Category Archives: Oceanography

The Turbulence of Van Gogh and the Labrador Shelf Current

Posted on May 8, 2013 by | 3 comments

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

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Oceanography and Icebergs in Baffin Bay: LCDR Edward “Iceberg” Smith

Posted on March 27, 2013 by | 4 comments

In 1928 Edward H. “Iceberg” Smith took the 125 feet long Coast Guard Cutter “Marion” on an 8,100 mile journey from Boston, MA to New York City, NY via Disko Bay, Greenland. Along the way he defined operational Arctic Oceanography to explain and predict iceberg entering the busy sea lanes off North-America. The Titanic was sunk in 1912, the International Ice Patrol was formed in 1914, and LCDR Smith sailed to Greenland in 1928. The data are priceless 85 years later still. I used them to place modern observations from 2003 into a context of climate variations. First, however, let me give credit to one of the pioneers on whose scientific shoulders I stand:

Edward H. "Iceberg" Smith of the US Coast Guard with reversing thermometer.

Edward H. “Iceberg” Smith of the US Coast Guard with reversing thermometer.

“Iceberg” Smith entered the Coast Guard Academy at age 21 in 1910 and served during World War I as a navigator on Atlantic convoy escort duty. After this war his ship was detailed to the International Ice Patrol and he became one of its first scientific observers at age 32 in 1921. As such he was sent for a year to Bergen, Norway in 1925 to learn the latest theories in physical oceanography. Scandinavian explorers like Nansen, Ekman, Sverdrup, Bjerknes, and Helland-Hansen defined physical oceanography at this time by applying physics on a rotating earth to phenomena that they observed from ships sailing at sea or ships frozen in Arctic ice. Much of this revolutionary work is now elementary oceanography taught in introductory courses, but then, nobody knew much about why ice and ocean move they way they do. It was time to put ideas to a thorough test which is what “Iceberg” Smith did, when he got his ship and orders to explore in 1928.

USCGC Marion built in 1927 [from http://laesser.org/125-wsc/]

USCGC Marion built in 1927. Note the scale indicated by a person standing on the lower deck. [From http://laesser.org/125-wsc]

Armed with new ideas, knowledge, and the tiny USCGC Marion “Iceberg” Smith set to out to map seas between Labrador, Baffin Island, and Greenland to explain and predict the number of icebergs to enter the North-Atlantic Ocean. During his 10 weeks at sea he mapped ocean currents from over 2000 discrete measurements of temperature and salinity at many depths. This was before computers, GPS, and electronics. In 1928 this was slow to work with cold water collected in bottles with “reversing thermometers” that cut off the mercury to preserve temperatures measured in the ocean at depth to be read later aboard. Salinity was measured at sea by tedious chemical titrations. Imagine doing all of this from a rocking and rolling shallow draft cutter that bounces in icy seas for 10 weeks within fog much of the time. No radar to warn of icebergs either, and you want to study icebergs, so you move exactly where they are or where you think they are coming from. And they though that the Titanic was unsinkable.

Iceberg in the fog off Upernarvik, Greenland in July of 2003. [Photo Credit: Andreas Muenchow]

Iceberg in the fog off Upernavik, Greenland in July of 2003. [Photo Credit: Andreas Muenchow]

USCGC Healy in northern Baffin Bay in July 2003 with iceberg. Ellesmere Island is in the background.

USCGC Healy in northern Baffin Bay in July 2003 with iceberg. Ellesmere Island is in the background.

The 1928 Marion Expedition was the first US Coast Guard survey in Baffin Bay while the last such expedition took place 2003. Unlike “Iceberg” Smith we then had military grade GPS, radar, and sonar systems. These sensor systems allowed me to directly measure ocean currents from the moving ship every minute continuously from the surface to about 600 meters down. Oh, we also took water samples in bottles, but temperature, depth, and salinity are all measured electronically about 24 times every second. As a result we can actually test, if the physics that had to be assumed to be true in 1928 actually are true. As it turns out, the old theory to estimate currents from temperature and salinity sections works well off Canada, but not so well off Greenland. Furthermore, we found several eddies or vortices in the ocean from the current profiling sonars.

And finally, it took Edward H. “Iceberg” Smith only 3 years to publish most of his data and insightful interpretations while I am still working on both his and my own data 85 years and 10 years later, respectively. Sure, I got more data from a wider range of moored, ship-borne, and air-borne sensors, but I do wonder, if I really consider my data and interpretations as careful and think as thorough as LCDR Smith did. Furthermore, he had no computers and performed all calculations, crafted all graphs, and typed all reports tediously by hand. I would not want to trade, but all this makes me admire his skills, dedication, and accomplishments even more.

