The wind is strong and the waves are high. The ship is rolling, up and down, up and down… It is difficult to walk straight down the hall way and we hold tight to the hand rails when walking down the stairs for dinner. I do hope they are not serving soup…
Maybe you’ve heard about the roaring forties, the furious fifties and the screaming sixties? That’s where we are right now… and at these latitudes (between 40-60 S) there are almost constantly strong winds from the west.
Everyone knows that if there is wind, there will be waves. And the stronger the wind, the larger the waves. The waves and the forces involved are impressive – you really do not want to be out on deck right now (and I figure that’s what the sign in Korean on the closed door tin the back is saying…) But there is more to the wind than waves! When the wind blows over the sea surface it “pulls” on the water (the physical term is that it excerts a “stress” on the surface) in the surface and cause it to move in the direction of the wind. But the water below it, which is not (yet) moving will “pull” in the other direction (friction). If the Earth did not turn, things would have been simple: The water had moved in the direction of the wind. The speed had decreased with depth, but all of the water had moved in the same direction. But the Earth do turn – one rotation per day – and with that everything gets more complicated… and more interesting! The results of wind blowing over water is not that the water moves with the wind, but it moves perpendicular to it!
The “warm” water that invades the ice shelves in the Amundse Sea is called “Circumpolar Deep Water” and most of it is water that sank down to the bottom of the North Atlantic a long time ago (a thousand year ago or so) and then started a slow voayage towards Antarctica. When arriving in the south, it gets caught by the Antarcic Circumpolar current, the world’s largest current which (unhindered by land) contourns the Antarcic continent. The winds in this region causes the surfacewater in the south to move southwards, and the surface water in the north to move northward… so in the middle there is no surface water left! The surface waters are then replenished from below, the (relatively warm) deep water is pumped upward by the wind and can then flow onto the shallow (400-500 m) continental shelf in the Amundsen Sea.
We are on our way home now and we’ve left Antarctica behing us. No more sea ice, no more penguins…. and very few ice bergs. A few of the scientist onboard are studying the eddies formed in the Antarctic circumpolar current, and their work ahs just begun… but I’m done. My instruments are in the water or up on the ice shelf* and all there is left to do is to write up the cruise report and cross our fingers that the icebergs will let the moorings be so that they are there when we come to pick them up, two years from now…
There’s still two weeks left of the cruise, and days are long now that they are not filled with work on deck. Someone brought along a bunch of styrofoam cups and (waterproof) pens, and last night was “decorate your cup night”… on one of the deeper CTD stations further north we’ll attach them to the CTD and send them down to the bottom. What do you think will happen?
[slr-togglebox title=”Guess before you click here to see how the cups looked like after being lowered to 3800 m”]
This is how the cups looked like after following the CTD to 3800 m depth! By what percentage has the volume of the cups shrunk?
* We’ve installed four ice radars on the floating shelf ice – every other hour they measure the thickness of the ice below them and then we can find out how quickly the ice shelf is melting. When Karen and Povl flew out with the helicopter to install them (I was busy deploying my moorings, so I could n’t go) they also tried to determine how much sea water there is below the ice. How to do that? It is simple! (At least in theory) All you need is a (special) microphone, a metal plate and a big sledge hammer… The you hit as hard as you can with the hammer on the metal plate… and listen for the echo! The first echo is the one reflected from the bottom of the ice, and the second one is the one refelcted from the sea bed! If the first echo arrives after 0.24 s and the second after 0.4667 s, how thick is then the ice? How far is it down to the seabed? The speed of sound is 1500 m/s in water and 4000 m/s in ice. To get a better result you use more than one microphone, placed along a line with a known distance from the sound source. When does the first echo reahc the microphone placed 100 m away? When does the second one get there? Don’t forget what mr Schnell said!