How a Mountain Chain Steers Antarctic Sea Ice: Hidden Drag Mechanism Revealed

Antarctic sea ice is one of the most stubborn puzzles in modern climate modeling. For decades, fully coupled climate models have struggled to reproduce its extent, its variability, and its sometimes counterintuitive trends. Now, a new study published in the journal Climate Dynamics has traced a surprisingly deep root of that problem to an unexpected place: the way a model handles the drag that the Antarctic Peninsula exerts on the winds that crash into it. According to the research team led by Maria-Vittoria Guarino of ENEA and the International Centre for Theoretical Physics, together with Jeff Ridley of the UK Met Office and colleagues, a seemingly technical tuning choice inside one climate model can shift the timing of a simulated Antarctic sea ice decline by as much as two decades.

The study focuses on HadGEM3-GC3.1-LL, the low-resolution configuration of the UK’s CMIP6 climate model, which simulates the atmosphere, ocean, land, and sea ice as a fully coupled system. The atmosphere component, the Met Office Unified Model, runs on a grid of roughly 135 kilometers, which means that much of Antarctica’s rugged terrain is too small to be resolved directly. Instead, the model relies on a parameterization of orographic gravity wave drag, or OGWD, to represent the forces that sub-grid-scale mountains impose on the flow. This parameterization splits the total drag into two components: flow-blocking drag, which arises when air is too stable to climb over an obstacle and is deflected around it, and gravity wave drag, which arises when air does surmount the terrain and generates waves that break aloft and deposit momentum.

Which of these two regimes dominates depends on a delicate balance captured by the non-dimensional parameter Nh/U, where U is wind speed, N is atmospheric stability, and h is obstacle height. When Nh/U greatly exceeds one, blocking prevails; when it is near or below one, the flow passes over the mountain. In the Unified Model, the effective sub-grid mountain height is set by a tuning constant called n-sigma, with a default value of 2.5 that is among the least constrained parameters in the entire model. The researchers exploited this lever to run two sets of sensitivity experiments over Antarctica: one, called F_BLOCK, in which n-sigma was doubled to 5, making virtual mountains effectively taller and forcing the flow into a blocking regime, and another, called F_OVER, in which n-sigma was reduced to 1.5, nearly halving the barrier height and letting winds sweep across the terrain while generating strong gravity waves. Crucially, the modifications were applied only over Antarctica, so any circulation change in the simulations could be attributed unambiguously to the southernmost continent.

The results were striking. In the baseline historical simulation, pan-Antarctic sea ice area begins a sustained decline around the early 1980s, driven largely by the Weddell Sea, one of the continent’s main sea ice factories. In the flow-blocking experiments, that decline is delayed by roughly ten to twenty years, with the ensemble mean holding steady until just before 1995. In the flow-over experiment, the decline begins about ten years earlier still, around 1972. Because the delayed decline appears consistently across all four members of the flow-blocking ensemble, the team could rule out natural variability as the explanation: the differences are a systematic response of the coupled climate system to how orographic drag is represented.

The immediate physical mechanism operates through the atmosphere’s vertical structure. In the flow-over configuration, larger-amplitude gravity waves break in the lower and mid-troposphere, decelerating the background westerly flow and weakening the tropospheric polar vortex. In the flow-blocking configuration, weaker wave amplitudes mean less drag and stronger zonal winds aloft. A stronger polar vortex insulates the Antarctic continent, exchanging less heat with the mid-latitudes, so surface air temperatures drop in F_BLOCK and rise in F_OVER. Colder conditions strengthen the katabatic winds, the rivers of cold, dense air that drain from the high interior toward the coast, which push sea ice offshore in the Weddell Sea and promote the formation of fresh ice. Within the first one to two years of simulation, sea ice area rises by about one million square kilometers in F_BLOCK and falls by a similar amount in F_OVER relative to the historical run.

The long-term story, however, unfolds in the Weddell Sea, and it hinges on the Antarctic Peninsula acting as a formidable barrier to the prevailing westerlies. In the flow-blocking experiment, the incoming flow cannot clear the ridge, so westerly winds both upstream over the Amundsen and Bellingshausen Seas and downstream over the Weddell Sea are weakened. Mean sea level pressure anomalies generated by the blocking propagate vast distances around the continent, echoing earlier work by Sandu and colleagues showing that orographic blocking leaves fingerprints far beyond the mountains that create it. June-to-November westerly winds at 850 hectopascals across the Weddell Sea are reduced by about half a meter per second, roughly sixteen percent, in F_BLOCK, and strengthened by a comparable margin in F_OVER.

