My name's Bill Shaw. I'm a research professor at the US Naval Postgraduate School. It's a graduate school for a naval and other military branch officers, we're in Monterey, California and my specialty is physical oceanography, and that basically means applying the classical laws of fluid dynamics to the ocean. So right now I'm standing on the half deck of the Russian research vessel Akademik Fedorov and we're on our way home from deploying some instruments as part of the Mosaic Project, which you hear all about is big project, and look at the evolution of sea ice over the course of a year. Okay. So now we just stepped into the captain saloon, a much better place to give a talk on ocean circulation than not there on the deck, and I'm going to break this down into two parts. The first is ocean circulation driven by the winds, and the second is ocean circulation driven by buoyancy. So the first component of ocean circulation is that, that's driven by the winds blowing against the sea surface and so to start to understand that, you have to look at the meteorological weather patterns. So in this map of the Arctic Ocean, we're looking straight down at it from the North Pole. I've sketched on the locations of the two predominant weather systems of the Arctic. There's a high pressure system in the Western Arctic, shown there in green. This is known as the Beaufort High and then North of Siberia on the lower right corner of the map is a large low-pressure system. So now I will show you a diagram what these pressure systems do in terms of winds and ocean currents. So on the top there, again, this is just a sketch of the, for example, the Beaufort High, you have what those green circles represent, contours of atmospheric pressure, so the highest in the middle and then decreasing as you go outwards and that makes a pressure gradient that's flowing, that's pushing I should say, from the middle of the pressure pattern to the outside. Now this is where one of the little strange things about geophysical fluid dynamics kicks in, is that usually for flows that were used to, usually water or air will flow from an area of high pressure to an area of low pressure. But at the very large scales of ocean and atmospheric flows, the effect of the Earth's rotation is significant and it changes that simple law. So for these large flows that occupies significant parts of the planet, instead of flowing straight down the pressure gradient, the winds or the currents that will flow to the right of the pressure gradient, indicated by the black arrow on my diagram that's perpendicular to the green arrow that represents the pressure gradient.. So for the Northern Hemisphere, the flow is 90 degrees to the right of the pressure gradient and for what it's worth in the Southern Hemisphere, it's just the opposite and it's 90 degrees to the left. So now I drew a line across the middle of that pressure pattern mark a, a prime. Now down below that, now I'm going to show you a cross-section of what the atmosphere and the ocean looked like through that pattern. So starting at the top of the cross-section, I've tried to sketch that on the right-hand side, the atmospheric winds are blowing out of the page, whereas on the left-hand side, they're blowing into the page. So it's the same principle here is that you have a forcing on the surface of the ocean. The ocean, it will respond by flowing to the right. So you have excess. Ekman transport is pushing water into the middle of now what we're calling an ocean gyre. Gyre just means the circulating pattern, and by converging water into the center of the gyre, it actually pushes up to sea level, that's indicated by the blue line in the diagram. So once we push up the see level like that in the middle, and now this is creating another, now pressure gradient that's pointing outwards from the gyre. Again, now this is going to start creating ocean circulation that is going around in a circle, just like the atmospheric circulation does. Okay. So now let's talk some about the buoyancy driven component of Arctic ocean circulation. Now this may sound like a little bit of a strange analogy, but it's common to talk about the Arctic Ocean as basically being a huge estuary, and the reason this analogy works is because there's huge fresh water inflows to the Arctic Ocean from the major Siberian rivers and also from the Mackenzie River of Northern Canada. These flow into the Arctic Ocean and in this little cartoon diagram here, you see they're forming a layer of fresh but cold water on the surface of the Arctic Ocean. The Arctic is connected, on the left, it's connected to the Pacific Ocean through the shallow Bering Strait and on the right of the diagram, you can see it's connected to the North Atlantic through the much deeper Fram Strait. You have this cold but freshwater which is more buoyant than the water of the Atlantic Earth Pacific on the surface of the Arctic Ocean and when it wants to flow out into the Atlantic and Pacific. So you get an exchange flow through both the Bering Strait and the Fram Strait with a cold, fresh arctic water leaving and relatively warm and salty Atlantic water coming in. The Atlantic layer enters the Arctic to the depth of the Fram Strait, which is almost 1,000 meters or so, and you got a lot of water coming in through there and a lot of heat. Now we can see in this map again of the Arctic Ocean, we see what these inflows look like as they enter the Arctic Ocean. So the Atlantic layer is coming in from the bottom side of this map. It goes around the Islands of Svalbard, but a lot of it's going through Fram Strait, some to the Barents Sea. There's another little geophysical principle at work here, is that once this water has found a certain water depth, it wants to stay at that water depth, and it's called topographic steering. So that's why as this water comes into the Arctic Ocean, it follows around the edge of the shelf break and it'll go all the way around the Arctic and actually they'll be little diversions coming across the Lomonosov and Markov Ridges. So there is a huge transport of heat associated with this and it's people are still not quite sure where all that heat goes. We're just coming back from our field program where we're deploying a set of instruments around the main site where the Polarstern is. One of the things we were deploying, and you can see it in this picture. I'm pretty much blocking it there with my bag. But there's an acoustic Doppler current profiler that we're putting down through that ice hole into the water. So that will measure the upper ocean currents of a type that Ekman was studying. It'll provide a great profile of those currents, and we also put out 30 or more buoys just to have GPS positions and they will act as trackers showing how the ice is moving. So we get the velocity and the positions of the ice and then when we're also measuring down to 50 or 80 meters what the current structures like. So I've especially as that whole installation moves towards Fram Strait, well, I think we'll see some pretty exciting patterns and the currents there. So I want to end up with just a, I'll do a little bit discussion about connections between Arctic circulation and climate change, and I think I shall do this in a form of three questions. What is the fate of a large quantity of heat entering the Arctic Ocean through Fram Strait? There's enough heat entering the Arctic Ocean through Fram Strait to more than melt all the ice cover, but a lot of it as that diagram, and actually, a lot of it just hugs the shelf break, and then goes all the way on the on the Arctic and somebody would actually just comes right back out, but not all of it. I think people say about half of that heat that enters actually makes it all the way around and comes back out into the Atlantic Ocean. The fate of the heat that is lost as actually quite unknown. Now there's a much smaller amount of heat coming in from the Pacific Ocean but people are starting to think that even though it's relatively small amount of heat, that it might be important for triggering seasonal ice loss in the Western Arctic and Beaufort Sea. It just turns out that if you start looking at the patterns of where this heat goes in the Western Arctic, it aligns fairly well with in some years where the big seasonal ice melt back occurs. It's thought that there's enough heat there that it might be enough to just boost the melting that's caused by solar heating during summer. Finally, how does storage of fresh water and sea ice in the Beaufort gyre affect sea ice cover and ocean circulation? You remember I was saying that for a high pressure system, it's a convergence zone. So it's holding in freshwater from ice melting and ice itself. The position of the Beaufort High changes or if its strength changes, it'll change the position where that water is being released. So it's thought that a weakening of the Beaufort gyre would lead to more ice loss through the Transpolar Drift. It's also possible that they'll be a sudden outflow of more freshwater than usual through the Transpolar Drift, and that it might even affect ocean circulation in the North Atlantic, like maybe even the formation of deep water off of Greenland.
