Tag Archives: turbulence

East Greenland Current Instabilities

The coast off north-east Greenland is a grey, cloudy, and icy place. I spent 4 weeks on a ship earlier this summer to place sensors on the ocean floor to measure water currents, salinity, and temperature. The data shall uncover the mystery of how ocean heat 300 m below the surface gets to glaciers to melt them from below year round. My contribution is a small part of a larger effort by German, Norwegian, Danish, American, and British scientists to reveal how oceans change glaciers and how oceans impact Greenland’s ice sheet, climate, and weather.

So, for months now I am watching rather closely how this ocean looks from space. Usually it is cloudy with little exciting to see, but for 4 days this week the clouds broke and displayed a violently turbulent ocean worthy of a Van Gogh painting:

Satellite image ocean current instabilities on Aug.-19, 2014 as traced by ice along the shelf break, red lines show 500, 750, and 1000 meter water depth. Small blue triangles top left are ocean moorings.

Satellite image of ocean current instabilities on Aug.-19, 2014 as traced by ice along the the shelf break, red lines show 500, 750, and 1000 meter water depth. Small blue triangles top left are ocean moorings.

A wavy band of white near the red lines indicates the East Greenland Current. The red lines show where the water is 500, 750, and 1000 m deep. All waters to the left (west) of the red lines are shallow continental shelf while all waters to the right (east) are deep basin. Some islands and headlands of Greenland appear on the left of the image as solid grey. The image covers a distance about the same as from Boston to Washington, DC or London to Aberdeen, Scotland. Black areas are ocean that is clear of ice while the many shades of white and gray are millions of ice floes that act as particles moved about by the surface flow. Using a different satellite with much higher resolution shows these particles. The detail is from a tiny area to the north-west of the red circle near 77.5 North latitude:

Individual ice particles as seen on the north-east Greenland shelf from LandSat 15-m resolution from Aug.-21, 2014 near 77.5N and 10 W.

Individual ice particles as seen on the north-east Greenland shelf from LandSat 15-m resolution from Aug.-21, 2014 near 77.5N and 10 W.

Strongly white areas indicate convergent ocean surface currents that concentrate the loose ice while divergent ocean currents spread the ice particles out in filaments and swirls and eddies.

This is how many real fluids look like if one takes a snapshot as satellites do. Stringing such snapshots together, I show the fluid motion as comes to life for about 3 days:

Output

Notice how the large crests seaward of the red line between 74 and 75 North latitude grow and appear to break backward. This is an instability of the underlying East Greenland Current. It starts out as a small horizontal “wave,” but unlike the waves we watch at the beach, the amplitude of this “wave” is horizontal (east-west) and not vertical (up-down). The mathematics are identical, however, and this is the reason that I call this a wave. As the wave grows, it become steeper, and as it becomes too steep, it breaks and as it breaks, it forms eddies. These eddies then persist in the ocean for many weeks or months as rotating, swirling features that carry the Arctic waters of the East Greenland Current far afield towards the east. The East Greenland Current, however, continues southward towards the southern tip of Greenland. The wave and eddy processes observed here, however, weaken the current as some of its energy is carried away with the eddies.

I could not find any imagery like this in the scientific literature for this region, but similar features have been observed in similar ocean current systems that transport icy cold waters along a shelf break. The Labrador Current off eastern Canada shows similar instabilities as does the East Kamchatka Current off Russia in its Pacific Far East. And that’s the beauty of physics … they organize nature for us in ways that are both simple and elegant, yet all this beauty and elegance gives us complex patterns that are impossible to predict in detail.

Beszczynska-Möller, A., Woodgate, R., Lee, C., Melling, H., & Karcher, M. (2011). A Synthesis of Exchanges Through the Main Oceanic Gateways to the Arctic Ocean Oceanography, 24 (3), 82-99 DOI: 10.5670/oceanog.2011.59

LeBlond, P. (1982). Satellite observations of labrador current undulations Atmosphere-Ocean, 20 (2), 129-142 DOI: 10.1080/07055900.1982.9649135

Solomon, H., & Ahlnäs, K. (1978). Eddies in the Kamchatka Current Deep Sea Research, 25 (4), 403-410 DOI: 10.1016/0146-6291(78)90566-0

The Turbulence of Van Gogh and the Labrador Shelf Current

Vincent Van Gogh painted his most turbulent images when insane. The Labrador Current resembles Van Gogh’s paintings when it becomes unstable. There is no reason that mental and geophysical instability relate to each other. And yet they do. Russian physicist Andrey Kolmogorov developed theories of turbulence 70 years ago that Mexican physicist applied to some of Van Gogh’s paintings such as “Starry Sky:”

Vincent Van Gogh's "Starry Sky" painted in June 1889.

Vincent Van Gogh’s “Starry Sky” painted in June 1889.

The whirls and curls evoke motion. The colors vibrate and oscillate like waves that come and go. There are rounded curves and borders in the tiny trees, the big mountains, and the blinking stars. Oceanographers call these rounded curves eddies when they close on themselves much as is done by a smooth wave that is breaking when it hits the beach in violent turmoil.

