The End

The ship approaches the Bahamas once more, heading for Freeport where the scientists and technicians shall disembark. For this exceptionally long cruise, the end is a strange feeling. We are on day 47/48, and you can certainly tell going by the state of some of the scientists! Lots of long and busy days and nights, even a fair amount of steaming with little to do takes its toll. We are certainly all looking forward to stepping back on land with all the luxuries that come with it.

We deployed our last mooring early this morning, and it was actually raining… the only mooring we have had to do in the rain. But it was only a quick one - just lob it over the side (gently with a crane). All our data processing is complete, the cruise report is ready, and most importantly, we have the updated MOC time series! It looks beautiful. It is certainly very rewarding to see the last 18 months of data added on as a consequence of all the hard work done by the whole of the ships crew. 

Tension and excitement reached fever pitch in the main lab as the new MOC timeseries is produced.

Lets not forget that as well as the end of this cruise, it is the end for RRS Discovery. She did her last CTD this morning, we had estimated it was the last of the thousands of CTDs she has performed in her lifetime. Her final job will be to sail the crew back to Southampton. 50 years at sea is certainly an impressive feat, and we are all honored to be a part of her last voyage! So whilst the new RAPID array looms in the dark, we look forward to seeing more unique data from deep in the Atlantic Ocean in 18 months time.

Alex, Rafael, Charlotte, Ben, Darren, David and Gerard

 

The crew, technicians and scientists of Discovery Cruise 382–Disco's Last Dance

Ocean Velocities from the ADCP

Aside from the CTDs and the moorings, there are also many instruments mounted to the ship that are constantly collecting data as we go. One of these is the Acoustic Doppler Current Profiler (ADCP), which measures how fast the water is moving under the ship. Much like a sonar, the instrument sends out a sound pulse, which is then scattered by small particles or plankton in the water. The ADCP listens to the echo and uses the so-called Doppler effect to calculate the velocity with which the particles (and thus the water) are moving. In this way, we can monitor the currents under the ship almost in real time!

The Antilles Current, just east of the Bahamas. This warm, shallow current flows north in the top 800 m with speeds of up to 60 cm/s (1.2 knots if you prefer) in this example. You can see the subsurface maximum of northward velocities measured by the ADCP.

This figure shows us crossing the Antilles current earlier this week. The ADCP shows strong subsurface currents down as far as 800 meters! 

Charlotte

Final Week

We're into our final week of Disco's last dance. And what a final week it is! In comparison with the long steams that broke up the work earlier in the trip, we now have lots of moorings to do–all in close range of one another.

This week we are doing the Western Boundary moorings. These moorings that measure the Antilles Current and the Deep Western Boundary Current in the North Atlantic. These are arguably the most important moorings in the whole of the RAPID array.

Tuesday will be a very exciting day. We recover WB2 the keystone mooring in the array. That evening, we should be able to look at an updated timeseries of the Atlantic Overturning circulation. This will be the first update since April 2011 and promises to be very interesting. I, for one, am very excited.

The last update showed us some dramatic interannual variability. There is a nice synopsis of those results here.

Meanwhile, before heading off on this leg, we stopped in Nassau, Bahamas. We picked up a new crew and some new technicians. Some of the scientist managed to get on land for a bit. Here's the ubiquitous picture of Ben and Darren in the sea with one of our hard working technicians, Ian.

Ben, Ian and Darren enjoying themselves during our brief stop in Nassau last Friday

Nassau is primarily a port for cruise liners and we think that Discovery may have felt slightly out of place at the dock–can you spot Discovery in the picture below?

Spot the Discovery

Gerard

Lats, longs, knots, points and cables

Life on a ship has a lexicon all of its own. Many might know the bow or the stern. You might know that starboard is the right-hand side of the ship facing the bow and that the port side is the left. On board, you don't talk of walls and floors–you're in the world of bulk heads and decks. Some of the terminology has it's basis in the old setup of a ship. The fo'c's'le/forecastle/foredeck was traditionally where the castle towards the bow on a ship was. And while heads are located all around the ship these days, in times past they were also on the bow. These days the forecastle deck is a more pleasant place. But I digress, what I wanted to talk about was the nautical terminology for distances, speeds and directions.

A picture of the usually peaceful foredeck on Discovery taken from our rival blog: Memories of Discovery

Latitude and longitude is a good place to start. These are the two measures used to break globe into north, south, east and west. Latitude is zero at the equator and ±90 at the poles. Longitude goes from 0 to 360º. The zero of longitude is arbitrary but nowadays is accepted to be at the Greenwich Meridian near London. The same place that gives us the Greenwich Mean Time–GMT–that Darren described. The prime meridian wasn't always at Greenwich and, if you're a keen fan of Jules Verne, you'll have to take Paris as your prime meridian to figure out where he's talking about. Paris Mean Time–PMT–didn't last long and, primarily because it was very grumpy and irritable, was eventually superseded by GMT, which is now universally accepted.

One degree of latitude is 60 nautical miles. Or one minute of latitude, 1/60 of a degree, is one nautical mile. Longitude isn't spatially consistent*. 1º of longitude is equal to 60 nautical miles at the equator but elsewhere you have to take a cosine of latitude to get the distance. For example, at 52ºN, the latitude of Cork, one degree of longitude is 37 nautical miles. It is possible, depending on your location to change longitude very quickly. For example, in the picture of Morrissey with a cat on his head (below), should the cat spin around, he/she will go through 360º of Morrissey. The same would be true of a human at the north pole.