Dr Helen Johnson on acoustic Doppler current profiler (sonar to measure ocean velocity) watch aboard the USCGC Healy in Baffin Bay in 2003. [Photo credit: Andreas Muenchow]

Dr Helen Johnson on acoustic Doppler current profiler (sonar to measure ocean velocity) watch aboard the USCGC Healy in Baffin Bay in 2003. [Photo credit: Andreas Muenchow]

P.S.: The New Yorker has three stories on the subject published in 1938, 1949, and 1959. I eagerly await to read those.

ResearchBlogging.orgSmith, E. (1928). EXPEDITION OF U. S. COAST GUARD CUTTER MARION TO THE REGION OF DAVIS STRAIT IN 1928 Science, 68 (1768), 469-470 DOI: 10.1126/science.68.1768.469

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Oceanography, Technology, and Ships

Posted on March 15, 2013 by | Leave a comment

Sea-going oceanography is in transition. Times are exciting as we developed new tools, sensors, and ideas on how to observe the ocean and the stuff that lives in it, floats on it, and is submerged below it. I just learned about an awesome interview with Eli Kintisch which is posted as a podcast at the American Association for the Advancement of Science:

Better technology, but less money: Eli Kintisch discusses the crossroads facing U.S. oceanography.(Podcast)

I will write more about this, but I have to run off to meet with an electrical engineer to discuss ideas on how we perhaps can get data from bottom-mounted sensors out of the ocean in ice-covered seas instantly, rather than waiting 2-3 years to get instruments back with a ship.

Kintisch, E. (2013). A Sea Change for U.S. Oceanography Science, 339 (6124), 1138-1143 DOI: 10.1126/science.339.6124.1138

Seal with ocean sensor.

Seal with ocean sensor.

Elephant seal off Antarctica with ocean sensor transmitting data via satellite [Credit Lars Boehme]

Elephant seal off Antarctica with ocean sensor transmitting data via satellite [Credit Lars Boehme]

CCGS Henry Larsen in thick and multi-year ice of Nares Strait in August 2009. View is to the south with Greenland in the background. [Photo Credit: Dr. Helen Johnson]

CCGS Henry Larsen in thick and multi-year ice of Nares Strait in August 2009. View is to the south with Greenland in the background. [Photo Credit: Dr. Helen Johnson]

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Shades of White as the Sun Rises over Nares Strait

Posted on March 5, 2013 by | 5 comments

After four months of total darkness the sun is back up in Nares Strait. It transforms the polar night into thousand shades of white as mountains, glaciers, and ice take in and throw back the new light. Our satellites receive some of the throw-away light as the landscape reflects it back into space. During the long dark winter months these satellites could only “see” heat, but this will change rapidly as Alert atop of Arctic Canada receives 30 minutes more sun with each passing day.

Surface temperature in degrees centigrade over northern Baffin Bay on March-4, 2013 16:20 UTC from MODIS Terra.

Surface temperature in degrees centigrade over northern Baffin Bay on March-4, 2013 16:20 UTC from MODIS Terra. Warm colors (reds) show thin and/or ice while cold colors (blues) suggest thick ice stuck in place.

A very strong ice arch at the southern entrance to Nares Strait separates thick (and cold) ice to north from thin (and warm) ice to the south. The thick and cold ice is not moving, it is stuck to land, but the ocean under the ice is moving fast from north to south. The ocean currents thus sweep the newly formed thin ice away to the south. This ice arch formed way back in early November just after the sun set for winter over Nares Strait.

Now that the sun is up, we can also “see” more structures in the ice by the amount of light reflected back to space. A very white surface reflects lots while a darker surface reflects less. We are looking at the many shades of white here … even though I color them in reds and blues:

Surface reflectance at 865 nm in northern Baffin Bay on March-4, 2013 16:20 UTC from MODIS Terra.

Surface reflectance at 865 nm in northern Baffin Bay on March-4, 2013 16:20 UTC from MODIS Terra. A true color image (which this is not) would show only white everywhere. Hence I show the very bright white as red and the less bright white as blue. This artificial enhancement makes patterns and structures more visible to the eye.

Zooming into the area where the ice arch separates thick ice to the north that is not moving from thin ice in the south that is swept away by ocean currents, I show this image at the highest possible resolution:

Surface reflectance at 865 nm at the southern entrance to Nares Strait on March-4, 2013. Contours are 200-m bottom depth showing PII2012 grounded at the north-eastern sector of the ice arch.

Surface reflectance at 865 nm at the southern entrance to Nares Strait on March-4, 2013. Contours are 200-m bottom depth showing PII2012 grounded at the north-eastern sector of the ice arch.

Note, however, that the sun is far to south and barely peeking over the horizon. This low sun angle shows up as shadows cast by mountains. And since the sun is still far to the south, the shadows cast are to the north. This “shadow” makes visible the ice island from Petermann Gletscher that anchors this ice arch as it is grounded. I labeled it PII2012 in the picture.

