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After pursuing the waypoint set at the end of last week through the weekend, Challenger has continued to see a consistent push to the north by the current.  Late yesterday to try and better counter this force, the waypoint was shifter further east and slightly more to the south to try and have 29 hit the edge of the eddy at a nice angle to ride it around to the south when the glider reaches that point a little later this week.

In the RTOFS forecast, the model is showing the Challenger has nearly reached the edge of the elongated eddy and says that the current should now be flowing north to south-south east representing the south west sector of the warm eddy.  However, the currents as they are reported by the glider seem to be going in the opposite direction flowing to the north with a little bit to the west.

In Copernicus, the model matches up much closer to the gliders report, showing a weak counter current to the rotation of the eddy whose presence is still a little ways away.

Over the past few days, Dave and Antonio have further discussed the change in vertical velocities seen by the glider over the past few months.  Starting in October, the glider had seen more time at the surface due to the trouble shooting of the pump which needed to be done.  This increased surface time then allows for a greater chance of  successful settlement of larval stage organisms such as barnacles on the hulls.

If we are seeing the effects of biological growth on the gliders, combined with issues still being dealt with with the pump, it will build up a case to increase pitch angles (faster and less oddities for pump) and fly a little deeper, perhaps longer too in order to  slow down the growth.  Either way, we believe if we can complete the journey by April the glider should be in good shape

Force wind sea & honor

Challenger, now under 2000 km from Cape Town is finally on the cusp of the eddy we have been positioning the glider for over the previous weeks.

In RTOFS, Challenger is right on the west-southwest edge of the more elongated eddy, however what the model shows of the current flowing to the south-southeast is nearly opposite to what the glider is reporting of to the north.

Similarly in Copernicus, although the eddy is more defined and compact a couple hundred km to the east, the model is again showing a southward flowing current contradictory to what the glider is reporting.

Over the next couple days, as 29 enters the eddy ahead the way point will be moved to the south east as we ride the southern edge of the eddy across the Prime Meridian.

Force Wind Sea & Honor

Now less than 2000km from the shores of South Africa, Challenger is continuing to inch her way across the map towards a more immediate goal: the Prime Meridian.

Last Thursday afternoon we pushed the waypoint further east in order for 29 to fly more perpendicular to the weak northward flow she had encountered.  Through the weekend however, this current has picked up a bit (from .03m/s to .08m/s) and rotated to the north west, providing 29 with some more head on resistance.  This change in direction of the currents is something we can see a few hundred km ahead in copernicus, but in the immediate area it almost looks like the sum of the east-west component of the copernicus model combined with the north-south component of the RTOFS model

Since the current is blowing more directly into Challenger’s nose, we moved the waypoint a little further north (to 35˚ S, 1˚ 30′ W) to try and fly a better angle against the current.

Over the weekend we also saw Challenger surface due to an abort.  After looking into what happened, it was discovered that during the previous surfacing Challenger did not maintain the connection long enough to reset her timer, so then when she dove again and continued to fly she eventually hit her time limit and came to the surface with the abort for no-comms.  Since this happened on Saturday afternoon she has continued to fly without any issues.

Looking ahead, over the next week 29 will continue to make her way towards the warm core eddy sitting at the Prime Meridian.  From there we will likely aim for the southern edge of the two weaker warm core eddies as we ride them to the north east before making our final approach towards Cape Town.

One last note, looking at Dave’s vertical velocity plots, we are noticing a downward trend from the past few months showing that the glider has lost about 10% of its original speed, largely starting since October.  This could potentially be due to biofouling especially since we have reduced our dive depth to 500m for the past 5 weeks.  One suggestion so far has been to weigh the options of if we were to dive deeper again, the pump slipping vs the distance lost to being slowed down by barnacles.  But for now this is just speculation and will be discussed further in the future.

