| 27 Jul 2026
Extreme Amundsen Sea Low (ASL) events and climate of West Antarctica
Abstract. The Amundsen Sea Low (ASL) is a regional atmospheric circulation feature that significantly influences the climate of West Antarctica through its impact on the meridional wind field. An observed deepening of the ASL over the satellite era has raised concerns about the possibility of more extreme ASL conditions through the end of the twenty-first century. Here we use ERA5 reanalysis and passive microwave satellite observations to examine the impact of extreme ASL events on West Antarctica during the period 1979–2025. In total, we identify 13 extremely strong and 8 extremely weak events that vary geographically based on magnitude. Strong events tend to be concentrated poleward and centered in the vicinity of the Amundsen Sea while weak events are found farther north and display large zonal spatial heterogeneity. Composite maps of surface anomalies suggest that the location of extreme events serve to explain a regional dipole in surface temperatures and sea ice concentration, both of which are co-located with anomalous wind velocities modulated by ASL strength. Given the link between the regional ASL and additional large-scale modes of atmospheric variability in the Southern Hemisphere, we also investigate extreme ASL events in relation to the Southern Annular Mode, El Niño Southern Oscillation, and Zonal Wave 3. We find that nearly all extremely strong events occur when the three modes of variability are positively in-phase; there is less agreement during extremely weak ASL events. We assess future ASL variability and extremes using CESM2 model output under middle- and high-emission scenarios. Future projections indicate a higher proportion of extremely strong ASL events relative to weak events through the end of the 21st century, suggesting an increased likelihood of more extreme meridional conditions across West Antarctica in the near future.
The manuscript provides an interesting analysis of past and potential future extremes of the Amundsen Sea Low and their impact on temperature and sea ice concentration. The text is clear and well-written. It describes well the extremes and their effects and provides interesting connections to previous work. However, I have some suggestions below to expand points and for additional precisions on some aspects.
General point
1/ The study analyses the results of one model (CESM2). It is always better to have a multi-model approach for such kind of studies but in this particular case I do not think it is absolutely necessary. However, the validation of the CESM2 run for the historical period is too short. It is mentioned line 144 that CESM2 reproduces well some aspect of the Southern Ocean circulation (with one reference) but a specific evaluation for the ASL is needed. Does it reproduce well the seasonality of the ASL strength and position, the timing of the extremes observed during the historical period ? Without those elements, it is impossible to evaluate if the differences discussed in section 3.7 are representative of a change in the system or just related to model biases already present in the historical period.
2/ I am also wondering why the ensemble mean of the three simulations is analyzed instead of the individual members (line 147). By construction, this ensemble mean will smooth the extremes as they do not occur at the same time in each member. I would strongly recommend to make the analyses of the extremes on each member and then eventually take the average to have a mean response rather than analyzing the ensemble mean itself.
3/ The link between the seasonal cycle of the ASL, its trend and the extremes could be discussed in more details. For instance, most of the strong ASL events occurs in September or October. Is it simply due to a different mean or a higher variance during these months ? Are the characteristics of these extreme events (like the location) due to the fact that they occur during these months or are the extremes different from other events occurring the same month ?
4/ A question related to the previous one is more generally about the characteristics of the extremes. Could we consider that the links with temperature and sea ice are the same for the extremes and more normal cases (as revealed for instance by a standard correlation analyses with ASL index) or are the extremes showing specific elements that are related to the larger magnitude of the perturbation ?
5/ The connection between ASL and ZW3 is mentioned but I wonder how the two indices are connected just by construction as one of the pole used to construct the ZW3 index is close to the region used to define the ASL index (lines 132-133).
Minor points.
Line 13. From the structure of the sentence, it is not clear to me whether the words ‘based on magnitude’ refer to ‘geographically’ or ‘events’.
Line 71. You mention ‘amplify baseline warming’ . Do you mean locally (as this can also damp the warming in other regions I guess) ?
Line 91. It is not clear to me how a fixed mean can account for the observed deepening.
Line 93. Is it parameterizing or rather characterizing ?
Line 116. I do not understand the wording ‘uses integers’ as the index can include decimals? The same remark is valid for line 138 that cites as an example a value of 0.9. Should it be ‘scalar’ instead of ‘integer’ ?
Line 347. Are the results similar if the series is detrended ? This diagnostic could help to disentangle the contribution of the trend in the behavior.
Line 351-360 . I am not sure I get the message from this part. Is it just the variability of the system ? Is it robust between ensemble members?
I suggest to group Fig 9 and Fig 10 in one figure.