Glaciology - Antarctic Ice Sheet (2) Atmospheric River

With reference to case studies, discuss the significance of atmospheric rivers for ice-shelf stability across Antarctica.

Fiona Fang, Trinity Hall

Antarctic ice shelves play a crucial role in regulating ice-sheet mass balance: by providing lateral and frontal buttressing, they restrain the flow of grounded ice and thereby limit contributions to global sea-level rise. Their stability is governed by a hierarchy of processes that operate over markedly different spatial and temporal scales. Recently, a growing body of work has identified atmospheric rivers (ARs)—narrow corridors of strong moisture and heat transport—as one of the key controls on the ice shelf stability. They typically originate in regions of strong subtropical evaporation and convection, which excite Rossby-wave trains that amplify the waviness of the polar jet (Gorodetskaya et al., 2023). As AR moisture condenses over the cooler Southern Ocean, latent-heat release further strengthens this jet disturbance (Terpstra et al., 2021). The resulting potential-vorticity anomalies promote high-pressure ridges and atmospheric blocking that can continue to funnel warm, moist air toward Antarctica even after the AR has dissipated (Pohl et al., 2021; Wille et al., 2024). This essay first examines the physical mechanisms through which ARs may influence ice-shelf stability, including their effects on surface mass balance, surface energy fluxes and sea-ice dynamics (Figure 1). It then evaluates three major ice-shelf collapse events—Larsen A (1995), Larsen B (2002) and Conger (2022)—to assess how ARs interacted with long-term structural preconditioning and the extent to which they acted as drivers, amplifiers or short-lived triggers of failure.

Figure 1 Conceptual sketch of an atmospheric river event and its potential impacts on Antarctic ice shelves. The example shown uses the Larsen Ice Shelf on the Antarctic Peninsula, but the illustrated processes are broadly applicable across the continent. Inspired by Wille et al (2022).

One of the primary impacts of atmospheric rivers on Antarctica is their capacity to deliver intense snowfall. Although ARs make landfall on Antarctic coastal regions for only ~3 days per year, they account for 10–20% of the continent’s annual snowfall (Wille et al., 2021; Gorodetskaya et al., 2014). In East Antarctica, they have generated 50–70% of extreme snowfall events since the 1980s (Wille et al., 2025). High-resolution ERA5 analyses further show that ~40% of extreme precipitation events in Dronning Maud Land arise from AR intrusions, which supply the highest daily snowfall totals (Simon et al., 2024). In principle, such large accumulation events increase firn-air content and inhibit ice-slab formation, both of which enhance resistance to hydrofracture (Dunmire et al., 2024) and could reduce future ice-shelf vulnerability (Veldhuijsen et al., 2024).

However, atmospheric rivers impact the Antarctic surface not only through snowfall but also through their strong perturbation of the surface energy budget, which can be expressed as:

Qnet=(SW↓−SW↑) + (LW↓−LW↑) + QH + QE + Qrain + Qground

First, downward longwave radiation (LW↓) increases sharply because ARs import deep, warm, moisture-rich air that supports mixed-phase and liquid-bearing clouds; these clouds have high emissivity and enhance nighttime surface temperatures by suppressing radiative cooling (Wille et al., 2022; Van Tricht et al., 2016). Second, ARs bring rain and drizzle – 40–60% of annual rainfall along Larsen and Wilkins occurs during AR events (Wille et al., 2022) – which transfers sensible heat directly to the snow or ice on contact, increasing the rainfall enthalpy flux (Qrain). Third, ARs enhance sensible-heat flux (QH) through adiabatic warming associated with Föhn winds: as airflow ascends the windward slope, it cools and loses moisture via condensation and precipitation; upon descending the leeward slope, the now drier air warms at the dry-adiabatic lapse rate, producing strong Föhn warming. This descent also reduces cloud cover, increasing shortwave radiation (SW↓) at the surface, a behaviour confirmed by in-situ observations and Polar WRF simulations by Zou et al. (2023). AR-linked Föhn warming typically raises leeward near-surface temperatures by 2–3 °C (Turner et al., 2021; Datta et al., 2019) and can exceed +10 °C under extreme cases (Zou et al., 2023). Finally, ARs elevate near-surface humidity, suppressing evaporation and sublimation such that latent-heat flux (QE) becomes less negative.

Combined, these modifications yield a strongly positive surface-energy balance that drives intense surface melt. Along the Larsen and Wilkins ice shelves, over 61% of melt rates above the 99.9th percentile occurred within 24 h of AR landfall (Wille et al., 2022). A notable example is the February 2022 AR, which produced an Antarctic Peninsula–mean melt of 6.2 Gt on 8 February – the largest February daily melt in the 1980–2022 MAR record (Gorodetskaya et al., 2023). However, ARs do not always generate strong melt; their impact varies with regional circulation and topography. During the 2020 “AR family” over the Thwaites Eastern Ice Shelf, for instance, the events delivered >100 kg m⁻² snowfall but only <5 kg m⁻² melt (Maclennan et al., 2023). Despite this variability, when ARs do produce extensive melt events, they become potent triggers of iceshelf weakening, with mechanisms discussed in detail below.

