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.