Stormy seas

Well – it had to happen in Drake Passage. After a few weeks of relatively quiet weather, we have finally hit stormy seas. With winds up to 40 knots and 6 to 7 metres of swell, science operations have been suspended. During a brief lull in the weather last night, the ship’s crew and technicians pulled out all the stops to get a CTD cast done in the middle of Drake Passage.

There have been some blog requests for a map of our progress, and I am happy to oblige, dear readers! Whilst the plot looks like a little bit of a spider’s web (including two trips to Stanley), you can see that we have effectively sampled both the North Scotia Ridge between the Falklands and South Georgia and the SR1 repeat line. Unfortunately, weather has meant that our tracer sampling of Drake Passage (where we are currently) has been coarser than we would have wanted but early results from the section are still encouraging.

Map showing the cruise track, starting in the north from Montevideo. The red dots indicate CTD stations and the yellow crosses are float positions.

Making the most of a weather window...

The cruise is now entering its final week, and with a Drake Passage storm due to hit tomorrow, we are working flat-out to get as much work completed as possible before we get stopped by the weather. It looks from the forecasts as if we could be put out of operation for at least a couple of days, meaning the work we do in the next 24 hours could be crucial to our success. We are attempting to complete a south-north Drake Passage section to analyse the cross-stream distribution of the tracer (dye) and find out how much has penetrated across the various ocean fronts that separate the different water masses of this region. Bearing in mind the weather down-time which we know is coming tomorrow, we are not currently doing any VMP profiles, though this could change if the weather ameliorates. I'll update you all soon with our progress...

Taking a dip!

No, this is not about any of us going swimming in the 3°C seawater! It’s instead about calibrating the Microcat sensors that were placed on our moorings. These instruments were put on the moorings to measure temperature, pressure and conductivity (from which we derive salinity) over a year. In order to check that these values are accurate, we strap the instruments (seen on the picture below) to the CTD frame for a so-called “calibration-dip”. The CTD is then stopped for 6 minutes at a number of different depth levels, during which we obtain measurements of temperature, pressure and conductivity from both the Microcats (which have a relatively slow response time) and from the carefully calibrated CTD itself. This ensures that we are able to detect any drifts or offsets in the Microcat data.

Weather or not to CTD ...

Unfortunately, 35 knot winds and high seas meant a period of 16 hours of science downtime today. The captain reassessed the conditions at 8 pm tonight and operations have since resumed. We aim to have completed our north-south section across the Scotia Sea by Monday evening. In the meantime, Uriel and Xinfeng took some great photos of the sunset last night, which they have kindly shared with us… 

Scientist Profile: Katy Sheen

I work at the National Oceanography Centre in Southampton, UK as a postdoctoral researcher for the DIMES project. I primarily look at the data that we collect using the vertical microstructure probes (VMP). These data are important as they provides 'on-the spot' estimates of the vertical mixing present in the ocean, in contrast to the time and space averaged values determined from the yearly spread of the tracer (dye). On the James Cook, my main role has been helping to deploy and recover the VMP, and to process the data. It can be a little nerve-racking, making sure that you have programmed the VMP correctly so that it doesn't crash into the sea bed!

Back in the office, I have been looking in detail at what kind of processes cause the variations in mixing that we observe in the VMP data. One source of mixing is the breaking of waves produced from the interaction of bottom currents with bumps in the sea-bed. These waves (internal waves) radiate through the ocean interior along density layers. When the internal waves lose enough energy, they  break, mixing up the water - just like waves breaking on the beach. High levels of ocean mixing are therefore associated with rough topography and strong currents, as in the DIMES region of study. The energy from internal waves contributes to pumping deep ocean waters back to the surface through vertical mixing, powering the global overturning circulation.

Internal waves typically have wavelengths of about 100 m, and information about the internal wave field can be obtained from the temperature, salinity and current velocity data collected by the CTD which is deployed at the same time as the VMP.  (The data obtained by the VMP is on the millimetre scales). These CTD data, along with theoretical ideas about how and when internal waves break, can be used to estimate mixing too. One of my main areas of research is to look at how well mixing estimates from CTD data compare to those measured directly by the VMP.

What do you enjoy about being at sea?

I enjoy working in a completely different environment , especially somewhere as beautiful and remote as the Southern Ocean - it's not a place where many people get the opportunity to visit! It's also a lot of fun meeting the other scientists and crew working on the project and exciting to be involved in such a dynamic and cutting-edge science programme. I normally manage to sneak in some travelling too - this year I spent a week camping in Uruguay before joining the ship in Montevideo.

What do you hope to gain?

Hopefully lots of data, new friends and I'd love to see an ice-berg!