Ice shelf calving and collapse cooccurrences with ARs
To examine cooccurrences between AR landfalls and iceberg calving along the Larsen ice shelves, we produced a climatology of AR landfalls along the AP from 1989-2020 using output from the AR detection algorithm with input from MERRA-2 reanalysis where instantaneous intensity is measured by integrated vapor transport (IVT) (see Fig. 1 and Methods for AR annual frequency and specifics on the AR detection algorithm respectively). We also examined calving and collapse events along Larsen A, B and C (between 64.5°S and 68.5°S) using MODIS-Terra and Aqua visible imagery from August-March 2000-2020. We observe that AR landfalls along the AP are slightly more frequent than other coastal regions of the continent 23 (around 1-5 landfalls per austral summer, i.e., between December-March) and occur during strong downstream blocking conditions thus agreeing with previous Antarctic AR climatology assessments 20,23. We found 21 major calving and collapse events between 2000-2020, all occurring before a decrease in AR activity after 2010. 13 out of the 21 calving events were preceded by an AR landfall within 5 days prior (The Larsen C calving in July 2017 is not considered here as it could not be retrieved during austral winter using MODIS although an AR was detected within 5 days prior of the supposed calving date; see Supplementary Data 1). A statistical analysis based on thousands of artificial AR occurrences at randomly selected dates between August and March from 2000-2020 confirms that ARs and these calving/collapse events were not independent at p = 5.3 10-5. The number of co-occurrences decrease if we consider AR landfalls within less than 5 days prior to calving events, but the co-occurrences are still statistically significant (except if we only consider cooccurrence on the same date) confirming that these ARs and calving/collapse events were not independent. In contrast with high swell events (swells within the 85th percentile of height), ARs are more likely to cause a calving/collapse event with only 9 out of 21 events cooccurring with 5 days prior of a high swell.
As an example of AR impacts on ice-shelf weakening processes, the intense AR that made landfall from January 24-26, 2008, is particularly striking. MODIS satellite imagery shows that this historically intense AR detection (IVT ~ 962 kg m-1 s-1, third highest intensity of all AR landfalls in our AR climatology) disintegrated and fragmented nearly all the land fast ice in the Larsen A and B embayments and generated 6.3 Gt (7.2 Gt) of runoff (meltwater) simulated by MAR likely leading to a calving event 6 days later (Fig. 2). This calving event (not included in our 13 cooccurring calving events) led to a 11 km retreat of the eastern region of the remnant Larsen B (named Scar Inlet ice shelf) 25. Other striking examples are the multiple AR events that preceded the collapse of the Larsen B in austral summer 2002 and the historically intense AR preceding the collapse of the Larsen A ice shelf and associated iceberg calving (~1700 km2) from the Larsen B in 1995 15,26 (Supplementary Fig. 3). In all these cases, the AR generated several Gt of meltwater and runoff, large reductions of sea-ice concentrations in the Larsen embayment, strong winds, and large swells (Table 1, Supplementary Fig. 3, see the supplementary materials for more examples of AR landfalls and their associated impacts on processes connected to ice-shelf weakening).
Atmospheric river impacts on processes detrimental to ice shelf stability
Hereafter, we demonstrate that ARs generate cooccurring extreme temperature, melt, runoff, and swell events, by analyzing values within the 99.9th percentile that occurred 24 hours before and after an AR landfall. Given AR landfall frequency on the AP, a result above 61% of cooccurrences is significant at the level of 1%. We also analyzed the rainfall percentage and composite sea-ice fraction change associated with ARs.
