13 September 2018

The marginal ice zone

After we wrapped up our shelf break surveys on Tuesday afternoon, the Sikuliaq mates cranked up the ship speed to 10 kts and made a beeline east toward the pack of sea ice we’ve been seeing in satellite images. These images, which we’ve been getting about once per day, allowed us to make a very general plan of where to go. They provide some information about where ice is lingering and where there might be warmer plumes of surface water, but they can’t tell us anything about what’s going on beneath the surface. We devised a three-step plan to find and target the subsurface mixing processes that we’re out here to study.

Step 1 - Ship survey

By Wednesday afternoon, we were approaching the ice patch and started paying close attention to the data streams coming in. As the ship travels, it pumps in surface water and measures its temperature and salinity, among other properties. Current velocities throughout the water column are always being measured from the downward-looking ADCPs (Acoustic Doppler Current Profilers, which use the Doppler effect to detect velocities) mounted on the hull. Using this information, we could identify where we started transitioning into a marginal ice zone.

As we transited through a region of increasing ice cover, we noticed some changes in the water: salty, fresh, less fresh, back to salty, back to fresh. Once the ice pack became too tight to easily maneuver through, it was time to retreat and come back toward the pack again, this time sampling throughout the water column to see what’s going on beneath the surface. In a similar way to the shelf break survey, we mapped out a zig zag pattern to follow.

Step 2 - Deploy the surface toys and the FastCTD

At the start of the zig zag, the University of Washington team threw some toys in the water. The SWIFT drifters float along with the surface currents measuring wave height, water temperature, salinity, and surface velocities. The Wave Gliders measure similar things but their heading can be controlled. The robots were left to ride the waves while we spent the day profiling.

Our instrument of choice for exploratory transects in the FastCTD. Deployed from a boom off the back deck, it flies down to a depth of 200 m and back in about 2 minutes. This speed lets us see a sheet of water beneath us with high vertical and horizontal resolution. We positioned the boom directly off the stern so that we wouldn’t be dragging our cable through the ice. The mates on the bridge weaved in and out of the chunks and warned us to hold the profiler out of the water if there was ever a big piece of ice coming to close to the line.

We passed through an area of anomalously warm water centered at a depth of only about 20 meters. The heightened temperatures reached all the way to the surface. The proximity of this water mass, plenty warm enough to melt ice, to the edge of the ice pack is probably not coincidental.

Water this warm in this region of the Arctic only comes from Bering Strait. It had meandered its way west and out into the Arctic Basin where the water depth is 3000 m. The heat coming out of the ocean here could be matching or even exceeding the heat radiating down into the ocean from the atmosphere, meaning most of the work of melting sea ice could be accomplished by the ocean. We wanted to get a closer look. As an adorable bonus, because of the way we’d sliced through the feature, the blob looked a lot like a crab. Going back to sample more closely meant we got to say things like, “let’s crab along here” [Ba-dum tshhh].

Step 3 - MMP survey

The blob of warm water we passed through had little to no sea ice at the surface making it a safer place to sample with our free falling profiler than the waters we were moving into. The MMP drops strait down as the ship moves forward and then has to be dragged along to reposition at the surface. This is not something we want to do in ice.

We found what looks like a strong front and we’ll be back tomorrow to look at it again, but first we need to make a detour…

Extra step - Pick up the toys

As we spent the day surveying the marginal ice zone, the SWIFT drifters moved swiftly to the west and were fast approaching the 50 mile radius around Barrow, AK that we’d promised to stay out of. There are native Alaskan communities that rely on subsistence whaling and we are out here during hunting season. Getting too close means we could scare off the whales with the noise of our ship or, worse, break up the ice pack that the whalers use as a hunting platform preventing them from getting safely back to land.

At our nightly science meeting, as we transited back west to pick up the drifters, we were puzzled by the great distance they’d traveled. We’re out here trying to figure out how the deep ocean responds to the surface, but the drifters were carried by surface currents moving at speeds far exceeding those below. How much do the surface and deep waters actually respond to one another? Are they completely decoupled or do changes in one affect the other? Are there thresholds beyond which they act entirely independently? There’s a lot we still have to learn.