What I'd like to talk to you today is about the Arctic Ocean. Let's first just go through some basics of the Arctic Ocean, basic features of the Arctic Ocean. Key points I want to get across is that the Arctic Ocean has a very complex hydrography. What we mean by that, there is a highly variable pattern of ocean depths across the Arctic Ocean. We have shallow shelf seas, continental shelf seas. There are deep basins were water depths are greater than 4,000 meters, very deep, and also restricted streets. The Arctic Ocean, of course, is a connection between the Pacific Ocean and the Atlantic Ocean.They are connected on both sides. But the Arctic Ocean is actually rather isolated because these connections are somewhat limited. In this sense, the Arctic Ocean has sometimes been described as a Mediterranean type of sea because of course, the Mediterranean Sea is rather isolated as well in terms of it's connections with the world ocean. There's a bit of an analogy there, but there are some very key differences and one of those is that, there are tremendous huge rivers that empty into the Arctic Ocean, some really big rivers. One of the things we'll see in a minute, this is very important because it keeps the service of the Arctic Ocean fairly fresh. This has a lot to do with the vertical temperature structure of the Arctic Ocean, the vertical salinity structure of the Arctic Ocean. These are key things and the river runoff is very important in these vertical structures of temperature and humidity. Now, also, the Arctic has a highly variable ice cover. Of course, that's the floating ice cover on top of the Arctic Ocean. Let's talk first about this complex hydrography. Extensive continental shelf areas where water depths are rather limited. Also, as I noted, we have these areas, these very deep bases, 4,000 meters or more of water. It's a hell of a lot of water. Of course, this Pacific connection to the Arctic Ocean, which is via Bering Strait, which is actually rather narrow and not very deep at all. We have a deeper connection to the Atlantic via Fram Strait. Now, what I'm showing here in this image is water depths. The hydrography of the Arctic Ocean. Just think of it as water depths. Well, the arrow I just brought in here, that blue one, is showing a continental shelf area that's in the red in this particular image. These are shallow seas, 30 meters deep, 40 meters deep, of course, very shallow near the shore until it gets to zero. We'll see the Arctic Ocean has a lot of these red areas, some really big continental shelves, wide continental shelves. Notably, on the Russian side of the Arctic, that's where a lot of these big shelves occur. Turns out there's a lot of oil and natural gas underneath there, which is something we'll talk about later. At the same time, there are these areas of very deep ocean, what we call deep basins. One of them is called the Canada basin, so named because it's north of Canada, 4,000 meters deep, more than that. Also, there's the Nansen and the Amundsen basins, separated by the Lomonosov ridge, which is an under ocean ridge going across the Arctic Ocean. Those are 4,000 meters deep or more. Pretty amazing the variations in ocean depth that we see across the Arctic Ocean. Now, what about these connections with the Atlantic and the Pacific? Well, I'm showing here the Bering Strait, that's then narrow strait that separates Russia or Eastern Siberia from Alaska and it's pretty narrow and is rather shallow as well. What happens here is, water from the Pacific enters the Arctic Ocean through the Bering Strait. There is also then the Fram Strait, that is a much deeper connection that occurs between the Arctic Ocean and the Atlantic Ocean. What happens there is that, Atlantic water, water from the south from the Atlantic moves into the Arctic via the Bering Strait. At the same time, there's water near the surface that exits the Arctic Ocean and moves out into the Atlantic Ocean. There's another connection through the Barents Sea, but that's in one of these shelf areas or a lot of it is in the shelf areas. It's a fairly shallow connection, but Atlantic water does move into the Arctic Ocean through the Barents Sea as well, it's called the Barents Sea branch, as you might imagine. A question. Were the Arctic oceans, continental shelves, these big continental shelf exposed during the last ice age, maybe 25,000 years ago? The answer is very much yes. During the last ice age, 20-25000 years ago, the sea level globally was something like up to a 135 meters lower than today. These continental areas were exposed. This is one of the reasons why we see what we call subsea permafrost. In the shelf areas, they've been exposed not just during the past ice age, but previous ones. Permafrost formed the water then rose and we still have permafrost in some of these areas. So it's quite fascinating that these areas were exposed during the last ice age. Now, what about these rivers that drain into the Arctic Ocean? I mentioned this, a number of really big ones. Well, there is the Ob River, the Yenisei, and the Lena. These are the really, really big ones that enter the Arctic Ocean from the Eurasian side. There's the Mackenzie River that enters the Arctic Ocean. But there are other ones as well. It turns out that the land area that drains into the Arctic Ocean is actually bigger than the Arctic Ocean itself. Big drainage area emptying right into the Arctic Ocean. This is key because this is one of the important things that maintains a fairly fresh, albeit cold surface layer. That surface layer is a fairly low density water layer, which inhibits mixing of waters from below, which are considerably warmer than the surface. That's very important, that the process of inhibiting mixing because it makes it easy for sea ice to form. This image is just showing some of the watersheds, these drainages, of these big rivers. The Ob, the Yenisei, the Lena on the Eurasian side, the Mackenzie, also the Yukon, and this other ones like the Colima, big rivers that drain into the Arctic Ocean as well as a whole bunch of smaller ones. As I said, this drainage area, the area of the land that drains into the Arctic Ocean is bigger than the Arctic Ocean itself. Key to maintaining that fresh surface layer. Let's take a look at that surface salinity we're looking at here a sea surface salinity in the Arctic Ocean. This is what we're going to see in summer. This is the summer pattern. If you look toward the bottom of that figure, what you see is a lot of green areas and the Arctic Ocean. Those are areas where the salinities around 35 parts per thousand. We use something called practical salinity units now, but just think of it as parts per thousand of salt in the water. Those are areas around 35 PSI or maybe even a bit bigger. Now that's