When sustained melt occurs, water initially percolates into the firn layers and refreezes, but repeated melt–refreeze cycles densify the firn and deplete its pore space, reducing the firn air content and therefore the capacity of firn to absorb additional meltwater (Veldbuijsen et al 2024). Once the firn becomes saturated, subsequent melt accumulates at the surface, forming ponds that further enhance local melt through reduced albedo and direct solar absorption (Donat-Magnin et al., 2021). As Wille et al (2021) demonstrates, there is a strong correlation (r ≈ 0.78) between AR frequency and observed melt-pond area on Larsen C. These ponds impose hydrostatic loads and, where they overlie surface or basal crevasses, the water pressure drives fractures downward through the ice thickness, a process known as hydrofracture. Pond filling and drainage also induce vertical flexure of the floating shelf, generating tensile and compressive stress anomalies capable of widening existing fractures and preconditioning the margin for failure (Banwell et al. 2019). Once fractures penetrate through the shelf, or once flexural stresses destabilise an already heavily damaged zones, large blocks would detach from the shelves. The resulting loss of frontal buttressing accelerates upstream ice flow, which further thins and stretches the remaining shelf, amplifying rift growth and promoting a positive feedback that drives rapid structural disintegration (Rignot et al., 2004). Given that currently 60 % of the Antarctic ice shelves (by area) both actively buttress upstream ice and are vulnerable to hydrofracturing if inundated by meltwater (Lai et al., 2020), atmospheric rivers are thus likely to become an increasingly important catalyst of future ice-shelf weakening and collapse under continued warming.

Another mechanism by which atmospheric rivers destabilise ice shelves operates through their impact on the sea-ice cover. When an AR reaches the sea-ice zone, its warm, moisture-rich air mass drives a strong positive surface-energy flux into the ice, which thins the ice, reduces its flexural strength, and inhibits nighttime refreezing, producing a fragmented cover characterised by weak floes and intermittent open water (Francis et al. 2020; Liang et al. 2023). Observational analyses show that in the marginal ice zone ARs can reduce sea-ice concentration by ~10% per day across all seasons, and individual AR events can remove up to ~50% of local concentration (Jena et al. 2022). Once the sea-ice belt becomes discontinuous, it permits long-period, large-amplitude swell from distant storms to propagate and reach the ice-shelf front with limited attenuation. These waves induce cyclic vertical bending of the shelf, generating alternating tensile and compressive stresses concentrated in the outer kilometres. In a pre-damaged or thinned shelf, such bending stresses are sufficient to propagate existing surface or basal fractures, trigger frontal calving, and remove the buttressing blocks that stabilise upstream flow. Subsequent reductions in back-stress then accelerate structural failure and can lead to rapid shelf disintegration (Massom et al. 2018).

These mechanisms outline the pathways through which ARs may interact with an ice shelf, but they do not, on their own, tell us how much ARs mattered in real collapses. Ice-shelf disintegration is a cumulative process shaped by long-term thinning, rift evolution, grounding-line retreat and ocean forcing; ARs intersect this preconditioning rather than replace it, as seen in the major ice shelf collapse events such as Larsen A (1995), Larsen B (2002) and Conger (2022).

For Larsen A (1995), the shelf was already weakened by multi-year surface-melt increase and rift development (Rott et al., 1996). With this state, an AR pulse on 24–25 January was thought to trigger the final collapse of the shelf, by bringing ≈3,987 kg m⁻¹ s⁻¹ cumulative Integrated Vapor Transport (IVT), >95th-percentile melt/runoff, and rapid sea-ice reduction that exposed the front to swell (Wille et al., 2022). Larsen B (2002) shared the same broad climatic trend but had developed a much more advanced internal failure mechanism: satellite records show a dense, hydrologically connected network of >3,000 supraglacial lakes that formed and then drained in the days immediately before collapse (Scambos et al., 2004; Arthur et al., 2020). This is through the run-away mechanism that once one lake drains, the resulting flexural stress triggered chain-reaction drainage of neighbouring lakes (Banwell et al., 2013). Wille et al. (2022) then place this cascade into a weather context: they identify a sequence of ARs from late December to late February, including a final event on 25–27 February 2002 (≈5,602 kg m⁻¹ s⁻¹ IVT) that produced >99.9th-percentile melt and runoff, complete sea-ice loss, and swell extremes about 3–5 days before disintegration on 2 March. In other words, at Larsen B the lake system and structural weakness pre-existed, and the late-summer ARs supplied the last, extreme melt pulse that loaded this system and triggered the observed lake-drainage chain.

In contrast, the Conger-Glenzer Ice Shelf (2022) failed with a different type of structural preconditioning. Walker et al. (2024) document ~80 m of multi-decadal basal thinning and loss of a pinning point, with no pre-collapse surface melt detected in Sentinel-1 backscatter, meaning the hydrofracture mechanism was inactive. Although a strong AR produced +30–40°C anomalies during the March 2022 heatwave (Wille et al., 2024), the immediate trigger was an embedded extratropical cyclone that removed fast ice and exposed the already-thinned shelf to swell-induced flexure (Walker et al., 2024; Teder et al. 2025). Together, these cases demonstrate a consistent pattern that ARs do not cause collapses on their own, but they can trigger failure once long-term structural integrity has already been critically reduced.

To sum up, this essay examined the processes through which atmospheric rivers can interact with Antarctic ice shelves, showing that their effects depend strongly on the background state of the surface and the shelf itself. Under colder conditions, ARs predominantly contribute snowfall, which can increase firn-air content and delay the development of impermeable ice slabs. However, when ARs deliver strong positive surface-energy anomalies—through longwave enhancement, rainfall enthalpy, Föhn warming and suppressed sublimation—the same moisture flux can drive intense melt, firn saturation, melt-pond formation and hydrofracture. ARs can also thin and fragment the sea-ice cover that normally buffers shelves from swell, enabling long-period waves to impose damaging flexural stresses on already weakened ice fronts. The Larsen A, Larsen B and Conger case studies demonstrate that collapses occur only where such processes intersect long-term preconditioning by basal thinning, rift growth and loss of pinning points. Since climate change will likely cause stronger ARs as anthropogenic warming increases atmospheric water vapour (Wille et al. 2025), the episodic stresses associated with extreme melt, rainfall and sea-ice disruption may become more frequent, particularly for shelves already near mechanical or climatic thresholds.

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