Anomalous collapses of Nares Strait ice arches leads to enhanced export of Arctic sea ice

Picture Nares Strait as a narrow turnstile between the Arctic Ocean and Baffin Bay. Each winter, ice arches—natural bridges of packed sea ice—tend to form at both ends: one up north where the Lincoln Sea meets Robeson Channel, and another down south in Kane Basin. When they hold, they act like seasonal gates. Thick, old ice piles up behind them, ages in place, and less of it spills south. Because the arches keep that heavy ice from clogging the southern end, winds can sweep thinner ice away near Smith Sound and help open the North Water Polynya, a recurring patch of open water that fuels a remarkably productive ecosystem. That's the classic picture. However, the Arctic is getting younger, thinner, and more mobile. Moore and colleagues set out to ask whether these gates are weakening—and what that means for how much multi-year ice we are flushing south. Historically, the pattern was dependable, if not perfectly regular. In the stretch from 1997 to 2009, the southern arch formed in most winters, while the northern arch showed up in about half of them. Then came 2007, a startling exception when neither arch formed. That winter, both the annual ice area and volume flux through Nares Strait jumped to about twice the usual for the period from 1997 to 2009. The new work takes that hint of instability and puts it in context: arch seasons are getting shorter, and both area and volume of ice exported through the strait have climbed sharply over the last decade. The picture is one of a regulator that's slipping, right as the pack itself gets easier to shove around. To make that claim stick, they needed to see ice move. Sentinel-1 satellites, which use synthetic aperture radar—think of it as all-weather, day and night imaging that can measure texture and motion on the ice—became the backbone. Moore and colleagues used the Komarov and Barber tracking algorithm to compare pairs of images taken about half to one day apart and pull out a motion field. Then they laid a measurement gate across the southern Lincoln Sea, buffered it by 30 kilometers to catch deformations, and sampled it every 5 kilometers along a span 139 kilometers wide. The idea is simple: the ice-area flux across that line equals the sum, piece by piece, of local ice concentration times the speed of the ice going straight across the line, times the width of each piece. Southward motion counts as export. The single-day uncertainty works out to roughly plus or minus 12 square kilometers per day for that gate. When averaged over a month or a year, those random wiggles practically cancel out. Motion alone tells you area. To translate that into volume—the stuff that matters for how much freshwater and old ice we are actually moving—you need thickness. Here, the team paired the area flux with mean ice thickness from the PIOMAS model, a widely used sea-ice model in this region, and cross-checked it with CryoSat-2 altimetry processed by teams at the Alfred Wegener Institute and University College London. Each source has its quirks; PIOMAS tends to underestimate thicker ice and overestimate thin ice. However, the independent CryoSat-2 retrievals track the PIOMAS story well enough to anchor the trends. The thickness story is stark: the Lincoln Sea's annual mean thickness fell from about 3.7 meters in the period from 1997 to 2009 to roughly 2.4 meters in recent years, with an uncertainty around three-quarters of a meter. Thinner ice should, all else equal, shrink the volume flux. The surprise is that the motion change outpaced the thinning. They were also careful about trend hunting in a noisy climate system. Sea-ice time series are red-noisy; they have memory, with more energy at slow, multiyear wiggles than at day-to-day bumps. To avoid fooling themselves, Moore's team generated one hundred thousand synthetic time series with the exact same power spectrum as the real record by shuffling the phases of its Fourier components and then inverting back to the time domain. For each fake series, they computed a trend. That gave them an empirical yardstick for what kind of trend you would expect just by chance, given the autocorrelated background. Against that yardstick, the observed changes—both the shorter arch seasons and the export jump—stand out as significant. Let's talk numbers, because they're the heartbeat here. The annual mean ice area flux through the southern Lincoln Sea gate roughly doubled, from about 42,000 square kilometers per year in the period from 1997 to 2009 to about 86,000 in the period from 2017 to 2019. Volume flux rose too, from 112 cubic kilometers per year, plus or minus 16, up to 190, plus or minus 55. So despite the Lincoln Sea thinning by more than a meter on average, more ice by volume still got flushed through. If you look across individual years, the run from 2017 to 2019 exceeded even the 2007 spike, when no arches formed at all. That's not a one-off; that's a regime nudge. One season makes that regime shift visceral. In winter and spring of 2017, the northern arch actually held late. On May 8, Sentinel-1 showed it intact, with ice barely inching along—well under a kilometer per day—on either side. Two days later, something gave. By May 10, the gate registered southward motion and peak speeds near five kilometers per day. Over the next four days, the whole structure unraveled, and velocities briefly surged to about 25 