Archive for June, 2008

Score one for the Altimeter

Monday, June 30th, 2008

I think it was Saturday night we gave up on heading north trying to get into the segment of the Stream heading northest.  We based that decision on the satellite altimetry.  Tonight we have a clear spot in the clouds that shows the northern side of that meander crest just ends around 40.5 N, and everything turns south. All we kept seeing all friday and saturday was currents to the south.

So score another one for the altimeter guru's.  Thats two days it saved us, which is really good.  People are looking at the glider velocities and seeing that westward component, wondering why we are going the wrong way.  I keep telling myself its based on the altimeter, and F still equals m*a, so it has to come around.   Guess we will know in a couple days how that decision plays out.  But in this case, the model, the altimeter, and the sea surface temperature all agree.  Its a big eddy out there.  Currents are increasing rapdily.  Some of the strongest we have seen in this part of the Stream. 

Link

Monday, June 30th, 2008

Here is the link to the Mercator Website...

Mercator Website

weather behavior; eastern atlantic

Monday, June 30th, 2008

Scott asked me to find the weather patterns for the eastern portion of the Atlantic Ocean and the coast of Spain, this is what I found.

The Mercator website allows you to view zonal maps.

  • While looking at the zonal maps if you observe areas D it shows the portion of the gulf stream.
  • Area B and E show the coast of spain
  • Area C narrows in on the coast of Spain

     Also while in the Mercator website if you click on the "sections" tab next to the zonal maps you will find cross sections of the different oceans. Cross section 7 and 4 seem to be around the area that we want to observe. Make sure that the date is adjusted...you can go as early as the prediction for July 9. These maps will give you temperature, salinity, and vectors.

http://www.stormsurf.com/page2/links/atlane.html

This is a link the Stormsurf.com which is a link off of NOAA. This site will give you the wave height and period for the Atlantic ocean.

I will post if i find more

             

That looks better

Sunday, June 29th, 2008

Now thats what a glider segment is supposed to look like.  Nearly identical ascents and descents, all yos to the same depth, all returning to the near surface depth of a few meters.

So we know what to do. Monday morning tiger team. Lets download the engineering files and look again.

What Was That? II – Return of the Remora remora Behavior

Sunday, June 29th, 2008

Whatever it is, it came back and left again.  Below is the time series plot of the glider depth (y-axis) as a function of time (x-axis).  You can see the glider move up and down in the watercolumn in a sawtooth pattern.  We now call it the Remora remora behavior till we figure it out.  Same behavior as last time, we suddenly switch to fast descents and slow ascents, sometimes not making it to the top of the yo.  It lasts for two 6 hour underwater segments, then goes away just as suddenly, returning to a normal series of approximately equal dives and climbs.  This time the oddity (as RU17 calls them when it reports back) of stopping RU17's upward motion occurred 7 times, causing RU17 to return to depth and try the ascent again. Luckily a robot never gets tired of trying again and again.

The puzzling thing is that there is not a thing that moves, or even thinks of moving, on RU17 without being sensed and recorded.  We'll download these engineering files again, just like we did last time.  Last time we found several related behaviors that occured in response to oddities, but nothing that appears to have caused the oddities.  We'll keep looking.

I always liked Plan B

Sunday, June 29th, 2008

That large cold eddy centered near 39 N, 47 W (see forecast below) continues to grow more distinct and is dominating the oceanography in the vicinity of RU17.  It is the strongest local feature here in the model forecast.  The forecast still has a smaller route northeast, but it looks like it heads off toward Norway.

A quick check of the satellite altimetry (below) and we see that the route to the northeast no longer exists. The region is pretty much dominated by the cold core eddy that is located between 45W and 48 W, just south of 39 N.

We spent a day (friday night through saturday night) with RU17 searching for the route northeast, but all we found was steady currents about 30 cm/sec to the south (figure below).  This is consistent with the Trust Your Altimeter philosophy we learned in the previous two Gulf Stream meander crests.  Now we see it applied to the very first eddy we encounter in the Gulf Stream extension region. Since RU17 can only fly about 30 cm/sec, continued searching for the potential route northeast becomes an exercise in station keeping and waiting for something to happen.  But we don't have time to wait.  So lets trust the altimeter, assume the northeast pathway is shut down, and follow an alternative path to the same eventual location.

 

So last night we turned RU17 into the currents and began heading south, starting a loop around the southern side of the cold eddy.  In the Sea Surface temperature image below, you can faintly see the eddy in the surface temperatures.  Its the oval you see between 46 W to 48W, just around and below 39 N.

The advantage of this route is that it uses the dominant oceanographic feature in the region as the route east, even if it may be a bit less direct.  If we want to use the oceanography to help us get across, we should probably start by using the dominant features that the models, satellite altimetry, and satellite surface temperatures all agree on.  Its always more comfortable flying these things when you know your enviroment based on the models and remote sensing data than if you are flying blind.

So below is the proposed route forward, flying eddy to eddy based on the altimeter field.  Our objective is to get into the 40 N to 41N band where the larger altimetric signals are found all the way east to 30 W.  With the northern route shut off, we turn into the southern route, loop around the southern side of the alpha-eddy, then back up the eastern side between the cold eddy's low height (blue) and the adjacent high (red) in the sea surface topography.  So we fly counterclockwise around the low and clockwise around the high, following what is known as the geostrophic currents.  We then spend some time on the southern sides of the next two cold eddies (blue).  We then see a small clockwise high that can boost us to a bigger clockwise high that is just off to the east of the image plotted here.  Just like the atmospheric weather, this ocean weather will evolve over the time as we travel along this route.  We will be increasingly relying on the broader oceangraphic community to monitor the ocean weather conditions in this region as we continue our journey.

