Archive for May, 2010

Cook plows forward

Sunday, May 30th, 2010

Cook is plowing into the head currents we saw in the previous post, and continuing to make good progress.  Cook is a fast glider, with a forward speed somewhere around 36 cm/sec.  Depth average currents are running just under 20 cm/sec.  In the image below, we see the glider currents (while flags along the track) line up well with the geostrophic currents from the altimeter.

And we see the same good agreement between the glider and the HyCOM model sea surface heights and surface currents.  With HyCOM we expect to cross into a burst of currents to the north, but according to the altimetry, it remains a head current.

If we switch to the Satellite sea surface temperature maps, we see a persistent region of cloud cover right over Glider Cook.  Why is that?

Switching on the clouds in google earth, we see a long line of clouds crossing the full North Atlantic.  If it is moving east, the sst imagery will start to fill in behind it with about a day or 2 delay. That will give us a good fresh look at the ocean surface.  So what is causing that long band of clouds?

For this we go to one one the Jet Stream forecast websites.  http://squall.sfsu.edu/crws/jetstream.html

Below is the present analysis with that long band of strong upper level winds running across the North Atlantic.

Overlaying that analysis on the visible cloud imagery, we see that that broad band of clouds is associated with the Jet Stream.  Our first lesson for the summer undergraduate internship program is now complete.

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

http://rucool.marine.rutgers.edu/deepwater/

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.

Is the Gulf Stream loosing some momentum?

Friday, May 21st, 2010

Hey all,

So I was just fooling around in google earth and happened to have open 27's path along with current sea surface temperature data (updated May 21, 2010 @ 13:03:27 GMT) when something pretty interesting caught my eye.  Surprisingly the path 27 followed once she hit the gulf stream matched perfectly with what todays updated SST map showed how the currents were moving.  When I looked a bit closer however, the date at which 27 was at the position that is currently where the very tail end of the Gulf Stream is extending to is only May 12, 2009.  May 21 of last year we were a good 1000 km ahead of the same spot.  Could this mean the Gulf Stream is a little over a week behind schedule compared to last year?  I guess it's a good thing we completed the mission last year otherwise we might have lost some time waiting for the Gulf Stream to catch up...

Nilsen

Cook Approaches the Center of the First Eddy

Sunday, May 16th, 2010

Cook is now approaching the center of the first eddy along the 26.5 N.  We experienced strong currents to the north up until now.

100516_cook_sst

Altimeter map below shows us in the center of the eddy.  We will soon hot a strong current to the south that brings us back to the 26.5 N line as we continue to head to the deep waypoint. Note the U shape to this current just to our east.  First we are pushed south, then north.

100516_cook_alt

Now check the HyCOM results below. The U-shape is not present.  It is now a straight shot to the southeast.  Cook will be flying into this region of disagreement.

100516_cook_hycom

And below is the overlay of HyCOM currents in white and altimeter currents in black.  Area of disagreement is the U-shape.  Other currents and eddies in the region appear to line up.

100516_cook_hycom_alt

We have noted this disagreement before.  It appears to persist in the model and the altimetry.  A good question for Cook to investigate.

Cook Turns East – Trans-Atlantic II Begins

Wednesday, May 12th, 2010

Yesterday the Thermal Glider Cook reached a latitude of 26 degrees, 30 minutes north, or 26.5 N as it has come to be known. We approached 26.5 N just east of 65 W.  Once we crossed the 26.5 N latitude line, Tod turned Cook east towards the Canaries.  Cook has flown just over 1300 kilometers since leaving St. Thomas on March 21.  Today we start our 2010 summer crossing along a more southerly route than RU27's 2009 crossing.   Only 5,000 kilometers to go till we get to the Canaries.  Timing is perfect.  Today we have a skype call scheduled with our partners in the Canaries on what we can do with this crossing.

100512_cook_big

Zooming into the region around Cook, we get three views of the eddy field.   First the satellite sea surface temperature (SST).  We are spinning around that clockwise eddy with some of the strongest currents we have seen on this trip pushing us to the northeast.

