Archive for May 25th, 2010

Back to Cook – for a moment

Tuesday, May 25th, 2010

If you have been wondering where we have been, its not because we have forgotten Cook.  We are part of the U.S. IOOS response to the Gulf of Mexico oil spill.  Check out the website

If you click on the DeepWater Blog, you will immediately recognize the format, the google earth images, the glider coordination and the bloggers names from the flights of RU17, RU27, Drake, Cook, the Middle Atlantic Bight and Antarctic Observatories.  It is truly amazing to see the collaboration in the oceanographic community's response.   With so many people willing to help, the need for a collaborative website to keep responders informed was identified.  The tools we developed for undergraduate education are now being used for that task.  It is a wonderful example of education feeding back to society.

So back to Cook, even if just for the moment before heading back to the Gulf.  Tod and Amelia both alerted me to the need for a new waypoint.  Good thing we have multiple eyes watching.  The waypoint will generally move to the east about every week or two this summer, but there will be refinements as we balance the desire to maintain the transect with the realities of flying a glider.

The first image below shows the altimeter data from Colorado.  As usual, the geostrophic surface currents (black arrows) calculated from the observed sea surface height (color fill) are in good agreement with the glider depth averaged currents.  This is always amazing to consider, that the average current over 1200 m depth is so well correlated with the surface current observed via a satellite.  The question we wanted to answer over the past week was the direction of the current along the 62.5 W meridian. Is that current flowing south as it is in the altimeter data, or southeast as it is in the HyCOM model.  Here we see pretty good agreement between Cook and the altimetry for the southward flowing jet.  Also according to the altimeter, Cook is entering a long stretch about 3 degrees of longitude wide of opposing currents.  Flying into a head current will be a good test for Cook, if the head current is really there.   The questions are what is the flow between the two clockwise rotating eddies circled in blue and magenta.  These are highs (reds) in the sea surface height contours.

Below we show the same glider track, currents, and blue & magenta ovals overlaid on the HyCOM forecast of currents (vectors) and sea surface height (color fill). The northern clockwise eddy outlined in blue is very close in both the HyCOM and the altimetry data. The Southern clockwise eddy outlined in magenta is very different.  Now the area in between the 2 eddies has a northward flowing current, a very different result than the headcurrent suggested by the altimetry.  All the more reason to fly into this region along the 26.5 N line. The thing that worries me is that if you look along 62.5 W, that jet that Cook crossed is flowing to the southwest in the model and to the south in the altimetry.  Cook's southward flowing currents in this region indicate that the southward flowing currents in the altimetry are likely more correct.  That means we are more likely to be flying into a head current.  But that is what we want to do.  These are deep glider profiles, so they should be exactly what the Navy needs for assimilation into HyCOM.  I hope we cn pass the data through to them.  We'll set the next waypoint for Cook right along 26.5 N and just on the other side of this region of uncertainty.  This is exactly what gliders do best, and why we want to augment the drifter array with gliders.  You can't tell a drifter where to go.  But a glider can be flown directly into the unknown, directly into the region of highest uncertainty.

To round out the guidance, here is the overlay of the HyCOM currents on the satellite Sea Surface temperature.  Some features do line up, the the guidance here is patchy.  The surface waters are likely warming with the coming of summer, masking the deep signals below.  This is when we hope for storms.  Some of the best satellite infrared images are found in the wake of a storm after the winds have mixed away the surface warming.