First, ARs are observed to strongly affect extreme high temperature events. Figure 3a shows that nearly all occurrences of 2 m air temperatures above the 99.9th percentile simulated by MAR (61-100% out of ~40 at a 3-hour time step) and of most temperatures above the 99th percentile (61-70% out of ~280 occurrences at a 3-hour time step) occur within 24 hours before and after an AR landfall (Supplementary Figs. 4a and 4b). In fact, the previous two Antarctic temperature records recorded at Esperanza Base in March 2015 and in February 2020 coincided with intense AR events that triggered foehn wind events 21,27 (see Supplementary Fig. 5). Higher nighttime temperatures from enhanced downward longwave radiation were also observed meaning that the melted water during ARs is less likely to refreeze in the firn and thus contribute to runoff 28.
Second, we observe (> 61 %) of melt rates at or above the 99.9th percentile along portions of the Larsen and Wilkins ice shelves and higher percentages on the glaciers in the mountainous regions occurred within the 24-hour period surrounding AR landfalls (Fig. 3c; Supplementary Figs. 4c, 4d). To complement our analysis of surface melt and the overall presence of liquid water on the snow surface based on MAR simulated results, we used passive microwave radiometer data from Scanning Multichannel Microwave Radiometer (SMMR) and Special Sensor Microwave/Imager (SSM/I). Results show that ARs typically engender neutral to largely positive melt extent anomalies in daily climatological melt extents along the Larsen C ice shelf especially outside of the peak firn saturation months of February and March (Supplementary Fig. 6). During these months, the presence of a saturated firn can obscure the occurrences of new melt detected by passive microwave remote sensing 29. Still, from October to March, we observe that two thirds of AR landfalls were associated with positive melt anomalies, suggesting that ARs generally produce anomalously widespread melt.
Third, related with melt water, rainfall events lead to periods of extremely high rates of firn saturation and runoff 30. Runoff here refers to liquid water beyond the saturation point of the snowpack that does not necessarily drain into the ocean and could remain on the ice shelf. From 40 to 60% of total annual rainfall along the Larsen and Wilkins ice shelves are linked to ARs (Fig. 3b) whereas ARs days also produced a statistically significant higher amount of rainfall than non-AR days (p-value < 0.0025). Runoff rates thus show similar relationships to AR activity as melt rates, the only difference being that runoff is predominately constrained to the lower-elevation ice shelves (Fig. 3d, Supplementary Figs. 4e and 4f). High runoff values, produced by melting and rainfall when the firn is already saturated with liquid water, are detrimental for ice-shelf stability 31. Indeed, these high melt and runoff rates lead to melt pond formation as demonstrated by the significant correlation between melt pond observations from MODIS and summer AR occurrences 32 (Supplementary Fig. 7).
Fourth, beyond processes related with melt and hydrofracturing, ARs are also observed to induce a rapid clearing of sea ice along the ice shelf fronts allowing cooccurring anomalously large swells to reach the ice-shelf and apply strain 14. This swell-induced stress leads to the calving of large icebergs and could result in the retreat of the compressive arch like what occurred on the Larsen B in 2002 14,33. On average, the strong ARs (those with integrated vapor transport (IVT) > 400 kg m-1 s-1 upon landfall) cause a two-day sea-ice fraction decrease of nearly 10% (i.e. about 5 sigma of sea-ice variation in this region) on the northern edge of the Larsen A and B embayment (Fig. 3e). This average sea-ice fraction decrease is slightly larger when considering four-day sea-ice change. Nearly all large and rapid sea-ice disintegration events of around a 10% decrease over two days east of the Larsen ice shelves (in the region labelled EAP) co-occurred with AR landfalls (Supplementary Fig. 8). The average two-day sea-ice change during intense AR days in this region (2.5% reduction) is statistically significantly higher than non-AR days (0.1 % increase; p-value < 0.0025). West of the AP, the average sea-ice decline in response to ARs is smaller, yet still notable around the Wilkins ice shelf and along the sea-ice front in the Bellingshausen Sea like during 2002 and 2005 when AR activity was elevated 29,31 (Fig. 1, Fig. 3e). At the same time, the winds found near the surface during an AR within the low-level jet are often extreme as noticed during the AR landfall coincident with the collapse of the Larsen A in 1995 (Supplementary Fig. 9a) and can lead to significant swell heights. When sea ice is depleted east of the Larsen ice shelves, 61-80% of swells in the 99.9th percentile of height co-occurred with an AR landfall (Fig. 3f).