kilometers per day as thick, multi-year ice surged into northern Nares Strait. Export didn't just resume; it pulsed. After that collapse, the area flux was large and erratic, then dipped before flipping back into a persistent, high-throughput mode by late June 2018. That seasonal choreography tells you what the averages can't: arches modulate export in a switch-like way, and when the switch fails, the system ventilates its old ice fast. Zooming out, the fate of those switches is trending the wrong way if you like stability. When Moore and colleagues stitched together the northern and southern arch histories, the combined duration of winter arch conditions declined at roughly seven days per year, and that slope held up under their red-noise test with a p-value less than 0.01. Some winters in the new decade offered no gates at all—2007 and again 2019—while 2018 managed only a short-lived southern arch. Even in 2019, when an arch-like form briefly appeared, it didn't actually stop the ice. Contrast that with the earlier regime, when the southern arch was almost a winter staple and the northern one still formed about half the time. We are moving from episodic brakes to a freer-spinning wheel. Now, why does any of this matter beyond the physics? Because those gates and the ice behind them set the stage for life. The North Water Polynya—a latent-heat polynya, meaning winds and currents keep sweeping new ice away as it forms—depends on arches to keep thick floes from bulldozing into Smith Sound. If arches fail more often, more multi-year ice can intrude, potentially muting the polynya's open water and tinkering with the timing and intensity of plankton blooms that support seabirds, fish, and marine mammals. Communities that rely on that productivity feel these changes first. Just north of the Lincoln Sea lies what's often called the Last Ice Area, projected to hang onto perennial ice longer than most of the Arctic. It's not immune to export. If the tap through Nares stays open, even that refuge can thin and fragment. There's also a broader plumbing lesson here. Fram Strait still dominates ice export by area, moving on the order of 900,000 square kilometers of ice each year. But Fram's recent changes in thickness don't produce the same clear rise in volume flux. In Nares Strait, by contrast, the area flux has climbed so much that it more than compensates for thinner ice, so the volume export climbs too. That nuance matters for freshwater budgets and for how quickly the Arctic's oldest ice can be drained from its strongholds. A word on confidence and caveats, because with remote sensing and models you always ask: how tight are the screws? On the kinematic side, Sentinel-1's revisit and the chosen gate geometry mean they are sampling deformation where it happens. The daily area-flux uncertainty—about 12 square kilometers per day over a 139-kilometer span with 27 samples—shrinks to near zero when averaged over seasons. On thickness, PIOMAS's biases are real but understood, and the independent CryoSat-2 products from the Alfred Wegener Institute and University College London broadly agree on the Lincoln Sea's thinning. That red-noise-aware Monte Carlo test, with 100,000 spectral clones of the data, makes the trend detection resilient to the kind of slow swings that can trick a straight-line fit. If you like equations made human: the area-flux calculation is just bookkeeping along a line. Take each five-kilometer segment, ask how much of it is covered by ice today, multiply by how fast that ice is moving straight across the line, and add up all the pieces. Southward is positive; northward is negative. Volume is that tally times how thick the ice is on average near the gate. The rest is careful choices—where to place the line, how to trust the motion, how to estimate the thickness—and a lot of cross-checks. Put the pieces together, and the arc is clear. The Arctic's gates at Nares Strait are faltering as the pack gets thinner and more mobile, cutting the duration of winter locks and boosting both area and volume of export. The 2017 collapse turned that into a live demonstration, with velocities spiking from barely a crawl to highway speeds and a rush of multi-year ice flushing south. Across the period from 2017 to 2019, the annual area flux doubled relative to the period from 1997 to 2009, and the volume flux jumped from about 112 to about 190 cubic kilometers per year despite the Lincoln Sea thinning from roughly 3.7 to 2.4 meters. That is a system re-tuning itself. Where does it go from here? The safe bet, echoed by Moore, Howell, Brady, Xu, and McNeil, is that arch formation will become rarer as warming continues, simply because thinner ice is easier to deform and break. That likely means more frequent and sustained export of multi-year ice through Nares, with cascading effects on the North Water Polynya and on the Last Ice Area's role as a refuge. The toolkit they built—Sentinel-1 kinematics, a clean flux gate, thickness from PIOMAS and CryoSat-2, and red-noise-aware statistics—now gives us a way to watch those gates season by season, year by year. And that may be the deeper point. We are not just counting ice; we are watching the Arctic's mechanical heartbeat change. A natural brake is slipping. The wheel spins more freely. What was once a seasonal pause is becoming a brief hesitation, and what lies downstream—ecology, communities, and the last reservoirs of ancient ice—feels the difference.