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Entries in oceanography (3)


Taking mapping to a whole new level

When you set out to drive somewhere, or to sail a boat, you put a lot of faith in maps.  You trust that things are where the maps says they are and that there’s nothing where the map says there’s nothing.  Can you imagine the chaos if the map didn’t sync up with reality?  It’s relaitvely easy to make faithful maps for roads because, for the most part, they’re man made, and someone planned and engineered them, so they have good survey data.  But for oceanic charts, its a bit different.  Charts are made by sounding (measuring the depth to the bottom at a specific point) and then joining up all the points of the same depths into contour lines (isobaths).  But because you can’t usually see the bottom over which you sail, you really have to trust that whomever made the chart did a decent job of it.

In this overlain image of two charts, you can see how one (the black lines) does not match the other (coloured lines)

Bill Baxley noticed that some of the charts for the area of the Abrolhos shelf where we are working don’t even match up with each other very well.  That means someone is right and someone is wrong, so Bill - the Harbor Branch director of technical operations for this expedition - is using downtime in between sub dives to better map the bottom.  At night, while the science crew grabs some much-needed shut-eye, Bill and the ship’s crew sail back and forth across the shelf break, taking super accurate readings of the depth using sonar equipment and precisely geolocating the soundings by GPS.  From all these data, he has not only created a much more reliable map for use on future research trips to this area, he’s even able to use GIS software to render the bottom in three dimensions.  In the movie below, Bill animated his map so that you can quite literally see the north shelf break of the Abrolhos platform rotating before your eyes.  He’s even put two submersible tracks on there.  The deeper of the two in red is the one where the bottle was seen, and on which this photo and video post was based.  If this isn’t just about the coolest thing ever, I don’t know what is!


Good old-fashioned home-grown oceanography!

If oceanography had a classic bread-and-butter technique, CTD casts would have to be it.  The C stands for conductivity (basically salinity), the T for temperature and the D for depth.  The “cast” refers to the fact that you measure these three properties as the instruments descend to - and return from - the sea floor.  CTD casts tell scientists about the structure of the water column beneath them.  How can water have structure?  Well, differences in temperature and salinity can lead to layers in the water and these can tell you about how the water is or isnt moving and also have implications for animal life living there.  If you’ve ever swum in a lake where your body was warm but your legs were cold, then you’ve experienced a structured water column, or water layers.  (Strong structure like that often happens in summer when surface waters are warmed by the sun, which makes them less dense, so that they are even more buoyant.  When winter comes, the surface layer cools until it is denser than the underlying water, at which point the surface water sinks and the water column “turns over”)

In a world of side-scan sonar, ADCP and satellite sensing, CTD casts still play a really important role in understanding the water column, so they are still a core part of any oceanographer’s toolkit.  Let’s take a look at one.  This CTD/rosette sampler is part of an instrument package belonging to the Rosenstiel School of Marine Science at the University of Miami:


This rather expensive bit of kit stands about 6 feet high and consist of 24 sample bottles arranged in a ring, with the actual CTD instrument package underneath.  Together, this equipment can make accurate measurements of not only salinity, temperature and depth, but also dissolved oxygen and chlorophyll concentration, AND it can take a 10L sample at any depth using the “rosette” of bottles.  In the following video, U. Miami oceanographers deploy the CTD, then I discuss the acquisition of data with Cepemar oceanographer Carlos Fonseca, and finally graduate student Nelson Alves collects water samples from the sample bottles for a study on bacteria and virus diversity


So far here in Brazil we’ve seen a typical “surface mixed layer”, where the temperature and salinity is the same throughout the top 10-20m.  Below that, temperature drops sharply 4-5 degrees C to a colder underlying layer; this transition is called a thermocline (thermo = temperature, cline = a gradient) and is a standard feature of that well-layered water column.  Below that, temperature drops more gradually, but with some jagged steps that result from “salt fingers”.  These are small-scale turbulence features that tell oceanographers (like Carlos in the video) about mixing processes taking place in the water column.

The CTD - long time friend of oceanographers the world over!


Putting the world's commercial ships to work

The University of Miami’s Peter Ortner calls the Royal Caribbean cruise ship Explorer of the Seas” the world’s most luxurious research vessel”.  That’s because he and his colleagues affixed instrument packages and even built a small lab on the luxury liner.  Why do this?  Well, if you think about it, cruise ships and commercial ships are criss-crossing the oceans all the time.  What an awesome opportunity to collect data!

Commercial shipping lanes of the world - the ultimate scientific transects?

One of the best sorts of data that Peter’s team collects is called ADCP, for Acoustic Doppler Current Profiling.  Its a sonar method of sorts, but not for measuring the distance to the bottom.  Instead, it can tell you the direction and strength of the current (i.e. its vector) at every depth under the ship.  That’s because the speed that sound travels through the water is distorted by current the same way that the speed of sound through air is distorted by speed (you hear this as, for example, the change in pitch when an ambulance goes by). 

ADCP current vectors (black arrows) recorded by a ship and mapped on temperature of the Gulf Stream. You can see how well they match up

By putting a bunch of ADCPs on a bunch of different ships that cruise regular paths, oceanographers can build up a very detailed picture of currents across ocean basins, on a scale that individual oceanographic vessels could never match.  Along the way, they have discovered new features, especially eddies of various sorts in some unexpected places.  An eddy is a circular current, sort of like a gentle cyclone in water; sometimes they form by themselves, but more often they spin off the edge of a current as it passes through another body of water; these are called frontal eddies.  Eddies can go clockwise or anti-clockwise and they can have a warm core or a cold core or a ring-like structure, depending on how they form.  Eddies are important because they profoundly affect the biology within them - either stimulating or dampening productivity.  They can also be really important for weather and climate, because an eddy can take a lot of heat energy from a warm current like, say, the Gulf Stream, and move it somewhere else.  Climate and weather prediction models work much better when eddies are properly accounted for.

What an eddy looks like by ADCP. The ship travels left to right across the top. Red pixels is where water is coming towards you out of the screen, while blue is it going away from you, into the screen


ADCP also gives you BIOLOGICAL data. Here, backscatter shows variations in the distribution of plankton as the ship crosses an eddy like the one in the previous figure

The idea of using commercial ships to collect oceanographic data has proven popular and now a UN committee is working on an implementation plan that would see many ships constantly gathering oceanographic data in all the oceans of the world.  That program is called Oceanscope, and when it reaches maturity, Peter’s dream would have become and reality and he can kick back and watch the data roll in.