September 19- understanding mixing waters

Good morning! We are slowly headed southwest completing our transect. Our last mooring recovery will be tomorrow. On the September 5 blog entry, I explained that a station may include any of the following:
  • Mooring recoveries and deployments
  • Conductivity, Temperature, and Depth (CTD)
  • Underway, Conductivity, Temperature, and Depth (UCTD)
  • Micro Structure Profiler (MSS) casts
  • Bongo and net hauls.
I have talked about everything except an MSS cast. I had the opportunity to hang out with the MSS team on two occasions. Today’s post highlights Markus Janout and his work with the MSS. Scientist Spotlight: Physical Oceanographer Markus Janout, (CATS) Alfred Wegener Institute for Polar and Marine Research. During this cruise Markus is focusing on the MSS. The MSS is a little specialized instrument that measures small scale eddies and turbulence. A CTD is not capable of doing these measurements. The MSS has the standard CTD sensors- conductivity, pressure (salinity) and a very fast temperature sensor. In addition to these sensors, there are two shear sensors. The sensors are connected to a silver cylinder that helps the profiler to be more stable in the water. There are also some electronics in the cylinder. An orange buoy that helps stabilize the MSS is connected to the cylinder. My favorite part is the green-looking hair on top of the buoy. The ‘hair’ helps break-up and prevent the formation of eddies when the MSS is falling through the water. On the very top are connectors to keep everything on the cable. The idea of the MSS is to have it free fall through the water column. For that purpose, the ship is simply drifting with the currents, not moving on its own. One of the reasons for the free fall is so there isn’t any interference from the cable that would create drag on the MSS and spoil the measurements. There is 400 meters of cable, but 300 meters is usually the maximum depth for the cast. You can tell once the MSS has reached the bottom or the end of the cable when the depth measurement doesn’t change anymore. The cable is important because this is where the profiler transmits the data, in real time to a computer on the deck. The shear sensors go in first to ‘feel’ the undisturbed water, taking around 1,000 samples per second. The sensors measure little disturbances in the water, turbulence. To avoid damage from hitting the bottom of the ocean floor, there is a conical-shaped quadpod that protects the sensors. In an ocean with strong currents or after strong winds, there is usually a lot of mixing in the water, small eddies measuring from millimeters to a meter in size. The shear sensors are very sensitive, and when they detect turbulence, they start to wiggle. You can imagine, on calm days without any wind there is not much turbulence in the air. The same holds true for water, the weaker the currents, the less turbulence in the ocean. One subject to study is how the warm water from the Atlantic Ocean mixes with the cold upper layer. Without this cold layer (-1.8°C) below the sea ice, the warm Atlantic water would melt all the ice. Considering that the ice in the Arctic Ocean is getting less and less, one question for oceanographers is: How much does the ocean contribute to the sea ice retreat? We can imagine how warm air melts the surface of the sea ice, and oceanographers want to understand how the warm ocean water melts the ice from below. The MSS can help with this question by quantifying the heat fluxes that reach the underside of the sea ice. Besides heat fluxes, we can also quantify, for example, how nutrients are transported vertically, which is what the chemists and biologists are interested in. Overall, the MSS can help us to learn a lot about the important processes that take place in the ocean, so hopefully we’ll get the chance to do many more measurements before the ocean is frozen over again. (Written with a lot of help from Markus). Breakfast: porridge, meat and cheese Lunch: chicken vegetable soup, meatballs, and fried rice Tea Time: lingonberry, strawberry doughnuts Dinner: baked chicken, mashed potatoes and pickled red cabbage
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