Consequently, AR landfalls prompt a state of high stress on the ice shelves through the combination of surface melt induced melt pond formation leading to hydrofracturing and wave strain along the ice shelf front. The tendency of ARs to create widespread melt pond formation makes them possible precursors of ice shelf collapse via hydrofracturing cascades like what was observed on the Larsen B in 2002 13. This combined with wind stress and radiative forcing leading to sea-ice clearing thus allowing swells to apply strain along the ice shelf fronts 14,22 makes ARs a unique forcing of ice-shelf weakening. Figure 4 provides a visualization summarizing all the aforementioned processes during AR landfalls that are linked to ice shelf weakening. The AR example in the schematic showcases an AR making landfall perpendicular to the ice shelf and generating a foehn wind. However, there are cases of ARs approaching the AP from different angles that do not result in foehn but still generate all the other illustrated effects.
We studied other possible connections between ARs and ice-shelf weaking and did not find conclusive evidence for a direct relationship between AR activity and ice-shelf basal melting, one of the long-term precursors of ice shelf collapse 34. Indeed, looking at ocean hindcasts from 1979-2018 35, we did not find any significant correlation between either yearly AR frequency or yearly cumulative maximum IVT and either the ocean temperature at the depth of the ice-shelf drafts or Ekman pumping (a driver of basal ice-shelf melt variations according to Etourneau et al., 2019, 36 near Larsen or Wilkins ice shelves. For a single weather event, Francis et al. (2021) showed that a significant sea surface height (SSH) anomaly in front of the Amery ice shelf may have contributed to trigger a calving event 37. However, looking at SSH anomalies in the global ocean simulations from 1979-2015 produced by Merino et al. 2018 35, we did not identify any significant SSH anomalies associated with ARs on both sides of the AP.
Characterizing cumulative impacts of atmospheric river events
The previously described ice-shelf weakening processes associated with AR landfalls become more impactful and likely as the AR intensity increases as a function of maximum IVT upon landfall. In fact, these most intense ARs often have the greatest surface melt potential. The relationship between maximum IVT upon landfall and both total melt and runoff along the Larsen ice shelves is exponential in shape with the average 3-hourly melt and runoff increasing from < 0.05 Gt 3h-1 for the weakest ARs to ~0.4 Gt 3h-1 for the strongest (Fig. 5; melt and runoff is calculated in the areas highlighted in Fig. 5c). These most extreme ARs (IVT > 800 kg m-1s-1) only occur around once every five years but have the capability to alter the cryosphere dramatically and quickly like observed in the January 2008 example (Fig. 2), the collapse of the Larsen A in 1995 (Supplementary Fig. 3), and in the March 2015 melt event/temperature record described in Bozkurt et al., 2018 21. When AR strength and impacts are characterized by considering the maximum IVT upon landfall and the landfall duration, like the scale used to classify ARs in western North America 38, consistent relationships between AR intensity and impacts emerge. This includes a significant correlation between annual AR cumulative intensity and annual melt/runoff across portions of the Larsen ice shelves (Supplementary Fig. 10).
As the cumulative maximum IVT of the AR increases, the more likely a temperature extreme, melting extreme, sea ice disintegration or high swell event occurs. This is visible in Figure 6a, where AR intensity is assessed by summing the maximum IVT values throughout the day when an AR is detected. Taken as a whole, all these events represent a physical state which promotes ice shelf weakening as confirmed by the cooccurring calving and even collapse events with AR landfalls. For the most intense ARs at landfall, there is a nearly 20% probability of a daily temperature, melt, or runoff extreme occurring over at least half the lower elevation portion of the northern AP (blue line and area; see Fig. 6c for domain). There is an increase in probability when the same calculation is repeated but with the added option of a significant sea-ice decline occurring (up to 40%, red line and area), and with the option of a high swell event east of the AP (green line and area).
