Tag Archives: eddies

Sea Ice, Oceanography, and Nature’s Way to Paint

Posted on October 2, 2016 by | Leave a comment

I am going to sea next week boarding the R/V Sikuliaq in Nome, Alaska to sail for 3 days north into the Arctic Ocean. When we arrive in our study area after all this traveling, then we have perhaps 18 days to deploy 20 ocean moorings. I worry that storms and ice will make our lives at sea miserable. So what does a good data scientist do to prepare him or herself? S/he dives into data:

Map northern Chukchi Sea with mooring locations (red and blue symbols), contours of bottom topography, and radar backscatter from space.

Map northern Chukchi Sea with mooring locations (red and blue symbols), contours of bottom topography, and radar backscatter from space. Slightly darker shades especially in the bottom segment are interpreted as sea ice. The offset in grey scale between top and bottom is caused by me using different numbers for two different data segments to bring the data into a range that varies between 0 and 1.

The image above is my first attempt to determine, if our planned mooring deployment locations are free of sea ice or not. The darker tones of gray are sea ice with the white spots probably thicker or piled-up ridges of rougher sea ice. The speckled gray surface to the north is probably caused by surface waves and other “noise” that are pretty random. There is a data point ever 40 meters in this image. It also helps to compare these very high-resolution ice data with products that the US National Ice Center (NIC) and the National Weather Service provide:

Ice Chart of the Alaska office of the National Weather Service (link)

Ice Chart of the Alaska office of the National Weather Service

The above is a wonderful map for general orientation, but it is not good or detailed enough to navigate a ship through the ice. The two maps agree, however, my patch of ice to the south of the moorings are represented as the orange/green patch on the top right (north-east). The orange means that 70-80% of the area is covered by ice and this ice is thicker than 1.2 meters and thus too thick for our ship to break through, but there are always pathways through ice and those can be found with the 40-m resolution maps.

In summary, on Sept.-29, 2016 all our moorings are in open water, but this can change, if the wind moves this math northward. So we are also watching the winds and here I like the analyses of Government Canada

Surface weather analysis from Government Canada for Oct.-2, 2016.

Surface weather analysis from Government Canada for Oct.-2, 2016. The map of surface pressure is centered on the north pole with Alaska at the bottom, Europe on the top, Greenland on the right, and Siberia on the left.

It shows a very low pressure center over Siberia to the south-west and a high pressure center over Arctic Canada to our north-east. This implies a strong wind to the north in our study area. So the ice edge will move north into our study area. If the High moves westward, we would be golden, but the general circulation at these latitudes are from west to east, that is, the Low over Siberia will win and move eastward strengthening the northward flow. That’s the bad news for us, but we still have almost 2 weeks before we should be in the area to start placing our fancy ocean moorings carefully into the water below the ice.

While this “operational” stuff motivated me to dive into the satellite radar data that can “see” through clouds and fog, I am most excited about the discovery that the radar data from the European Space Agency are easy to use with a little clever ingenuity and a powerful laptop (2.5 MHz Mac PowerBook). For example, this hidden gems appeared in the Chukchi Sea a few days earlier:

Close-up of the ice edge in the northern Chukchi Sea on Sept.-23, 2016. The mushroom cloud traced by sea ice and associated eddies are about 10-20 km across.

Close-up of the ice edge in the northern Chukchi Sea on Sept.-23, 2016. The mushroom cloud traced by sea ice and associated eddies are about 10-20 km across.

It is a piece of art, nature’s way to paint the surface of the earth only to destroy this painting the next minute or hour or day to make it all anew. It reminds me of the sand-paintings of some Native American tribes in the South-West of the USA that are washed away the moment they are finished. Here the art is in the painting, just as the pudding is in the eating, and the science is the thinking.

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East Greenland Current Instabilities

Posted on August 22, 2014 by | 4 comments

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

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Posted in Greenland, Oceanography, Uncategorized

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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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Posted in Oceanography

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Petermann Ice Island(s) 2010 through 2011, Part-1

Posted on January 6, 2012 by | 1 comment

An ice island 4 times the size of Manhattan spawned from a remote floating glacier in north-western Greenland the first week in August of 2010, but it quickly broke into at least 3-4 very large pieces as soon as it flowed freely and encountered smaller, but real and rocky islands. A beacon placed on the ice transmit its location several times every day. It shows a rapid transit from the frigid, ice-infested Arctic waters off Canada’s Ellesmere, Devon, and Baffin Islands to the balmier coasts of Labrador and Newfoundland:

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.

Initial progress was slow as it took the new ice island almost 30 days to wiggle itself free of the narrow constraints of Petermann Fjord:

Petermann Glacier discharges its large ice island into Nares Strait on Aug.-30, 2010.

As soon as it left its home port, it hit broke hit tiny Joe Island on Sept.-9, 2010 and broke into two pieces, PII-A and PII-B for Petermann Ice Island A and B. Not a good start for a new island setting out to sail all the way to Newfoundland where PII-A arrived a year later, but I am getting ahead of my story.

Petermann Ice Island breaks into two segments on Sept.-9, 2010 as seen in this radar image provided by the European Space Agency. Greenland is at the bottom right, Canada top left, the Arctic Ocean is at the top right.

Once in Nares Strait both ice islands experienced a very strong and persistent ocean current. PII-A, about 1.5 the size of Manhattan went first followed by the larger (about 2.5 Manhattans) and thicker PII-B. Their tracks follow each other closely and they almost kiss on Oct.-8, 2010 when both are caught in the same eddy or meander of a prominent coastal current flowing south along Ellesmere and Devon Islands.

