Tag Archives: coriolis

Travels by Mind to the Glaciers and Oceans off North-East Greenland

Our minds travel easier than the body. My eyes have never seen East Greenland, but I moved across its white Continue reading

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

Arctic Sea Ice Cover and Extreme Weather Explained

Addendum Sept.-24, 2012: A New Climate State, Arctic Sea Ice 2012 (video by Peter Sinclair).

I just discovered an outstanding interview that Dr. Jennifer Francis of Rutgers University gave to a non-profit community radio station out of Vancouver, British Columbia.

Jennifer Francis Interview 20120910

She connects and explains global warming, its much amplified signal in the Arctic, the extreme record minimal Arctic sea ice cover this summer, and how the warming Arctic and its disappearing sea ice impacts our weather in the northern hemisphere by slowing down the atmospheric jet stream separating polar from mid-latitude air masses. She explains all of this in non-technical language without loss of accuracy.

Dr. Jennifer Francis, Rutgers University [Photo Credit: ARCUS]

If this program piques your interest and you want to read more, Andrew Revkin of the New York time has led an informed discussion at his New York Times blog Dot Earth. And finally, Climate Central presented and illustrated Dr. Francis’ observations and ideas rather well with graphics and videos.

New Ocean Observations in Petermann Fjord

A new ice island separated from Petermann Glacier on July 16, 2012 as reported here first. Less than 4 weeks later, the Canadian Coast Guard Ship Henry Larsen reconnoitered the ice island on Aug.-9 when it blocked the northern half of the entrance of the fjord.

Petermann Ice Island 2012 (PII-2012) as seen Aug.-11, 2012 at the entrance of Petermann Fjord. The view is to the north-west. [Photo Credit: Canadian Coast Guard Ship Henry Larsen.]

I was aboard this ship when Captain Wayne Duffett decide to break into the largely ice-free fjord behind the ice-island after consultations with Ice Services Specialist Erin Clarke. The ice observer had just returned from her second helicopter survey in 2 days with pilot Don Dobbin to assess both ice cover and its time rate of change. From the time the ship entered the fjord behind the ice island, hourly flights to a fixed point at the south-western corner of the ice island ensured that its movement would not cut off the ship’s exit. This approach worked and it gave the science crew of 8 aboard about 18 hours to conduct the very first survey of a previously ice-covered ocean:

Petermann Glacier, Fjord, and Ice Island as seen by MODIS at 865 nm on Aug. 07, 2012 overlaid with survey lines of CCGS Henry Larsen on Aug.-9/10/11, 2012 in red.

We were not funded to do enter the fjord, but our main mission to recover an array of ocean moorings with 3-year long data records covering the 2009-12 period about 100 km to the south in Nares Strait has already been accomplished. So, what does a physical oceanographer do when in uncharted and unknown territory? He drops a number of CTDs, that is, measuring conductivity (C), temperature (T), and depth (D, pressure, really) as the instrument (the CTD) is lowered at a constant rate from the surface to the bottom of the ocean at a number of stations. The results from such work next to the present front of Petermann Glacier was a surprise for which we do not yet have a satisfactory explanation: The waters inside the fjord are much warmer at salinities 32.5-34.25 than they are outside the fjord:

Temperature as a function of salinity from 9 stations across Petermann Fjord next to the current seaward edge of Petermann Glacier on Aug.-10, 2012 in red. For comparison I show in blue a station done outside the fjord on Aug.-9, 2012. Note that temperatures increase with increasing salinity which is expected for waters that are a mixture of cold and fresh polar and saltier and warmer Atlantic waters. Density deviations from 1000 kg/m^3 are shown as solid contours along with the freezing temperature that decreases with increasing salinity.

Another way to show the same data is to actually plot the section, that is, the distribution of temperature and salinity in physical space across the fjord as a function of depth:

Section across the seaward edge Petermann Glacier on Aug.-10, 2012 for salinity (left panel) and temperature (right panel). Symbols indicate station locations from which color contours are drawn. Note that the display is cropped to the top 300 meters while real recordings extend to the bottom which exceeds 1000 meters. The view is eastward towards the glacier with north to the left.

Note the doming salinity contours which to classically trained oceanographers suggest a flow out of the page on the right and into the page on left with maximum at about 90 meter depth relative to no flow at, say, 500 meter depth. Another way to view this distinct property distribution is that the flow above 90 meters is clockwise (outflow on left, inflow on right) relative to the more counter-clockwise flow below this depth. This feature, too, comes as a surprise and requires more thought and analyses to explain.

