In late April, Jeff Bowman of the Scripps Institution of Oceanography and John Cassano of the University of Colorado Boulder returned from a three-month stay on the Polarstern, a German icebreaker locked within the Arctic Ocean ice pack. The scientists were part of a 20-nation research effort known as the Multidisciplinary Drifting Observatory for the Study of Arctic Climate (Mosaic), which is led by Germany’s Alfred Wegener Institute. The Polarstern is to spend a full year trapped inside its drifting floe while scientists observe how the ice, atmosphere, and undersea ecosystem evolve over the course of the seasons. They have deployed a network of buoys, sensors, and other instrumentation across the ice at a distance of up to 40 kilometers from the vessel.
Polarstern departed Tromsø, Norway, on 20 September 2019. Though the original plan was to employ six two-month rotations of personnel, the coronavirus pandemic necessitated combining the last four legs into three extended ones. A total of 600 scientists and support staff, including nearly 100 from the US, will have participated by the time the ship extracts itself from the ice in October. The sponsoring agencies from the US are NSF, the Department of Energy, NASA, and NOAA.
Bowman and Cassano, along with Frank Rack, the NSF Mosaic program manager, spoke to Physics Today last month.
PT: What was it like aboard a ship locked in ice?
BOWMAN: We had to take a Russian icebreaker to get up to the Polarstern. That transit took a month in the late winter, and it was completely dark. Just when we got to the Polarstern we started to get into twilight. After a couple of weeks we saw the Sun rise.
PT: You were there during the coldest part of the year.
CASSANO: I was expecting to see –30 °C, but we got down to –40°. Those 10 degrees make a difference when you are out working.
PT: Can you describe a typical day on the ship?
CASSANO: At 8:00am there was a weather briefing. We had an all-hands meeting for the scientists at 8:30 to reconfirm plans for the day: who was going where on the ice and who was serving as polar bear guards for each of the science groups. After lunch we’d be back on the ice from 12:30 to 5:30, followed by dinner. We had another all-hands meeting at 7:30 each night to talk about plans for the next day. We worked six days a week on that schedule.
BOWMAN: For me there were no two days alike. Some days we would be entirely contained in a lab on the ship or processing samples or conducting an experiment. Other days we would be out on the ice to get more material and make observations and experiments, and other days we’d be repairing or maintaining equipment.
PT: What were your specific research projects?
CASSANO: My group was flying drones to make temperature, humidity, wind, and turbulence measurements of the lowest 1 kilometer of atmosphere. Our main intent was to sample how the bottom part of the atmosphere changes as we move from winter through the melt season into summer. It was exciting for us to be, in the middle of the winter, in the central Arctic, where there are almost no other weather observations made, certainly no profile-type measurements that give you a sense of how the atmosphere is interacting with the underlying ice surface. From the initial look at the data we collected, we observed very strong inversion conditions and conditions that were a bit colder than we expected at the surface. We observed very rapid warming as storms passed by and brought up warm air to the polar latitudes. We’re still trying to make sense of all the data.
PT: What is the significance of inversion conditions?
CASSANO: Normally in the lowest 10 kilometers of the atmosphere, the temperature decreases with height. During the extended polar night, the ice is radiating lots of energy, it becomes very cold, and the air in contact with it gets very cold. So you get temperature increasing with height. Inversions are relevant because they suppress mixing and reduce turbulence.
Turbulent processes are one of the ways the atmosphere communicates with the ice, ocean, or land. Understanding turbulence is important for coupled climate-system processes, and it’s one of the weak points in our climate models. They don’t do a very good job of representing turbulence and coupling to the surface when inversions are present. We saw very strong inversion conditions at the beginning of leg 3 and then the transition away from those conditions over the course of the leg as the temperature profile changed.
BOWMAN: I’m a biological oceanographer. My group is largely interested in the dynamics of the microbial community in the water column throughout the year. We look at the impacts of that microbial community in terms of the different trace gases they emit, particularly methane and carbon dioxide, that end up in the atmosphere. A lot of our observations take a very long time to develop. They are based on DNA sequence analysis of the microbial community. It’s a multiyear process.