 

The Transition to a New Regime

Saturday, June 28th, 2008

RU17 is making slow but steady progress towards the northern branch of the Gulf Stream we discussed last night.  RU17 is steering toward the X in the plot.  We have very quickly transitioned into a new type of environment.

Current speeds have dropped significantly in the last day.  The time series of current speed indicates we dropped down nearly to the line where we can just about navigate like a ship rather than move like a drifter.

So we are still trying to not fight this current, still treating it like a swimmer swimming perpendicular to a rip current.  Right now the glider is flying on a bearing of 58 degrees, so we are heading 58 degrees measured clockwise from north.  The depth averaged current is flowing towards 148 degrees, again measured clockwise from north. The difference between the two is exactly 90 degrees, meaning RU17 is flying exactly perpendicular to the current.  This is the most efficient path to get out of the area we are now in, and move towards the northern branch of the Gulf Stream where we want to be.  All we can do now is sit back and wait for the magic to happen.

OMG – There it is!!

Friday, June 27th, 2008

 We left RU17 with a bit of a plan, we knew we wanted to go northeast, but the exact way to get there was still uncertain. There are lots of different current patterns you can encounter on the way northeast, and we were feeling our way through these based on the glider velocity measurements alone.  The problem with this approach is that you never know when you'll hit a dead end. Then right after dinner this image pops up. Wow.  There is the oval ring we saw in the altimetry. Its faintly visibile between 45 W and 48 W.  And there is a route north.  My favorite part about Satellite Sea Surface temperature is that it is synoptic.  That means it is a clear snapshot at a single time, not a smeared average over some time interval.  Its not always representative of the true subsurface currents, but when it is, there is nothing better to to steer a glider by. 

Below we zoom out a bit on the google earth image.  Again you can see the oval eddy, with one branch going south of the oval eddy, and the other branch of warm water turning to the north.

Now lets insert the global 4 km dataset over the blue area at the right of this image.  This shows the interface between our locally acquired 1 km data and the global 4 km data.  The northern branch of the Stream heads north and turns east towards Spain, running just above 40 W.

Now lets zoom out to view most of the hemisphere.  We are right in that bifurcation zone of the Gulf Stream. We are trying to take the northern branch that them heads all the way across with the maximum gradients near 40 N.  Thats the latitude we saw the strong altimetric signals extending the farthest to the east.  Thats the zone we need. Strong fronts and Sea surface height gradients mean strong currents.   The thin yellow lines, many of which look like circles around the islands, are the boundaries of the Exclusive Economic Zones, the EEZs. This route will skim along the northern side of the Azores EEZ, staying in international waters until Spain.

Looks like The Pathfinder earned his keep today.

The Pathfinder steers a hard left.

Friday, June 27th, 2008

Here is the picture we had in the morning after the 5:30 am surfacing. We made it to the top of the meander crest, and we were heading SE.  But which way to Spain?  Northern route, southern route? All we see is warm water.

Below is the Navy forecast. We see a long stretch of continuous current running to the southwest between 50 W and 45 W.  Looks like a good route to there, but then it dead ends, turning back on itself and heading to Norway.

Still with some uncertainty in our next move, all the facutly and senior staff headed to the conference room for our montly call with the MARCOOS network. MARCOOS is the Mid-Atlantic Bight Coastal Ocean Observtion System. It is the Regional onserving system for the Mid Atlantic Coastal Ocean Obseving Regional Association known as MACOORA.  It is part of the U.S. Integrated Ocean Observing System (IOOS). The undergrad remained behind, searching for a path.  The MARCOOS call was running long today.  Our plan for the HF radar network was due today, and we were discussing the final tweaks to the last 2 weeks of work.  While we were on the call, we get an email from Justin, our undergraduate pathfinder.  All he said was the satellite Sea Surface Temperature and altimetry agree, and he was turning RU17 to follow a developing ridge in the sea surface height.  In the google earth display we saw he had made a sharp left hand turn, 90 degrees to the current, exactly the move you pull in an emergency when you have to make progress in the face of a strong current.  Where was Justin suddenly taken us?

Below are a couple shots of the satellite altimetry data showing the sea surface height in color with red being the high and blue the low. Between 50 W and 45 W we see the strong current to the southeast, simialr to the Navy forecast.  There is the low blue oval centered about 38 N, 48 W, and several filaments heading north.  The altimetry picture says the the filaments on the north extend east to nearly 30 W.  The strong current to the southeast just ends, dropping you at 45 W.  So the southern route is faster the 45 W, but then what?

Below is the satellite altimetric measurement of sea surface height from 30 W to Spain. Its flat.  Nothing there. We get no help beyond 30 W, just a slow, steady slog.  So the goal becomes the fastest route to 30 W, and thats the northern route.  Justin's hard left in the morning was our attempt to find that route north in that featureless Sea Surface temperature image.

 

 

HF Radar Site

Friday, June 27th, 2008

Today, Hugh, Eric and I started to put together a High Frequency Radar Site outside of the IMCS building. This is a 13 MHz system that is kept insulated; this is more convenient to access while using less energy to receive signals.

We will continue working on this Monday.