Force Wind Sea & Honor

Over the past 4 days, Challenger has been countering the northward current by flying towards perpendicular to the east. Since then, the currents let up a bit allowing Challenger to fly more quickly through the water, shown but the increase of 20 to 23 on the figure below:

However, now the current although still relatively weak, is rotating back to our nose, so we have pushed the waypoint a little further away to 35˚23’S, 1˚30’W.  This will keep the glider flying along the white line shown below which is the direct path from Challengers latest location to the shores of cape town.  This path also does a nice job of lining the glider up to fly smoothly through the eddy that is resting on the prime meridian while steering us clear of any unfavorable bathymetry.

Force Wind Sea & Honor

Through the weekend Challenger continued to fly towards the waypoint assigned towards the end of the week at 35˚30S, 4˚W.  With the current correction off, this allowed the glider to ride the northward flux along the arcing red path proposed in the previous post.

There is a counter clockwise eddy in both RTOFS (green) and copernicus (yellow)
in nearly the same locations.  In the figure above, the oval RTOFS eddy is outlined in green, and the perfectly round copernicus eddy is outlined in yellow.

In the two figures above, the straight green line is the line straight to Cape Town.
We want to keep RU29 between the straight red line to the south and the straight orange line to the north, while keeping as close to the green line as possible.

The waypoint decision we made last week revolved around how to approach the eddy near the Prime Meridian prominent in both models.  We had 2 options, the northern curved route (red) or the southern curved route (orange). The glider currents were persistently pushing us north and the currents along the northern track were consistent through both models, so we made the decision to go with this flow, and pursue a track along the northern red line.

For the foreseeable future, the bathymetry looks unobtrusive so as Challenger continues across the Prime Meridian (for the third crossing of the Challenger mission) and on towards Cape Town, we can rest easily when it comes to dodging sea mounts.

And finally below we have a field of the locations of the Argo floats that lie ahead.  Through the remainder of the mission we will attempt at approaching at least a few of these sampling devices for comparison and quality control purposes.

Force Wind Sea & Honor

Challenger has now been at sea for over 160 days and flown 4,100km and is continue to push onwards towards her goal of the shores of Cape Town South Africa.

Yesterday the team put their heads together on whether to turn to the north or south to navigate through the currents that lie ahead.

Despite the RTOFS (in green above) and Copernicus (in yellow below) disagreeing about the northern flux currently affecting 29, both models agree that if we continue pursuing the latest waypoint, we will enter a pretty persistent head current.

As seen in the figures above, the Copernicus shows a pretty promising route to the south that RTOFS shows as being more of a mess.

To the north however, both models show what seems to be a number of more defined eddies that have meshed together.  This is the option that the team ultimately decided upon using the red path shown in both figures.  Beginning today and following through the weekend, Challenger will start pursuing the new way point at 35˚30S, 4˚W to swing her way to the north and through the eddy field.

As for the troublesome pump, we are continuing to take a few precautions to reduce the slipping we have seen over the past couple of months.  The slipping we have seen is caused by the oil that dictates the buoyancy of the deep glider leaking within the pump section of the glider, resulting in the pump having to move multiple times over the course of a dive/climb in order to maintain it’s flight path.  In order to reduce the number of times the pump has to move (which increases the wear and tear on the pump), Dave has reduced the depth 29 dives to to 500m and has adjusted the pitch angle on the dives and climbs.  By adjusting the angle, we are then able to change how quickly the glider moves through the water column with the aim of reaching the inflection depth before the pump has a chance to slip.  The issue has been variable over the past few months, but the team has been hard at work diagnosing and testing the issue along the way to find the best way to fly under the circumstances.

Force Wind Sea & Honor

Now over two months and roughly a quarter of the way into the mission, Challenger is continuing to make good time consistently reporting speeds of over 25km/day as she weaves her way through the stream of eddies across the South Atlantic.

