by Penny Holliday
I’ve mentioned a few times now that the EEL programme has been running for 40 years, so in this post I thought I’d briefly explain what we have found from the records. I’ll focus on the research that I am doing: looking at changes in temperature, salinity and ocean circulation. There is plenty of other research based on the programme on different topics, including chemical and biological processes and changes, and with any luck we will have time to cover some of those during the course of this cruise blog too.
The section that we observe every year on the EEL crosses the deep ocean between Iceland and Scotland, measuring the relatively warm “Atlantic water” that flows northwards from the subpolar region (roughly between Canada, Greenland, Iceland and NW Europe) into the Nordic Seas and eventually the Arctic. I’ve been looking at how and why the warm water changes in character from year to year. There are a few different reasons we care about this, and they are reasons relevant not just to scientists, but to society in general. The most obvious reason is that it tells us something about our climate system; how it works and what we humans are doing to change it by adding carbon dioxide and other greenhouse gases to the atmosphere. As ocean currents change, our weather and climate changes too, and long term measurement programmes help us recognise what the changes are and give us clues as to why the change come about.
The second broad reason is that changes in ocean currents and temperature, nutrient and carbon content has a huge impact on the marine ecosystem. As the ocean warms or becomes more acid, or has different amounts of nutrients available to use, so different plants and animals thrive or struggle to survive. Even a change in water temperature of perhaps half a degree (which seems small in the context of air temperature or heating in your house), can mean different species of whales are observed west of the UK and Ireland. Similarly a change in the currents and the nutrients they carry can mean an entire fishery (blue whiting or herring for example) can move into a different region, with significant economic impacts.
Four decades of observations on the EEL show that the top layer of the ocean (here around 0-800m) shows an overall trend towards higher temperatures and salinity (the first image below). Within that long-term trend though there is plenty of wiggling about on the graph, and this is an indication of changes on shorter timescales. Some of this is because of the well-known seasonal cycle (warmer and fresher in the summer and cooler and more saline in the winter), and the extent of that cycle is shown by the thin lines on the graph. Any changes that are larger than that band around my data are changes caused by something other than the seasonal cycle.
The salinity graph shows that there are also periods of highs and lows in salinity, each a few years in length. In fact temperature generally follows the same pattern as salinity, but the pattern in much clearer in the salinity record. Look closely and you will see that the most recent years (since about 2010) have been slightly cooler and fresher than the mid 2000s, though values are still high compared to the first half of the time series.
Through the analysis of this data set, as well as models and theory, we have learned that the ups and downs of salinity and temperature we see here are not simply the atmosphere warming and cooling the water, but are changes in ocean currents. When the EEL section is warmer and more saline, the currents are carrying more water from far-off subtropical regions. During periods of cooler and fresher conditions (as we appear to be seeing now) the currents are carrying more water from the western subpolar region. Scientists are now investigating why the currents change in this way, and how the ocean and atmosphere work in tandem to induce this effect.
Images by Penny Holliday