmorino et al.]

Peeping at the boils from afar with GeoSat-1 satellite imagery. Zoomed in images in panels (b) and (c) show they form just downstream of the ridge (red bathymetric contours).[source: Figure 4 from Marmorino et al.]

But the formation of boils is not the only consequence of this interaction between tidal currents and undersea ridges. The flow of water up and over the sill also creates my beloved internal waves!  These internal waves radiate away from the lee side of the sill, draining energy from the tides and redistributing its energy. Pretty rad.

In nearby Knight Inlet, the Canadian Coast Guard vessel Vector has been commandeered by scientists chasing an internal wave crest that has escaped from the local sill. [source:http://www.eos.ubc.ca/~rich/knight/index.html]

What we don’t know is how important these boils are in a larger context. Sure the boils are big and the turbulence is whomping, but they are short lived and aren’t a normal phenomena in most places. While the authors of the study were able to find some satellite imagery of boils, it was only because they knew they were there. So it isn’t clear how prevalent this form of charismatic mega turbulence is and whether they are an important source of mixing globally. Nonetheless, they are probably important locally, mixing up water, transporting nutrients and feeding the sea beasties that live in the rich coastal environment.

But you don’t need a satellite or a boat to see charismatic mega turbulence, you can head on over to Deception Pass to see if for yourself. Deception Pass is narrow constriction at the north end of Whidbey Island in Washington state with whipping tidal flows and a bumpy bottom. Even better, there is a walkable bridge spanning it so you can safely look down at the fluid action while staying completely dry. Or if you dare (and are experienced), you can rent a kayak or paddleboard and experience this wicked turbulence in situ! While some may prefer their charismatic mega features to be of the cetacean kind, I would rather hang with out with my mega turbulence any day of the week. And who knows, maybe one day someone will be lucky enough to see both together!

SOURCES:

Marmorino, George O.; Smith, Geoffrey B.; Miller, W. D. 2013. “Surface Imprints of Water-Column Turbulence: A Case Study of Tidal Flow over an Estuarine Sill.” Remote Sens. 5, no. 7: 3239-3258.

Rick Pawlowicz’s research page http://www.eos.ubc.ca/~rich/research.html

 

 

 

Kim Martini (77 Posts)

Kim is a Physical Oceanographer at the Joint Institute for the Study of Atmosphere and Ocean at the University of Washington. She received her Ph.D. from the University of Washington in 2010. Her goal in life is to throw expensive s**t in the ocean. When not at sea, she uses observations from moored, satellite and land-based instruments to understand the pathways that wind and tidal energy take from large (internal tides) to small scales (turbulence).





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