Those wind changes translate directly into the ocean. The total surface stress exerted on the Weddell Sea, the combined push of atmosphere and sea ice on the water, is much weaker under flow-blocking conditions and much stronger under flow-over conditions, partly because stress scales with the square of wind speed and partly because thinner or reduced ice allows more efficient momentum transfer. Stronger winds spin up the cyclonic Weddell gyre, whose depth-integrated circulation, measured by the barotropic streamfunction, strengthens steadily through the historical run. A stronger gyre makes the surface flow more divergent, and by continuity, relatively warm, salty water is pumped upward from depth, with Ekman pumping in the historical simulation rising from roughly 55 meters per year in 1980 to about 85 meters per year by the end of the run.

Near Maud Rise, at about 66 degrees south and 3 degrees east, weak stratification and favorable topography set the stage for open-ocean deep convection, the vertical mixing that draws heat from the deep ocean to the surface. In the model, these events are diagnosed when the ocean mixed layer exceeds 2000 meters in depth, and polynya-scale events are flagged when the convecting area surpasses 80,000 square kilometers, the size of the real Weddell Sea polynya of 2016 and 2017. The correspondence with sea ice is dramatic: every cluster of deep convection years coincides with a sharp drop in sea ice area. In the historical simulation, convection recurs almost every year from 1989 onward, and in F_OVER it appears even earlier and more often. In F_BLOCK, by contrast, deep convection is nearly absent, with only a handful of small events across sixty-five years, and the ice survives largely intact.

The team then turned the correlation into a diagnostic tool. Plotting annual sea ice area against ocean surface stress reveals a strong negative relationship, with a correlation coefficient of minus 0.75, and the historical simulation migrates over time from a low-stress, high-ice state toward a high-stress, low-ice state that mirrors the flow-over experiment. Using the observational range of sea ice area from the HadISST1 dataset as an emergent constraint, the researchers estimate that plausible surface stress values lie between 0.042 and 0.066 newtons per square meter, a range that the flow-blocking simulation satisfies best. That result suggests the default model may underestimate atmospheric blocking by the Peninsula, and that a calibrated tuning of the sub-grid orography there would be advisable.

The broader implications reach beyond model tuning. The study shows that a more positive Southern Annular Mode does not necessarily mean stronger winds over the Weddell Sea, because the Peninsula’s barrier effect can override the large-scale annular signal. It also suggests that the springtime strengthening of westerlies over the Weddell Sea seen in the historical run, which is consistent with ERA5 reanalysis, may partly reflect a long-term shift in the Peninsula’s flow regime toward weaker blocking, driven by declining static stability, rather than greenhouse and ozone forcing alone. And because the spurious deep convection is fueled by wind-driven Ekman pumping, a large-scale process the model resolves explicitly, simply tweaking vertical mixing parameterizations will not fix it. The authors point instead to revising how ocean surface stress is represented, potentially through variable drag coefficients that respond to the evolving state of the ice and atmosphere, an approach already under development for the next version of HadGEM3. In the end, the message is humbling and exhilarating in equal measure: a single tuning constant, buried in the treatment of invisible mountains, can decide whether a flagship climate model loses its Antarctic sea ice a decade early, a decade late, or not at all.

Subject of Research: The influence of orographic gravity wave drag and flow regimes over the Antarctic Peninsula on Weddell Sea sea ice and deep convection in a coupled climate model

Article Title: Southern Hemisphere sea ice response to different flow regimes over the Antarctic Peninsula

Article References: Guarino, M.-V., Ridley, J. K., Farneti, R., Kucharski, F., & Tompkins, A. M. (2026). Southern Hemisphere sea ice response to different flow regimes over the Antarctic Peninsula. Climate Dynamics, 64(9), Article 410. https://doi.org/10.1007/s00382-026-08350-6

Image Credits: AI Generated

DOI: 10.1007/s00382-026-08350-6

Keywords: Antarctic sea ice, Antarctic Peninsula, orographic gravity wave drag, Weddell Sea, deep convection, polynyas, HadGEM3, CMIP6, wind stress, Ekman pumping, Southern Annular Mode, climate modeling