typical of the world ocean. But if you go to and move up into the Arctic Ocean, you don't see much of that green anymore. But what you see is near the coastal areas, very, very low solidities. That is the impact of these rivers that are draining into the Arctic Ocean. So the rivers drain into the Arctic Ocean, coastal areas very fresh, but then that freshwater circulate around the Arctic Ocean keeping the surface of the Arctic Ocean fairly fresh. What does the vertical structure of temperature and salinity look like? The figure shows in red, the vertical structure of temperature. Temperature is read on the x-axis on the bottom, and depth of the water column is in the y-axis. What you see is near the surface, temperatures are fairly low, in other words, it's cold. But then as you see as we move down there is this rapid increase in temperature to a maximum around maybe 400- 500 meters depth depends on where you are. That has a name, that area of rapid increase of water temperature with depth it's called a thermocline. So we have the fairly cold water at the top. Then we have the rapid increase in temperature to a maximum depth of a maximum temperature down below. Now, that warm water down below, that's what comes into the Arctic Ocean from the Atlantic. Now, the blue figure there, or I should say the blue line is showing salinity. What do we see there? Now the x-axis there is on the top. So you look at the top and now you move from left to right with salinity increasing. What it's showing is near the surface, it's fairly fresh. A big determiner of that is the river run off there's more things going on. There's actually an excess of precipitation minus evaporation over the Arctic Ocean. So there's other things going along here, but the river input is really key. What we see is fairly low salinities at the surface and then layer where the salt increases, the salinity increases rapidly with depth. That area of rapid increase with depth has a name as well. It's called a halocline. Here's the important thing. Is that the density of water in the Arctic is very, very strongly tied to the salinity. With less salt, the water tends to be less dense. What we find is that the least salty freshest water is right near the surface. That means that water is very light, it does not have a lot of density. It wants to stay on top. It wants to inhibit the mixing of the warmer waters from below. If it wasn't for the halocline, we could have a mixing of those warm Atlantic waters up to the surface and it would be hard to form a lot of sea ice, but we do have that fresh layer at the top. This is important in making it easy for sea ice to form because there's not mixing and not a lot of mixing of that warm water from below. Now, let's turn to that sea ice cover. We've already introduced the sea ice cover in some previous videos. This is the floating sea ice cover of the Arctic ranges in thickness from a few centimeters maybe, 2, 3, 4, even 5 meters these days. Well, the ice extent, that's the number of square miles or square kilometers of ice it waxes and wanes with the season, of course as a mid-March maximum, and then there's a mid-September minimum. The sea ice grows through autumn and winter, a maximum around mid-March, and then shrinks through spring and summer to a minimum in September. But there's a wide range of thicknesses across the Arctic ice thicknesses and a wide range in concentrations. As I mentioned, it could be a thin veneer of ice to a five meters thick or maybe more, concentration ranging from 0-100 percent. Hundred percent meaning if you looked at it say a square kilometer, it would be all ice. A 50 percent concentration would be that it's 50 percent ice, 50 percent open water, zero percent it's just all open water. These two figures side by side are showing the extent of ice at the March maximum. This is looking at 2019 on the left and at the September minimum for that year. You can see this tremendous seasonal cycle in the extent of ice cover between the two seasons. On the image on the right, note that red orange line that is showing where the sea ice ought to be extending. In September in a typical year and we see were greatly retreated. Of course, that's something we've been seeing for quite some time now, this is overall retreat of the ice cover. It turns out September 19th ended up with a second lowest sea ice extent in the satellite record time 2007 and another year which I forget what it was at the moment. But this really shows you that strong seasonal circle and sea ice. This also shows that seasonal circles of sea ice, it is moving through the year and we get that maximum around mid-March, the sea ice shrinks to a minimum around mid-September. This figure is showing all the different years together. Some years is higher, some years are lower but all years are going to follow this seasonal cycle of sea ice extent. As I noticed that September ice extent in particular is really going down. Arctic sea ice thickness, as I mentioned it's highly variable. What we find is that the thickest ice and that's shown in the red areas here which the arrow is pointing to, that's the thickest stuff. What's happening there is that there's a motion of ice up against in part the coast of the Canadian Arctic Archipelago and it's squeezed together at big ridges form, and that's why it's really thick, thinner on the Eurasian side of the Arctic. By really at any parts of the Arctic Ocean, you can find thin ice, you can find thick ice. This particular image here was actually based on retrievals from satellite data. A question, for an ice flow three meters thick. Let's imagine a three meter thick ice flow with no snow on top, how much actually sticks above the Arctic Ocean? The answer is about 30 centimeters, because the density of ice is about 90 percent of that of water. About 10 percent sticks up, be same case with an iceberg. The proverbial tip of the iceberg follows the same thing. The density of sea ice is about 90 percent of that of water. Same thing with fresh ice like an iceberg. A three meter thick ice flow about 30 centimeters sticks up. There's snow on top of course can be complicated, but it's about that. I mentioned ice concentration, the concentration of ice. Those areas on the black are where the ice concentration is very high, like 95-100 percent, almost all of the areas covered with ice. But as you see as we move toward the south, there is lower ice concentrations like in the orange areas. This particular image was just average for 5-11 January of 1994. This is actually from a national ice center that does these analyses because knowing what the sea ice conditions are is very important to things like navigation. We'll continue with understanding of all the Arctic Ocean and sea ice in our next videos. Thank you.