100512_cook_sst

Next the  satellite sea surface height (SSH).  The altimeters say we have a long ribbon of current heading east as we fly eddy to eddy to eddy.

100512_cook_alt

Last is the Navy HyCOM forecast.  The HyCOM model assimilates the SST and SSH satellite data, uses our knowledge of ocean physics to produce calculate the dynamical balances based on Newtons laws (F=ma), and then uses these balanced these fields to produce a forecast. It gives us a third look at the deep ocean eddy field.

100512_cook_hycom

In the last blog I noted the great agreement between the SST and SSH satellite products.  Today I am amazed by how well the HyCOM model (white arrows) agree with the altimeter SSH data (color contours and black arrows).  I know they are supposed to, since the model is assimilating the altimeter data. But as we learned from RU27, that is not necessarily the case.  There is still much to learn in the field of data assimilation, and these Trans-Atlantic flights give us that opportunity.

100512_cook_hycom_alt

One thing we will be talking about with our friends in the Canaries is how we use Cook to study heat transport.  Below is the section of temperature derived from Cook on this leg of the journey.  On the x-axis is distance alongtrack. On the y-axis is depth of the glider, with the ocean surface at the top and 1200 meters depth at the bottom. As we head north, the depth of the warm surface layer shallows.  We will be looking at these temperature profiles, and the currents derived from Cook to see which way the warm water is flowing.

cook_100421t1320_100512t0101_twrc-ru_temp_xsec

100 Kilometers to the Start

Saturday, May 8th, 2010

Today Cook surfaced 100 km southeast of the 26 30 N, 65 00 W, the starting point for our second Trans-Atlantic section.  We'll be there in a few days, and from that point we turn east towards the Canaries.

Below is the satellite altimetry data we get from Colorado.  it is a plot of the Sea Surface Height (SSH) and the resulting geostrophic currents. Its a prime dataset for the open ocean.We are looking at the two  clockwise eddies up in the northwest corner of the coverage.  They are colored red, meaning they are a sea surface high.  We are getting a boost from the western eddy, and the glider currents in green agree well with the satellite altimetry.

100508_cook_alt1

Below we add the satellite Sea surface temperature (SST).  The agreement between the many eddies in the altimetry and the resulting response in the SST is clear.  It improves your confidence in both products.

100508_cook_sst_alt

Next is the Navy's HyCOM model.  It produces a global forecast, and we pull out a section.  Here we show the model forecast sea surface height and surface currents.

100508_cook_hycom_alt

Here we zoom in to the two eddies in the altimetry, western eddy circled in green, eastern in blue.

100508_cook_zoom_alt

We see areas where it agrees with the sst below.

100508_cook_zoom_sst

And now the model comparison.  There is good agreement between all the products on the western eddy.  But the eastern eddy is different.  Its located farther north in the altimetry data than it is in the model.  It will be the first mystery Cook will investigate.  It will use the good agreement on the western eddy to whip around the start point, and then head across to this eddy and find out who is right.

100508_cook_zoom_hycom

On Wednesday next week we will skype with our partners in the Canaries.  Alvaro, one of the students that visited Rutgers for last year's flight of RU27, will be on the line.  We'll link up the students on both sides.

Crossing the Tropic of Cancer

Wednesday, May 5th, 2010

Depth-average currents are low with the most recent report being only 6 cm/sec. After all, Cook is diving to 1200 m, so we expect the currents to be much less than we are used to seeing from near surface gliders like RU27. Cook is also fast, about twice as fast as RU27. Still we like to use the currents to our advantage. To do that, we have now flown about 120 km to the Northeast along the green line. The most recent surfacing just crossed over onto the northern side of the Tropic of Cancer. We'll spend the entire crossing north of this line, hopefully returning south to it on the other side.

100425_cook_alt

Two days from now is the 1 year anniversary of the launch of RU27. What shall we do to mark this occasion? Any suggestions?