Date: Sat, 15 Feb 1997 22:07:21 +0000 (GMT)
From: Jason Goodman <sci5@knorr.whoi.edu>
To: seadiary@MIT.EDU
Subject: Tow-Yo and off to Greenland
Message-Id: <Pine.SUN.3.91.970215213741.15618A-100000@mike.knorr.whoi.edu>
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February 16, 1997
59 59' N, 55 03' W
Temp: -6 C, Winds: 20 m/s
Seas: Calm, but should increase with this much wind.

  Exciting stuff, yesterday!  A bit of background, first.  The water in 
the Labrador Sea doesn't change much with depth in the wintertime.  In 
fact, we've been observing a "mixed layer" 400-600m thick at the surface 
whose temperature is constant to within .01 C.  It forms because when you 
cool the top surface of the water, it becomes denser and sinks until it 
reaches water of like density.  In doing so, it stirs up the water it's 
sinking through.  We've noticed a sharp jump in density at the base of 
this mixed layer.  Yesterday, we saw a place where there wasn't a mixed 
layer: the cold mixed fluid had been covered over, we think, by warm 
water which surrounds the edge of the Lab Sea.  Even more surprising, 
there was a sharp jump in density nearly 2 kilometers below the surface.
This is extremely unusual, and the sharpness of this jump suggests that 
it was created very recently.  Could it be the mark of deep convection 
(the process we're hunting for) which happened last month?  Could it mark 
the top of a blob of warm, salty deep water carried from elsewhere?  
We're not sure.
  To tie things down, we did a "Tow-yo" survey of the area.  We took a
very small CTD package with no sample bottles, 200 lbs of lead weights,
and a "rudder" on one side and lowered it over the side.  Usually, CTD
casts are done while standing still: here, we moved the CTD up and down
through the water while sailing at a few knots forward, so the Tow-yo CTD
made a zig-zag path through the water.  This lets us map out the horizonal
and vertical extent of the deep density jump.  It's a tricky operation,
because the tow-yo trails about 2 km behind the ship, and you have to
balance ship speed and winch speed very carefully to make the tow-yo go
where you want it to.  Also, you can only go in certain directions, or the
wind will push the ship's hull against the taut cable, whose motion will 
saw a hole in the ship.  It's like fly-fishing from a moving pickup truck 
in a hurricane.  With 4 kilometers of fishing line.
  This feature changed radically over just a few kilometers north-south, 
much faster than ocean features generally do.  We're still looking at the 
data to figure it out.  (Turning a bunch of zig-zag paths into a map is a 
real trick, which I'll be working on for the next few days.)
  We're now sailing northeast, toward the coast of Greenland.  We should 
be there tomorrow night.  Ice permitting, we may get within 20 km of the 
coast, so it should be visible if we're still there during daylight.  I'm 
hoping for some great pictures at the expense of lost sleep.

						Jason