by Penny Holliday
At present we are working a line of CTD stations northwestward from Rockall, across the Hatton Bank then northward along 20°W to an end point just south of Iceland. This is the northern part of something called “the Extended Ellett Line”; a CTD section that is surveyed every year. There has been a programme of marine measurements here for over 65 years, and you can find out more at http://projects.noc.ac.uk/ExtendedEllettLine/ or for a potted history, you can just read on.
The history of the Extended Ellett Line started in 1948, when scientists realised that the supply ships servicing the Ocean Weather Ships dotted around the North Atlantic could usefully make measurements of the temperature and salinity of the ocean surface along the way. A programme of these opportunistic measurements was established by a man called Jack Lumby and it ran until as recently as 1996. The routes went across the deep ocean basin west of the UK called the Rockall Trough, and the scientists discovered early on that there were seasonal cycles in both temperature and salinity, and that the range of those cycles changed from year to year. They also realised that changes across the whole subpolar North Atlantic had some coherence, suggesting they were responding to large scale phenomenum.
In the mid-1970s an ocean scientist called David Ellett set up a programme to measure temperature and salinity of the full ocean depth in the Rockall Trough. He reasoned that they could expect to see similar year to year changes below the surface too, and that changes observed there would relate to those seen in the very small number of places where full-depth ocean temperature and salinity were repeatedly measured. So began a programme of regular surveys to the region – they happened at least once a year, sometimes as many as 6 times a year, and a new ship the RRS Challenger was commissioned by the UK to carry out this and other deep ocean work. The programme eventually became known as the Ellett Line after it’s dedicated founder and champion. David Ellett retired in 1994, and died in 2001.
The Challenger did her last Ellett Line cruise in 1996 and later in the same year the Ellett Line became extended by reaching past Rockall and out into the Iceland Basin. The scientific rationale for this extension was to survey the warm North Atlantic Current heading into the Nordic Seas, and to survey the returning dense overflows (see the post on 5 July). The Extended Ellett Line has been run annually since 1996. Results from the section were some of the first evidence that variations in the temperature, salinity and nutrient concentrations west of the UK are dominated by changes in the subpolar gyre circulation. The data have been compared with the few other similar repeat surveys in the North Atlantic and have shown that the changes observed on the section persist along the pathways of the main currents all the way into the Arctic ocean. Our latest results show that long-term changes in carbon content of the ocean water along the Extended Ellett Line are representative of changes across the whole subpolar gyre.
These results emphasise the need for us to keep on measuring the conditions on the Extended Ellett Line. One of the pre-eminent scientific issues of the twenty-first century is understanding and quantifying our changing climate, and in order to that we must understand how and why the ocean changes over time. We must also examine the impact those changes have on ecosystems and marine resources. Programmes like the Extended Ellett Line are a crucial element of global ocean observations because they contribute measurements of the highest accuracy over a long time period.
So here we are today, on the 78th repeat of the CTD section known as the Extended Ellett Line, carrying on the good work started by Jack Lumby and David Ellett all those years ago, and finding out new things about the ocean as we go.
Images: CTD coming out of the water, the CTD closest to Rockall (by Penny Holliday). Map showing the Extended Ellett Line (black solid and dotted line) and the location of some of the old Ocean Weather Ships (black dots, no longer in service) from Holliday and Cunningham, 2013, Oceanography, http://dx.doi.org/10.5670/oceanog.2013.17)