There is much more work to be done to figure out what all this means. I feel like scratching the surface of a large iceberg half-blind. The data from below 300 meter depth, too, contain clues on how some this glacier interacts with the ocean. As for the purpose of this post, I merely wanted to report that the ice island is presently having a hitting or scratching tiny Hans Island. The latter is unlikely to move, but Petermann’s Ice Island will slow on impact, swivel counter-clockwise, bump into Ellesmere, and pretend nothing has happened on its merry way south. This is the latest image I have:

Petermann Ice Island 2012 on Aug.-22, 2012 as seen by MODIS Terra at 21:45 UTC. The tiny red dot marks Hans Island, the location of a weather station in the Kennedy Channel section of Nares Strait. Petermann Fjord is towards the top right out of view.

ADDENDUM Sept.-1, 2012: PII-2010B had a maximum thickness of at least 144 meters as it passed over a mooring that measures ocean currents from the Doppler shift of acoustic backscatter that is shown here for one of four beams:

Time-depth series of acoustic scatter from a bottom-mounted acoustic Doppler current profiler for 24 hours starting Sept-22, 2010 9:30 UTC. Red colors indicate high backscatter from a “hard” surface like ice. The vertical axis depth in meters above the transducers while the horizontal is ensemble number into the record (0.5 hours between ensembles). The 2010 ice island from Petermann Glacier (PII-2010B) passed over the mooring. When PII-2010B was attached to the glacier it was adjacent to the segment that became PII-2012 this year.

Ice Island Flotilla From Petermann Glacier Continue Southward Flow

More icebergs and ice island from Greenland are heading south along northern North-America this year. Petermann Glacier’s first piece arrived last year off Newfoundland causing a local tourist sensation for a stunning display of ice along its shores. There are many more pieces from Petermann to come for a few more years.

Track of Petermann Ice Island from Aug.-2010 through Aug.-2011 traveling in shallow water from northern Greenland along Baffin Island and Labrador to Newfoundland.

April 29/30, 2012 locations of Petermann Ice Island 2010 on their way south along northern North America. [Credit: Luc Desjardins, Canadian Ice Service]

Yet, how come that these arrivals are both so predictable in their pattern, but are almost impossible to pin down for an exact location and time? The answer involves mystical and fake forces, stunningly beautiful experiments, elegant mathematical equations, and, most important of all: spin.

The earth spins rapidly around its axis and neither ocean nor glaciers leave the planet for outer space. The obvious answer that gravity holds all the pieces in place is neither the correct nor the full answer. A subtle balance of several other forces makes Planet Earth the perfect place to keep us supplied with water to drink and air to breath. Additional forces besides gravity relate to the difference in pressure between the top and the bottom of the ocean as well as the rotational force that forces our car off the road if we speed too fast around a curve. The net effect of these is that earth fatter at the equator than at the North Pole. There appears to be more of gravity pulling us in at the North Pole than there is at the equator. Put another way, a scale measuring our own weight dips almost a pound more in Arctic Greenland than it does in the tropical forests of Borneo even if we do it naked in both places. Lose a pound of your weight instantly, travel to the far north. (GRACE)

This makes no sense intuitively, but common sense and intuition help little when it comes to how the ocean’s water and the atmosphere’s air move on a rotating planet. For example, we all know intuitively that a down-pour of rain flows down a slope into the ditch. It requires work to bring water up to the top of a hill or into the water towers to make sure that water flows when we open the faucet. Not true for the ocean at scales that relate to climate, weather, and changes of both. Here all water flows along, not down the hill. Better yet, it requires no work at all to keep it moving that way for all times. This is why Greenland’s ice keeps coming our way as soon as pieces break off. The earth’s spin makes it go around the hill, to speak loosely of pressure differences. Winds and friction have little effect. The ocean’s natural and usually stable state is in geostrophic balance. Geostrophy is a fancy word for saying that the ocean’s water flows along, not down a hill, because it is balanced by a fake and mystical Coriolis force that I will not explain. I teach a graduate class on Geophysical Fluid Dynamics for that.

In technical language, most of the oceans tend to flow along not down a pressure gradient. A kettle of boiling water discharges water from high pressure inside the kettle to the lower pressure in the kitchen. Yet the steam dissolved in the atmosphere moves around high or low-pressure systems. That’s how we read weather maps: Clockwise winds around high-pressure over Europe, North-America, and Asia to the north of the equator, counter-clockwise winds around low-pressure systems. If I apply this spin-law to Baffin Bay containing all the icebergs and ice islands, the spin rule states that these large and deep pieces flow along lines where the earth’s local rate of rotation, lets call it planetary spin f, divided by the local water depth, lets call it H, is a constant. So, to a first approximation, the icebergs and ice islands flow along a path where f/H is constant. If the planetary spin is constant, then the ice island follow lines of constant water depth H. There is more to the story, much more, such as the effects of waters of different densities residing next to each other, but I better continue this later, as I got a dinner date with a sweetheart and “Thermal Wind” can wait 😉