Pieces of Petermann Ice Island on Oct.-8, 2010 off southern Ellesmere Island about 600-km to the south of their origin. RadarSat imagery is courtesy of Luc Desjardins of the Canadian Ice Service, Government Canada.

Within a week the larger 136 km^2 piece PII-B breaks into three pieces of 93.5, 28.9, and 11.3 km^2 by Oct.-16 while PII-A stays largely intact at 73.6 km^2. These are all very large islands, the land area of Manhattan is about 60 km^2 for comparison. Some of these pieces approach the coast, some become grounded for a few days to a few weeks, some break off smaller pieces and spawn massive ice bergs that are not always visible from space. PII-A enters Lancaster Sound a week ahead of PII-B on Nov.-14, but exits it within 2 weeks:

Multiple pieces spawned from Petermann Ice Island as seen by RadarSat on Nov.-26 and Nov.-28, 2010 composited and anotated by Luc Desjardins of the Canadian Ice Service, Government Canada.

Notice also the evolution of a string of segments that Luc Desjardins of the Canadian Ice Service identified as pieces from Petermann Glacier. Glacier ice has a darker radar backscatter signature than the sea ice around it. All these pieces eventually enter the Baffin Island Current, a prominent large ocean current that extends from the surface to about 200-300 m depth. The Petermann pieces are moved mostly by ocean currents, not winds, because there is more drag on the submerged pieces of the 40-150 meter thick glacier ice. In contrast, the much thinner sea ice is mostly driven by the winds. This is also the reason one often finds areas in the lee of icebergs and islands free of older ice which is swept away by the winds as the iceberg moves slower as it is driven by deeper ocean currents. I will talk more of these in a later post.

As part of a large oceanography program in northern Baffin Bay and Nares Strait in 2003, we collected ocean temperature, salinity, chemistry, and current data along lines roughly perpendicular to both Baffin Island in the west and Greenland in the east along with the trajectory of PII-A in the fall of 2010 (red dots) and the almost identical track of a much smaller ice island from Petermann Glacier that passed the area in 2008:

Map of the study area with trajectory of a 2010 (red) and 2008 (grey) beacons deployed on Petermann Glacier ice islands over topography along with CTD station locations (circles) and thalweg (black line). Nares Strait is to the north of Smith Sound.

I will talk about these data and the subsequent tracks of PII-A and PII-B from 2010 into 2011 in Part-2 of this summary on how the first of this piece (PII-A) arrived off coastal Newfoundland in the late summer of 2011. Rest assured that there are many more pieces coming to coastal Labrador and Newfoundland in 2012 and 2013 where they put on a majestic display of abundant icebergs such as this last remnant of PII-A as seen from the air on Nov.-2, 2011 in Notre Dame Bay, Newfoundland.

Last surviving fragments of PII-A on Nov.-2, 2011 from a survey by air of southern Notre Dame Bay conducted by Canadian Ice Service, Government Canada..

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Swirling Ice in Coastal Waters off Eastern Greenland

Posted on September 12, 2011 by | Leave a comment

Nature provides us with art that is always changing in time and space. Delicate swirls and vortices give a rare glimpse of how the ocean’s surface looked today off eastern Greenland. The data originate from the MODIS/Terra satellite which from 440 miles above the earth captures light that is reflected from anything below. Here it shows the ice-free ocean (bottom right) and Greenland’s ice-free Scoresby Sound (bottom left) in very dark blues, lightly vegetated lands (left) in light blue, and a highly organized pattern of sea ice (top right) in white. The resolution of this image of light just beyond the visible, just beyond the red is about 300 yards and the swirls and elongated filaments are about 3-5 miles. To me, they vividly show the ocean’s surface circulation.

Swirling surface motion on the continental shelf off eastern Greenland Sept.-12, 2011 as indicated by sea ice. Black lines show contours of bottom depth from 300 to 1200 meters in 300 meter increments.

The physics of these motions are similar to those I was reading into another beautiful work of art to the north of Norway. The postulated physics involve the earth’s rotation as well as differences in density. The density of the ocean relates to its temperature a little and to its salinity a lot. Near the coast and at the surface, ocean waters are much fresher and thus lighter than they are offshore and at depth, because Greenland’s melting glaciers and sea ice are fresher than the waters of the Atlantic Ocean. The thin black lines show bottom depths to distinguish the deep Atlantic Ocean to the right in the image from the shallow continental shelf off eastern Greenland to the left in the image. Note that all the swirls, eddies, and filaments are within 30 kilometers (20 miles) off the coast in water less than 300-m deep. The same physics apply to the algal blooms off Norway which is the reason that the swirls and eddies are of similar size here and there as well.

Incidentally, the same physics also apply the discharges from rivers and estuaries such as the Delaware or Cheasapeake Bay. There, the pattern are not quiet as visible to the naked (satellite) eye as off Norway or Greenland, but if one takes measurements of the ocean, similar patterns of ocean salinity and velocity as, I speculate, they do here for the ice (Greenland) and algae blooms (Norway). While my academic journey of fresh water discharges started with the discharge of the Delaware River into the Atlantic almost 25 years ago, I am still fascinated by the many ways these patterns always come back to me. Physics and oceanography are beautiful in both their many natural manifestations and its unique balance of forces. There is so much more in how the oceans interact with the ice and glaciers off Greenland and elsewhere. To be continued …

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