We also analyzed if iceberg calving and ice-shelf collapse events were related with AR cumulative intensity within 5 days after the AR event. When defining AR intensity using the cumulative maximum IVT throughout a discrete continuous AR landfall event, we find around 60% of calving/collapse events (13 out of 21 total events over August-March 2000-2020 plus the collapse of the Larsen A in 1995 and the calving event from the Larsen C in July 2017), occurred when an AR with at least a cumulative IVT value of 500 kg m-1s-1 was detected within 5 days prior. Although, that percentage progressively decreases as the IVT threshold increases (see Methods for details on the statistical significance of the lag between AR and calving). This suggests that calving requires neither a large trigger nor large swell heights once hydrofracturing and other weakening processes have already weakened the stability of the ice shelf front. However, the major collapses of the Larsen A and B were preceded by intense ARs like the early January-February 2002 AR that caused widespread melt pond formation across the Larsen B and contributed to large swell heights (Table 1, Supplementary Fig. 2). Even more significant in terms of summer AR activity, the summer 2002 had the second highest IVT over 1980-2020, coinciding with exceptional sea-ice-free conditions around the AP and with large wave-induced flexure on the ice shelves 14 (Table 1).
Physical particularities of AR events
When ARs make landfall perpendicular to the mountains of the AP, their large moisture transport typically engenders a large latent heat release. These are ideal conditions for foehn winds, which are commonly proposed to trigger of intense surface melt along the leeward AP ice shelves 20,21,27,39–41. However, the enhanced moisture transport also triggers melt through clouds containing anomalously high liquid and ice water content resulting in high downward longwave radiation 20. Thus, it is important to analyze the differences between these distinct phenomena, ARs and foehn winds.
To compare AR and foehn frequency and surface melt potential, we compared AR landfalls events against instances of foehn winds detected by a dedicated algorithm 11(see Turton et al., 2018), which is based on relative humidity observations from six automatic weather stations (AWSs) and model output from the Antarctic Mesoscale Prediction System (AMPS). From 2009-2012, the amount of AR events per year to occur within 24 hours of a foehn detection ranged from 17%-63%. (see Supplementary Table 1), and these percentages increase when considering only intense ARs (IVT > 400 kg m-1 s-1 upon landfall). The foehn events that are related to ARs appear to have a greater duration and extent over the ice shelves than non-AR related foehn events. Indeed, 43% of all foehn events detected at more than four or more AWSs are associated with ARs (compared to 22% only detected at one station) and 60% of foehn events lasting more than 3 days are associated with ARs (compared to 23% only lasting 0-2 days). The greater impact area during AR-related foehn events can be explained by the higher windward flow velocity 10. ARs with their associated high wind speed are more likely to create a linear flow across the ice shelves, mechanically mixing down warm air aloft and transporting sensible heat to the surface. Weaker foehn events are typically associated with nonlinear flow over the mountains which results in hydraulic jumps shortly after descending thus limiting the warming extent over the ice shelf 10.
Thus, standalone foehn events are associated with less intense and widespread melt across the Larsen ice shelves. This is further observed with intense ARs (IVT > 400 kg m-1 s-1 upon landfall). If we compare all ARs and foehn events from 2009-2012 using MAR, ARs generated smaller positive 2 m temperature anomalies along the base of the leeward AP mountains, but higher anomalies across the rest of the AP (Fig. 7a), with greater melt along regions in the northern AP (Fig. 7b). Higher melt is explained by ARs producing greater downward longwave radiation flux anomalies (associated with higher cloud liquid/ice water contents) (Fig. 7c and 7d). We also see that ARs enhance the melting potential of the foehn when comparing foehn events co-occurring with an AR landfall against all foehn events. Supplementary Fig. 11).