Hi. I'm Don Perovich. I'm a professor at the Thayer School of Engineering at Dartmouth College. I'm also a member of the MOSAiC Sea Ice team. Today I'm going to talk about my second most favorite sea ice parameter and that's the sea ice mass balance, and I like it for two reasons. First of all, I like it because it's simple and I like simple things. All the sea ice mass balance is how much the ice grows in the winter and how much it melts in the summer, both on the surface of the ice and on the bottom of the ice. During MOSAiC, we're going to make many measurements of the sea ice mass balance using highly sophisticated tools. Well, maybe not so sophisticated because what they are are sticks, ablation sticks that we freeze into the ice, and next to them, we put thickness gauges, which are wires with crossbars on the bottom. Since these are so inexpensive, we can put out a lot of these. Then in the photographs, you can see an example of these ice mass balance sites. What we do is we go to the site after we install it and we make two measurements. We measure the position of the surface of the ice, and then we pull up the thickness gauge and measure the position at the bottom of the ice. Two simple measurements where the key is to make those measurements for a long period of time. When we do that, we get a wealth of data. We get information on how snow accumulates during the winter, and how it melts in the spring. We also get to see how the ice grows and how it melts. It's a simple observation. Of course, it's not enough to be simple. The second reason I like it is because it's a powerful observation. For the past four decades, satellites have been monitoring the Arctic ice pack, the extent of the ice cover, and these maps show examples from September of 1980 and September of 2012. The satellites do a great job of showing what's happened. They show us there's been a tremendous loss of sea ice. They also do a really good job of showing us where it happened. We can see that we've lost ice off the coast of Alaska in an area called the Beaufort Sea, and also off the coast of Northern Russia. What they don't do is tell us how these changes have occurred, and if we don't know how they occurred, we'll never figure out why they're occurring, and that's where the sea ice mass balance comes into play. It's a way we can attribute these changes. To look at that, let's just consider the results from a single mass balance site. We pick a location, we install our ablation stake, our thickness gauge, and then we can put together a record, a time series, as you can see in this graph, of how the ice is changing over time. When we do that, we can have a whole annual cycle of what's occurring. In this particular case, we started out with ice that was 180 centimeters thick. During the course of the winter, it grew 66 centimeters. In summer though, it melted 110 centimeters, 76 from the top and 34 from the bottom. So we can see right off, is that for this piece of ice, it lost thickness over the course of the year. We can also see there was around twice as much melting on the surface and on the bottom, indicating that the atmosphere was playing a greater role. In addition to just measurements of changes in thickness, we also get some indications of timing. The timing of events. We see that ice growth didn't start till November, and we see that melting started in May and early June. This timing can be also important in interpreting how these changes are occurring. Now, it's nice to have results from one site, but what's really nice is when you have a lot of observations, and we'll be doing that during MOSAiC. We also did it in a past experiment called SHEBA, which was another year-long drift back in October of 1997-1998. What you see in this photograph is a picture of one of our ice mass balance farms. We deployed 100 mass balance sites during SHEBA, where we were able to look at how things varied spatially. We looked at every different kind of ice we could find, different thicknesses, different snow depths. Even better, we were able to do that over a whole year, and we could see the changes, as we can see here in this slide, that happen as we went from fall to winter, to spring, to summer. We could take all these measurements from the 100 sites and plot them together to see what was happening over this whole area. What we see is that the ice grew during the winter and then in the summer, in every place we looked, we melted more ice than we grew, that in 100 sites, we lost the amount of ice that we had before. Then we can look in more detail and see that we had roughly the same amount of losses on the surface of the ice as we did on the bottom, indicating that the atmosphere in the ocean were playing roughly comparable roles in things. Now, this is really informative. We learn a lot from things like this. But there is a problem, it's one place at one time. To really understand how the changes are occurring over the entire Arctic, we want to do a lot of places at a lot of times, and with that in mind, we developed an autonomous sea ice mass balance buoy. It's the next best thing to being there. It's an autonomous buoy. Instead of having an ablation stake and a thickness gauge, it has two acoustic sensors. One that's above the ice looking down, measuring the position of the surface, and one that's in the ocean looking up, measuring the position of the bottom. Instead of having somebody go out there and write numbers down every so often, we have a data logger that's recording information every hour, and then sending that information back home to us via satellite. This is a way that we can look at a number of different sites for a number of different years. We've been doing this for the past couple of decades and we've been able to put together a climatology, if you will, of the sea ice mass balance. That's what's shown here in this slide. What we have are results from three different regions, the Beaufort Sea in green, the North Pole in red, an intermediate between those two in blue. The results from 41 different ice mass balance summers for over 55 years. Now, these weren't all autonomous buoys. Some of them were from ice camps and some were from ships. But we put it all together, we have a record at different places spanning the time period from 1955-2014. We can look at these in a number of different ways, and one simple way is to just say, "What's the average amount of summer melting at these locations?" That's what we can see in this figure. We look at surface melting and what we see is we had the largest amount of surface melting in the Beaufort Sea. Then next, in the intermediate, in the least, in the North Pole. It's almost nice steps downward. When we think about this, it makes perfect sense because we're going further north, and as we go further north, there's less sunlight, and sunlight is a major factor in summer melting. Now, when we look at the bottom, it's a little bit more complicated. We see that for the intermediate site and the North Pole site, they're almost the same, slightly more in the North pole. But the real story is in the Beaufort. It has roughly twice as much bottom melting than the other sites. Again, if you remember, this was an area where we've seen the greatest ice losses over the past decade or two. So this is something we want to look at in more detail. What we can do is look at the dataset from a slightly different perspective, instead of comparing different places, we can compare different times. So we take the data from the Beaufort and we divide it up into two time periods, last century and this century. Then we take the average amount of melting for each of those time periods, and we see looking at surface melting, there has been an increase in surface melting this century compared to last. It's not terribly large. It's not large when you compare it to what's going on, and by the melting. We see it in the Beaufort, this century, there's more than three times as much bottom melting than we saw before. So this tells us that the ocean is playing an increasing role and gives us insight into where we need to look to understand the why of these changes are occurring. Now, MOSAiC has tremendous promise to continue this work. I'm incredibly excited by that because it's a chance to look at the mass balance of the new Arctic sea ice cover. Now, we'll be doing that in detail. We're going to have 100 ice mass balance sites. There's also going to be a number of temperature strings. Look at temperature profiles from the air through the snow, through the ice, into the ocean. Surrounding the main MOSAiC Central Observatory, we're going to have remote sites that use autonomous sea ice mass balance buoys. So we're going to work to get a complete picture, to get an extensive, integrated dataset studying the new Arctic ocean. When we look at MOSAiC, we realize that the data will be our lasting legacy. One more thing, I mentioned earlier that the sea ice mass balance is my second most favorite sea ice parameter. You might be wondering what the first one is. Well, to find that out, I suggest you listen to Bonnie Light's lecture on the optical properties of sea ice, and when you do, pay particular attention to the discussion about albido.
