Friday, September 20, 2013

Snakes on a ship

Guest Blogger:  Dr. Ben Hodges (WHOI)

As the Endeavor steams along, seawater is drawn in continuously and analyzed with onboard sensors.  We left Ponta Delgada yesterday morning, heading south-southwest toward a mooring that needs a repair.  When we arrive there in a few days, we’ll have a record of changes in salinity, temperature, and chlorophyll concentration every second along a line over a thousand miles long.  This line is about 5 meters below the surface of the ocean, since the seawater intake in the bottom of the hull is about that deep.

Wind tends to stir up the upper layer of the ocean, keeping it homogenous, so the water 5 meters deep and right at the surface are usually pretty much the same.  On very calm days though, there are sometimes big changes near the surface, even in the top meter.   Right after it rains, there might be a “puddle” of fresher water sitting on top of the saltier water below; or, on a hot sunny day, evaporation from the surface might make it saltier than the deeper water. We’re using a “snake” to suck up water from just beneath the surface, so we can keep track of these changes in salinity as we steam along.  The snake is a hose that drags along from the end of a long boom, which keeps it away from the side of the ship—too close, and it might take in deeper water churned up by the Endeavor.

Seawater enters the snake from a few inches beneath the surface through small holes in the bottom of the hose.  Inside the ship, a pump moves the water through a system that removes air bubbles, and then measures the amount of dissolved salt it contains.  

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