In a previous post, we had mentioned one of these large eddies to our East as being an interesting test case to study as RTOFS interpreted the area as being a large cold core eddy while Copernicus showed the area being two warm core eddies rotating to the north and south of the region.  As we have approached this location and the forecast has evolved over the two weeks, the two models have come to a similar conclusion that there is a cold eddy solar system (cold eddy with a number of warm eddies surrounding it as seen below)

Looking at how the currents progress through this solar system, we unfortunately will have to take a southern route in order to keep our progress in the Eastward direction.

RTOFS Averaged Currents Sept 1 2015

Both models from our current location look to be agreeing to an extent as they are both showing the glider leaving the presence of a small warm eddy.

Copernicus Averaged Currents Sept 1 2015

From here, we plan to continue following the southern edge of this stream of eddies outlined by the  front in the salinity field (barrier between red and orange in the image below).  Over the next week or so we will shift the way point further to the south as we line 29 up to step to the south east along that path

Copernicus Averaged Currents overlaid on top of the Salinity field for 500m depth Sept 1 2015

Finally, Dave recently noticed we have been getting oddities in the readings of the oil pump used to drive 29 and will be investigating the issue further.

Force Wind Sea & Honor

Since the waypoint change last week, Challengers slowed down quite a bit as the currents have now shifted to the North, despite both models consistently showing more of a westward flux.

Looking ahead, as we continue to maintain our position between the orange line (direct route from Ubatuba to Cape Town) and the red line (angled path bringing 29 towards Tristan da Cunha ) there is an interesting system lying a few hundred kilometers to our east.

The Depth Average currents for RTOFS August 14

In the RTOFS forecast above, there appears to be a large cold core eddy spinning in the clockwise direction, while below in the copernicus forecast, there are two warm core eddies rotating in the counter-clockwise direction; both resulting in a westward flux.  As both models use similar baseline readings to then derive their forecasts from, it is very interesting to see the two different reasonings behind the same westward current, and so over the next few weeks as 29 makes her way in that direction, we will keep an eye out on the data to see the evidence in our data for which forecast will get this right.

The Depth Average Current forecast for the Copernicus Model August 14

In the meantime as we make our way towards this test area, Challenger has to snake her way through the seamounts immediately ahead of us.  With the current to the north of the seamounts being due west, we have decided to duck down to the south through the valley before cutting back to the east where the current look to be more favorable.

Global Bathymetry Product for the South Atlantic Region

Force Wind Sea & Honor

Since the way point change earlier this week, Challenger has been making excellent time.  As seen in the forecasted currents from both Copernicus and RTOFS for today, 29 has made it to the south easterly jet between the eddies to either side.

Copernicus Averaged Currents Forecast for August 06, 2015

RTOFS Averaged Currents Forecast for August 06, 2015

By managing to catch this jet, we are seeing some of the best speeds of the mission as we have averaged 18 km/surfacing since the waypoint change resulting in just under 30km/day!

RU29 Velocity Plot

In the next day or so, the way point will be adjusted again to account for reaching the end of the jet as we then continue to make our way from eddy to eddy on our way to the east.

Force, Wind, Sea & Honor

This past weekend Challenger crossed the 1000 km milestone on the trans-atlantic crossing from Brazil to South Africa.

RU 29 has completed the first 1000 km of the mission

Now at 1041 km into the mission, 29 is now roughly 1/6 along the projected path passing by Tristan de Cunha to Cape Town.

RU29 Crossing the South Atlantic

Looking to the forecasted currents, the two models are both showing that Challenger is flying down the eastern side of a cold core eddy which combined with the new way point from Friday has led to an extra ~3km per surfacing.  Despite the increased distance, Challenger is continuing to show a slight overall flux to the north through its calculation of depth average currents.

Copernicus Averaged Currents August 03

RTOFS Averaged Currents August 03

Moving at this pace, we will continue to have Challenger pursue the current way point for another 2-3 days before we move the way point back east to account for the edge of the eddy we are currently taking advantage of

Force, Wind, Sea & Honor