Waves come in many sizes at many periods. The wave on the beach has a period of 5 seconds maybe and measures 50 meters from crest to crest. Tides are waves, too, but their period is half a day with a distance of more than 1000 km from crest to crest. These are scales of time and space. There exist powerful mathematical statements to tell us that we can describe all motions as the sum of many waves at different scales. Our cell phone and computer communications depend on it, as do whales, dolphins, and submarines navigating under water, but I digress.

The Labrador Shelf Current off Canada moves ice, icebergs, and ice islands from the Arctic down the coast into the Atlantic Ocean. To the naked eye the ice is white while the ocean is blue. Our eyes in the sky on NASA satellites sense the amount of light and color that ice and ocean when hit by sun or moon light reflects back to space. An image from last friday gives a sense of the violence and motion when this icy south-eastward flowing current off Labrador is opposed by a short wind-burst in the opposite direction:

Ice in the Labrador Current as seen by MODIS-Terra on May 3, 2013.

Ice in the Labrador Current as seen by MODIS-Terra on May 3, 2013. Blue colors represent open water while white and yellow colors represent ice of varying concentrations.

Flying from London to Chicago on April 6, 2008, Daniel Schwen photographed the icy surface of the Labrador Current a little farther south:

Ice fields seen in Labrador Current April 6, 2008 from a plane. [Photo Credit: Daniel Schwen]

Ice fields seen in Labrador Current April 6, 2008 from a plane. [Photo Credit: Daniel Schwen]

Ice in the Labrador Current as seen by MODIS-Terra on April 6, 2008. Blue colors represent open water while white and yellow colors represent ice of varying concentrations.

Ice in the Labrador Current as seen by MODIS-Terra on April 6, 2008. Blue colors represent open water while white and yellow colors represent ice of varying concentrations.

The swirls and eddies trap small pieces of ice and arrange them into wavy bands, filaments, and trap them. The ice visualizes turbulent motions at the ocean surface. Also notice the wide range in scales as some circular vortices are quiet small and some rather large. If the fluid is turbulent in the mathematical sense, then the color contrast or the intensity of the colors and their change in space varies according to an equation valid for almost all motions at almost all scales. It is this scaling law of turbulent motions that three Mexican physicists tested with regard to Van Gogh’s paintings. They “pretended” that the painting represents the image of a flow that follows the physics of turbulent motions. And their work finds that Van Gogh indeed painted intuitively in ways that mimics nature’s turbulent motions when the physical laws were not yet known.

There are two take-home messages for me: First, fine art and physics often converge in unexpected ways. Second, I now want to know, if nature’s painting of the Labrador Shelf Current follows the same rules. There is a crucial wrinkle in motions impacted by the earth rotations: While the turbulence of Van Gogh or Kolmogorov cascades energy from large to smaller scales, that is, the larger eddies break up into several smaller eddies, for planetary-scale motions influenced by the Coriolis force due to earth’s rotation, the energy moves in the opposite direction, that is, the large eddies get larger as the feed on the smaller eddies. There is always more to discover, alas, but that’s the fun of physics, art, and oceanography.

Aragón, J., Naumis, G., Bai, M., Torres, M., & Maini, P. (2008). Turbulent Luminance in Impassioned van Gogh Paintings Journal of Mathematical Imaging and Vision, 30 (3), 275-283 DOI: 10.1007/s10851-007-0055-0

Ball, P. (2006). Van Gogh painted perfect turbulence news@nature DOI: 10.1038/news060703-17

Wu, Y., Tang, C., & Hannah, C. (2012). The circulation of eastern Canadian seas Progress in Oceanography, 106, 28-48 DOI: 10.1016/j.pocean.2012.06.005

Ocean Paintings off northern Norway

Someone is painting the ocean to the north of Norway and Russia in vivid colors, twirls, and twists for the last two to three days. The exuberance and rich detail reminds me of a Vincent van Gogh. While some may see a divine hand and design at work, I see tiny plants floating in a turbulent ocean. To see the ocean, one needs a satellite in space to cover this painting more than 1000 miles wide and long. To see the plants, one needs a microscope.

Algae blooms off northern Norway Aug.-16, 2011. Spitsbergen is seen in the top left, Norway bottom right, and Novaya Zemya to right.

The painting shown here is coarse and crude, because it is composed of dots that are two kilometers big, but anyone with a fast connection can download the same painting with dots that are two hundred and fifty meters big here. The canvas is bigger than your computer screen, so you will need to scroll around.

The colors are made by stuff in the ocean, called coccolithophores that reflects sunlight back to detectors (light catchers) on the MODIS satellite which for the last 11 years has cycled around the globe shooting a picture every five minutes as it flies overhead. It returns to the same spot every sixteen days, but since all tracks come always together over the North and South Pole, there are many images of the same region in the far north and south of this earth.

Scientists study ice and clouds and glaciers and small plants in and near the oceans using the numbers (digits) that the satellites sent back to earth. On land our soldiers in Afghanistan and Iraq use these and similar observations to prepare for sandstorms on land. Under water, our Navy Seals use them to prepare for visibility in coastal waters. So lots of stuff is done with these data, but the amazing thing to me, and I truly love my country for this, all these data are made available for all to see and for all to use as they see fit. If I download and crunch numbers, anyone can. I’ll teach you, too.