Thursday, 2 February 2012

Interview with the Chief Scientist

We are proud to present a double blog update today. Earlier in the day, I took a few minutes out of our schedule to talk to the Chief Scientist of the expedition Professor Andrew Watson F.R.S. of the School of Environmental Sciences at the University of East Anglia. He discusses the background to the project and some of his aspirations for the cruise:

What is the DIMES project and how did you become involved?
To explain the project, you have to know a bit about how the ocean works. Below the surface, the sea is stratified. That means it mixes only slowly vertically, but rapidly horizontally. It’s a bit like a lasagne where the water moves mostly along layers of different densities, light at the top and heavier as you go down. The slow cross-layer mixing is called “diapycnal” while the fast in-layer mixing is “isopycnal”. DIMES stands for “Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean” and we’re measuring and studying both kinds of mixing. We are doing that using a “dye” that is inert but which can be measured at incredibly low concentration, vertical turbulence profilers, and lots of neutrally-buoyant floats that drift along the density layers. The “dye” (which isn’t actually coloured) is a chemical chosen because it can be detected at a concentration of about 1 milligram in a cubic kilometre of sea water, so a very small amount goes a long, long way, and it’s almost the only way to directly measure the cross-density mixing.

DIMES is “big science” -- an ambitious and lengthy experiment, that has taken a long time to plan and to execute. I first got involved in planning it over 10 years ago, on a sabbatical to WHOI to work with my friend and colleague Jim Ledwell. It’s grown into a major joint US-UK experiment, with the field work started in 2009 and expected to run until probably 2014.

Why do you feel that the DIMES project is important?
It’s important for an understanding of the Earth’s climate and the ocean’s part in it. You might think that how the sea mixes over small distances isn’t important in the great scheme of things, but that turns out not to be true at all. For instance, how a climate modeller represents that slow vertical mixing in a global climate model, determines whether their model ocean has an overall top-to-bottom circulation at all, let alone whether it looks like the one that the real ocean has. And the Southern Ocean is the least well understood of any of the world’s oceans. We don’t for instance have an agreed theory for the overall circulation here. We think that small scale processes like eddies and vertical mixing are critical to it, but we don’t know how best to represent those in our models.

What are the main objectives of the cruise?
The dye and floats are now coming through Drake Passage and into the Atlantic sector of the Southern Ocean, in the Scotia Sea, which is the region between Drake Passage and the island of South Georgia. The Antarctic Circumpolar current flows across a very mountainous sea bottom here, and it squeezes through narrow passages as it exits the Scotia Sea. We suspect that much of the mixing, both horizontal and vertical, that occurs in the Southern Ocean happens here because of this rough ocean floor, and the object of this cruise is therefore to measure these. We already know from last year’s measurements that vertical mixing rates are very high in Drake Passage. This year we’ll find out if they remain high through the Scotia Sea.

What do you personally hope to find out on this cruise?
Getting measurements of the mixing in the Scotia Sea will be the culmination of 10 years of planning and work for me and for several of us involved with the project. And I also want to find out if I will still be sane after I’ve been at sea for 51 days – I’ve never been to sea for such a long cruise before!

No comments:

Post a Comment