Morrissey with a cat on his head. The cat is demonstrating how it is possible to go through 360º of longitude by spinning while located at the poles... or in this case Morrissey's head

Another measure of distance is the cable. It is one tenth of a nautical mile. After eagerly learning this new nautical term, I was determined to use it as often as I could. And, like every scientist who learns a new nautical term, I eventually use it incorrectly. Like one time when we asked our American colleagues to set up two cables from a mooring site and were greeted with blank expressions: Americans don't do cables. 

Is there a point in all this? Well, I'm glad you asked. The point is a unit of direction. While a point as a unit for direction seems a little arbitrary at 11º to begin with, the explanation has some sense to it. It is a point of a compass. If north is at 0º, west is at 90º. Then NW is 45º, NNW is 22.5º. Half of this again is 11.25º and that is one point.

Hard to fathom? Not really: a fathom is 0.01 of a cable and is used as a unit for depth.


Gerard


*Shrewd readers may now be thinking that latitude isn't exactly spatially consistent either due to the earth being an imperfect spheroid.

We've not gone away...but the internet has: Alex explains

For all you land dwelling folks working at stationary desks, it is often the case that refreshing your Facebook page can provide a small escape from reality and spur you on to do another 10 minutes of hard work, before another Facebook refresh is due. For us out at sea however, one cannot rely on the internet being there to comfort you, and this blog post comes about after 4 whole days without internet.

Discovery’s internet connection is provided by a geostationary satellite at 38ºW in the middle of the Atlantic ocean. Discovery’s Internet receiver is located in front of the main mast. Putting two and two together (this is the science part of this blog entry), one finds that after you have sailed sufficiently west, and keep a westward heading, the main mast blocks out all signals from the geostationary satellite. The last 4 days we have been steaming to the WB6 site, and so our westward course meant continuous internet black out. We had 15 minutes of relief when we stopped and turned into the swell to deploy our last Argo float. However, being the one who deployed it, by the time I reached the lab again internet had gone. Though some were lucky enough to send off a single email!

So with the topic of discussion on board being the lack of internet, what is there to do when you need a break from your swaying Matlab screen? Often you will find us refreshing internet pages anyway, despite knowing what the outcome will be. In actual fact, reading about how the DNS look up failed often provides about as much relief and stimulation as Facebook does sometimes. Some great alternatives to internet entertainment can be to take a pleasant walk out on deck in the hot sun, and watch some flying fish leap about. However the weather has been less than satisfactory these past 4 days, with bouts of heavy rain, choppy seas and much cloud. Some intense lightning storms have even made good shows in the evening.

One might ask, why is Discovery designed so poorly, such that internet is blocked by something so trivial as the ships main mast? Surely, in this modern day and age where you can check facebook 2000 miles from land, such engineering difficulties would be easily overcome. You have to remember that Discovery is 50 years old now, and back in her day, the internet didn’t exist. When it did become available, some tell stories of your email being delivered to you each morning on paper printouts, and any outboard email you wished to send had to be given to the purser on a disk so he could send the whole batch at once. Dark times indeed.

So why are we able to put this blog post up today then? Well we have reached the WB6 site and our altered heading means the mast isn’t getting in the way, and so it looks like we will have internet for a good portion of the day. However once we are done here the blackout will return, as we set sail for Freeport in the Bahamas for the crew change.

Alex

Movember Time!

Because moustaches grow at sea too you know!

Many of you may have heard of Movember before, but for those that haven't: it's the month formerly known as November that has been renamed to promote men's health awareness (and give an excuse to grow outrageous moustaches!).

Many of the scientific party, technicians and crew have decided to take part in this event - some going as far as shaving off already well established facial hair to keep with the spirit of growing a moustache in a 1 month period.

The Disco's Last Dance blog have setup a Movember team page for cruise participants where we shall be exhibiting our progress, and hopefully receiving some donations to the Movember cause.

More information on Movember and the programmes it supports can be found at http://uk.movember.com/about/

Darren and team

Moorings, moorings, moorings.

Seeing as the main purpose of this cruise is to service the RAPID-26N moorings, I thought I'd give a bit more information on what our scientific moorings are like.

They are basically long plastic-covered wire ropes (or synthetic fibre ropes), with a lot of floats to hold the top end up, and a big lump of steel to hold the bottom end down. The scale of them can be impressive though, with our tallest mooring rising approximately 5150m from the seabed, meaning it would dwarf a skyscraper if it were stood next to it .

The height of our mooring called MAR1 relative to some famous landmarks. We are currently steaming towards this mooring and will be working on it on Thursday.

Yet for all their length, the wire diameter is less than 8mm, and the majority of the buoyancy is provided by glass! The hollow glass spheres are packed in plastic "hard-hats" to protect them from impacts, but they are designed to withstand the pressure at a depth of 6000m (600 times atmospheric pressure). Sometimes, however, even these implode due to the enormous pressures.

Glass buoyancy recovered from a mooring today. Usually glass spheres reside inside plastic hard hats. Here the enormous pressure the spheres were under from the ocean has caused the glass to disintegrate into white powder. The force of the implosion can be seen in the damage to the plastic hard hats.

As the mooring is deployed, self-logging instruments - built into titanium or stainless steel pressure cases - and additional buoyancy are clamped to the wire ropes or inserted inline between joins in the wire. The mooring is assembled as it goes over the stern, and once all the wire, floats and instruments are streaming aft of the ship, the anchor is attached and dropped into the water. The whole mooring is then pulled down as the anchor sinks and it eventually stands upright with the top of the mooring stopping about 50m below the surface. The mooring is then left in place collecting data until we come to collect it in 18-months time.

To recover the moorings we use an acoustic release that when given the correct coded acoustic signals lets go of the anchor allowing the mooring to float to the surface. The ship manoeuvres in to pick the top of the mooring up, and then the rest is winched aboard with the instruments taken off and the data downloaded. A replacement mooring is deployed and we move to the next site.

Darren