| 23 Oct 2026
Seasonal reorganisation of the Asian summer monsoon response to projected Arctic sea-ice loss
Abstract. The extent to which projected Arctic sea-ice loss affects the seasonal evolution of the Asian summer monsoon, and the physical mechanisms linking Arctic forcing to regional rainfall through remote atmospheric teleconnections, remain unclear. We investigate how projected Arctic sea-ice loss affects the Asian summer monsoon, and in particular how the response evolves within the monsoon season. We use 100-member ensembles from eight CMIP6 models participating in the Polar Amplification Model Intercomparison Project (PAMIP) to examine June–September responses to present-to-future Arctic sea-ice loss. The prescribed sea-ice perturbation and near-surface Arctic warming strengthen through the melt season; precipitation and circulation responses over Asia change markedly between months. Stationary-wave anomalies and wave-activity flux indicate distinct Eurasian–North Pacific propagation pathways that develop within a seasonally varying upper-tropospheric jet. These changes coincide with shifts in upper-level divergence, vertical motion, lower-tropospheric circulation and moisture convergence, with circulation-driven changes dominating the moisture-flux convergence response. Thus, stronger late-summer Arctic forcing does not produce a progressively stronger Asian monsoon response; instead, the atmospheric pathway and regional hydroclimatic response vary substantially through the season. More broadly, the pronounced month-to-month and regional contrasts in the forced response suggest that seasonal averaging may obscure important Arctic-related rainfall signals, while internal atmospheric variability may further complicate their identification in observations.
Summary
This paper uses 100-member ensembles from eight PAMIP models to examine how the Asian summer monsoon responds to projected present-to-future Arctic sea-ice loss from June to September. The authors find that, although the prescribed sea-ice perturbation strengthens through the melt season, the precipitation and circulation response does not strengthen with it. Instead it changes markedly from month to month. Additional diagnostics (stationary waves, wave-activity flux, moisture-flux convergence, and the Eurasian upper-tropospheric jet) are used to argue that a seasonally evolving background flow conditions the pathway through which the Arctic forcing reaches Asia.
The topic is interesting and relevant. The large ensembles and the joint analysis of precipitation, moisture budget, and upper-level circulation are strengths, and the manuscript is clearly organised. However, I am not yet convinced that the Asian monsoon response is large or robust enough to support the paper’s conclusions, and the mechanistic interpretation is largely qualitative. I therefore recommend major revision.
Major comments
1. Magnitude and robustness of the response.
The precipitation response in Fig. 3 is at most about 0.4 mm day⁻¹ and is spatially patchy, with sparse stippling, whereas climatological precipitation reaches about 10 mm day⁻¹ (the contours in Fig. 3). It is therefore unclear that the response is large enough to matter for the monsoon. I suggest the authors quantify its significance relative to (a) the climatological mean, (b) the anthropogenic (greenhouse-gas-forced) monsoon response, and (c) internal variability, for example the ensemble spread.
The robustness analysis also needs strengthening. Requiring that at least seven of eight models agree on the sign of the multi-model-mean response says nothing about magnitude. It can be satisfied by responses that are negligible in most models, and it does not exclude a large response in a single model dominating the mean. It is also not a significance test. I would suggest the authors consider using other metrics for robustness analysis.
2. Mechanism: the role of the background circulation.
The authors argue that the response varies from month to month because both the imposed forcing pattern and the background circulation evolve. This is potentially the most interesting result, but the supporting analysis is thin. I expected a stronger July and August response given the same forcing, because the climatological jet is located further poleward. The results appear to show the opposite, with the clearest stationary-wave pathways in June and September.
The jet diagnostics in Fig. 7 only partly resolve this. The jet axis moves poleward from June to August (37.7°N, 41.6°N, 42.5°N), but the August-minus-July change (+0.93 ± 1.28°) is small relative to inter-model spread, as the authors acknowledge. The statement that the changing jet “supports the interpretation that the seasonal basic state helps condition the pathway” (Sect. 3.4 and Sect. 4) is not well supported.
I would suggest the authors add more discussion/analysis (waveguide diagnostics, and/or linear baroclinic model experiments) on how background atmospheric circulation might affect the circulation response.
Minor comments
The Results are long and largely descriptive, and rely on qualitative language (“pronounced,” “coherent,” “clear”). Quantitative statements would be more convincing.