From laser measurements we know that the ice islands stands about 20 meter (or 60 feet) above the rest of the ice field. This height is enough to cast a visible shadow towards the north (slightly darker = less red) as well as a direct reflection off its vertical wall facing south (brighter = more red) towards the sun. At its thickest point, PII2012 is about 200 meters (~600 feet) thick. For this reason, I also show the 200-m bottom contour that moves largely from north to south along both Ellesmere Island, Canada on the left and Greenland on the right.

The sun brings great joy to all, especially those hardy souls who live in the far north. The sun’s rise also shows the delicate interplay of light and shadows that we can use to solve puzzles on how ice, oceans, and glaciers work. At the entrance of Nares Strait the playful delights of the sea ice, ocean currents, and ice islands gives us a large area of thin ice. The thin ice will soon melt and perhaps has already started to set into motion a spring bloom of ocean plants. Ocean critters will feed on these to start another cycle of life. Whales, seals, and polar bears all depend on it for 1000s of years now.

Sketch of the biological pieces that a large area of open water near a fixed ice edge like that of a polynya may support. [From Northern Journal>/a>]

Sketch of the biological pieces that a large area of open water near a fixed ice edge like that of a polynya may support. [From Northern Journal]

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Storm Surges, Global Warming, and Delaware Beaches

Posted on November 4, 2012 by | 5 comments

ADDENDUM (Nov.-7, 2012): Time lapse video from Delaware Sea Grant.

Rising seas and flood waters cause most of the damage during storms such as Sandy did last week. Tides, waves, and storms all contribute. We can debate how global warming impacts any of the above, but the arguments are involved. So lets assume, that neither tides, waves, nor storms are impacted by global warming, but that the globally averaged rise in sea level over the last 50 or 100 years is. This global warming induced sea level rise is about half a foot in 50 years (3 mm/year), but why would we care about global averages, when we live in Delaware? Furthermore, why worry about the whimpy surges we get ever 2-3 weeks. We don’t, we worry most about the most extreme events like Sandy and want to know how often they occur. Below I show a Sandy-like event to occur about once every 10 years. Furthermore, over time Delaware’s most extreme storm surges are rising twice as fast as global averages do. So, how much does the global warming impact our local flooding in Delaware?

Market Street on the beach in Lewes is in one of the lowest lying areas of town and takes its good old-time draining. This photograph looks northeastward toward the beach, just west of the intersection with Massachusetts Avenue. [Credit: Cape Gazette]

More than I initially thought: the largest storm surge that has hit Delaware was the Ash Wednesday storm on March 6, 1962 which added 5.8 feet to the regular tides and waves. I wrote about this yesterday using public NOAA data. This same storm today would add 6.8 feet to the regular tides and waves. For comparison, Sandy’s storm surge added 5.3 feet. So Sandy was a weak storm by comparison. If it had hit in 1962, it would have added only 4.3 feet. The difference of 1 foot in 50 years is due to steadily rising sea levels:

Largest storm surge at Lewes, Delaware each year from 1957 to present. The red line is a linear fit to the data. The slope indicates that the largest storm surge increase by almost 3 inches every 10 years.

On average each year has a larger largest surge than the year before. While this steady increase by 2.8 +/- 1.7 inches each 10 years is statistically significant (95% confidence), picking the extreme each year is perhaps not the best statistic as extremes do not happen often. Please note that a 95% confidence means that there is a 5% chance that the true increase is either smaller than 0.9 inches/decade or larger than 4.5 inches/decade.

What about the mean or average surge each year? From hourly data, I pick the middle surge, that is, half the surges each year are larger and half are smaller:

This increase of 1.4 +/- 0.2 inches per decade (95% confidence) is more in line of the global average. The uncertainty in this trend is smaller than that of the trend for the extreme, because the median sea level varies little from year to year, while the extreme value varies more from year to year. So, from these results we can conclude, that while the mean or median sea level at Lewes increases by perhaps 1.5 inches in 10 years, the extremes increase twice as fast. So, storm surges like Sandy will become more common than they are today mostly because of global warming.

Over the last 50 years we had at least 5 such events in 1962, 1968, 1996, 1998, and 2012. So, on average we have a Sandy-type storm surge greater than 5 feet every 10 years. This contradicts a Wilmington News Journal article today which quotes John Ramsey to describe “… Sandy as a 1-in-200-years storm, unlikely to be repeated anytime soon. That could give coastal communities time enough to deal with the real threats and realities of sea level rise and climate change.” There is no such time, as it is mis-leading to describe Sandy as a 1-in-200-year event when it has happened about every 10 years during the last 55 years. Instead of a 0.5% percent chance of a Sandy-like event to hit Lewes each year, I would raise this chance to be larger than 10%.

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