Tank experiment on a typical circulation in a fjord.
Traditionally, a fjord circulation experiment has been done in GEOF130’s student practicals. Pierre and I recently met up to test-run the experiment before it will be run in this year’s course.
This is the setup of the experiment: A long and narrow tank, filled with salt water, a freshwater source at one end and an outlet at the other end. This sets up a circulation from the head towards the mouth of the fjord close to the surface, and a deep return flow.
Watch the movie below to see how different circulations are set up depending on the depth of the freshwater source. As in the picture, velocity profile 1 is for the case where freshwater is being added close to the surface, and in case 2 the freshwater is being added deeper down.
We think we observed Langmuir circulation, but we don’t understand the mechanism causing it.
Recently, my friend Leela came to visit Bergen and we went on a fjord cruise to make the most of a sunny October day. We observed foam streaks on the fjord. The structures were long and persistent, and being the oceanographers we are, of course we knew that they had to have been caused by Langmuir circulation.
Langmuir circulation on Østerfjorden, Norway.
But then we started wondering about the mechanism driving the Langmuir circulation. Textbook knowledge tells us that Langmuir cells are spiraling rows with convergences (the foamy stripes) and divergences (in between the foamy stripes) at the surface. They are, according to common knowledge, caused by wind that has persistently blown over the surface for more than some 10 hours, and by Ekman processes. Plus there might be some interaction with waves.
More Langmuir circulation
But that’s about where my knowledge ends, and I have absolutely no mechanistic understanding of Langmuir circulation. Literature research was unsuccessful (at least in the period of time I was willing to spend on this), a quick poll of my colleagues didn’t help, so now I am turning to you, dear readers: Do you have a simple mechanism for me that explains Langmuir circulation? Please help!