Archive for October 22nd, 2009

Persistence Pays Off – Scaling “The Wall”

Thursday, October 22nd, 2009


A week ago starting on October 14, Scarlet came upon a counter-current on her trip east.  The westward flowing currents we not present in the large-scale geostrophic currents derived from the satellite altimeter maps of Sea Surface Height, and the area was covered with peristent clouds, blocking the view of the statellite infrared imagers that give us Sea Surface Temperature.  There was a small scale feature out there, and our satellites couldn't see it.  All we had for guidance was the data Scarlet was collecting herself.  Over the next week we would use Scarlet as the explorer, and learn that this small scale feature was actually about 80 km long.  It also would be given a name by people working in the lab.  It came to be known as The Wall.


It took 4 attempts for Scarlet to cross The Wall. The first was on October 15 when she encountered strong currents to the west that slowed her progress towards Spain to a crawl.  We did have guidance from the satellite altimeters that this westward flowing current extended a couple hundred kilometers to the east, so there was no way to burst through.  We had to go around.  Going around to the south meant warmer water that promotes biological growth and increasing our distance from our targeted pick up point.  So Scarlet turned north, flying perpendicular to the westward current in a sweeping arc for a second attempt 3 days later on October 18.  This route showed promise, with a report of nearly zero currents at one point.  We expected this would be the front we were looking for, something that identified the strong sheer zone we expected to find between strong westward flowing currents and strong northward flowing currents.  But 8 hours later, these hopes were dashed with a report that the westward currents were back, and even stronger.  We went back to flying north, and letting the currents advect us west.  Two days later, on October 20, the first clear satellite image came in, it was digitally enhanced, and we got our first look at The Wall from space.  It was a clockwise rotating eddy that was not being resolved by the satellite altimeters.  We saw the eastern edge of the eddy was only a few 10's of kilometers away, so we again turned Scarlet east for our third attempt.  Again, we encountered strong currents that stopped us dead. We were pretty dissapointed by this news, but we have also been in these situations before. We turned Scarlet north again, persisting in our efforts to explore The Wall and find a way through.  Then earlier yesterday we began to detect a change in the currents.  We saw the current directions switch from flowing mostly to the southeast to almost due south.  That meant we were approaching the outer edge of the eddy on its western side.  With our hope renewed, and for a fourth time, we turned Scarlet east starting yesterday at noon.  This time we would be heading for a distant waypoint so that Scarlet would use all of her energy to just fly east towards what we hoped was the eastern edge of The Wall.  Then, this morning, at the 4 am surfacing, Scarlet reported a nearly 180 degree shift in the current direction.  Currents were now flowing nearly straight north, and are back in agreement with the larger-scale altimetry maps.  After a week of exploring, we had found the eastern edge of The Wall and crossed through it.   Surprizingly the sheer zone between the two water masses on either side is only a few kilometers wide.  Probably the location of a big biological party. 

Thanks to all our partners on both sides of the Atlantic that provided guidance data during this most difficult week since the Azores. Today we begin the recalculations of arrival times.