But we do have some real-time observations. We look at gases in the water that are indicators of the trace gases we want to observe in the atmosphere. We use a flow-through mass spectrometry system to get continuous measurements of the concentration of oxygen and argon in the surface water. That tells us something about the net productivity of the upper water column. The amount of photosynthesis or respiration that is dominant in the upper water column drives the amount of CO2 that’s in the water and getting into the atmosphere. We expect to see respiration dominating during the wintertime, when there’s not a lot of light to drive photosynthesis, and the opposite in the summertime. What we didn’t necessarily expect to see was a such a strong physical signal in the system as well.
PT: Why are you measuring argon?
BOWMAN: To disentangle the physics in the system from the biology. Photosynthesis produces oxygen, and respiration consumes it. But there are physical processes that are also injecting oxygen from the atmosphere into the water, such as when a lead [fissure in the ice] opens or waves break. Those sources of oxygen throw off our ability to distinguish photosynthesis from respiration. Argon is relatively abundant in the atmosphere, it partitions into water in a similar physical sense as oxygen does, and it is inert. So if you track oxygen and argon in the system, you can decouple the physical from the biological.
PT: If Mosaic is a first-of-its-kind expedition, what do you have to measure your data against?
CASSANO: We have a blend of short-term forecasts, satellite, and other observations that go back 50 years. My group is comparing that data with what we measure now to give us a sense of how much the Mosaic year differs from what we think happened in the past.
There was also a yearlong project north of Alaska in the late 1990s called Sheba [Surface Heat Budget of the Arctic Ocean]. That is a touchpoint to give us a sense of the amount of change that has taken place in the Arctic over the last 25 years. The most obvious change is that the sea ice is so much thinner and less extensive than it was 20 years ago. We saw that firsthand. The ice was much more dynamic, much more mobile than we would have expected for the end of winter. Almost daily, new leads were opening up, and ridges were forming that made it very difficult for us to work on the ice.
PT: How far did the ship drift in the ice?
BOWMAN: It certainly drifted more than expected. It headed north, then south, as we drifted past the North Pole. We don’t know how unusual that drift trajectory is.
CASSANO: The total distance traveled by the Polarstern [as of when the two scientists left the ship] was 7667 kilometers, including sailings to and from the ice. The drift is probably one-third of that, or a couple thousand kilometers since October.
PT: Will the ship remain locked in the ice until the expedition ends?
BOWMAN: That’s a big unknown. The Polarstern had to maneuver to maintain its position relative to the floe. And with the COVID-19 situation, the ship couldn’t be resupplied by ships or aircraft and had to come out of the ice and go to Svalbard to resupply and exchange personnel. The ship then reinserted itself in its old location in the pack ice. The idea is to stay with the old floe as long as possible, even as the ice is breaking up, to see how the ice has evolved throughout an entire year. That’s the most valuable set of observations we can get. If the ice disintegrates, there will be discussion about reinserting the ship farther north to continue observations for the rest of the year.
RACK: There were autonomous instruments left behind in the ice during the resupply in Svalbard, so the time series for some measurements continued throughout that process. Now is the interesting part of the season when you see melt ponds on top of the ice and the biological bloom conditions evolve.
PT: Was the experience worthwhile? Would you do it again?
RACK: It’s a great opportunity for grad students, postdocs, and early-career scientists. This is the best thing they’ve done in their careers to date. We’re hoping that this investment in young scientists will reap benefits.
CASSANO: I had a student with me on my leg. Judging from talking to her and reading her blog on the expedition, it was a life-changing experience for her. It made her excited to be a polar scientist and look forward to being involved in this type of work for the rest of her career.
As a grad student, it was going to Antarctica on an NSF-funded project that put me on the path to becoming a polar scientist. I had planned to be an atmospheric scientist, and I didn’t really think about polar work until I had the opportunity to go to Antarctica as part of my master’s. I said, “Yeah, I’ll go check it out, take a trip.” I never expected to go again. I’ve been there 14 times now.
BOWMAN: It’s imperative that the global scientific community figure out how to do this type of observational campaign on at least a decadal cadence. As rapidly as the Arctic is changing and as central as it is to the global climate system, it’s critical we understand how it’s changing.
