Excess ice refers to ground-ice bodies in permafrost whose volume exceeds the soil pore space, and its melting can cause abrupt permafrost thaw. However, only a few Earth system models (ESMs) currently represent excess-ice processes, with the physical effects remaining unclear. This study quantified how representing excess ice in ESMs affects pan-Arctic permafrost simulations using sensitivity experiments with separate configurations of CESM2 (sub-grid and grid-cell mean schemes) and CAS‑ESM2 (grid-cell mean schemes). Model evaluation showed that both ESMs reasonably simulated the spatial extent of continuous permafrost, but only CESM2 captured discontinuous permafrost. Excess-ice processes reduced the pan-Arctic permafrost extent by 0.16, 2.3, and 0.79 million km2 in CESM2 sub-grid, CESM2 grid-cell mean, and CAS-ESM2 grid-cell mean, respectively. The ALT (active layer thickness) responses also differed among the configurations: in both CESM2 configurations, ALT increased mainly near the southern boundary of continuous permafrost, whereas CAS-ESM2 simulated a more spatially widespread ALT increase. Comparative analysis indicated that using the grid-cell mean scheme yields a larger reduction in permafrost extent compared to the sub-grid approach. These responses suggest that excess ice melt may influence soil hydrothermal conditions by modifying soil moisture, soil thermal conductivity, and heat transfer into deeper soil layers. Compared with CAS-ESM2, CESM2 showed drier soil conditions, lower thermal conductivity, and reduced downward heat transfer in summer, which was consistent with lower soil temperatures and shallower ALT.
A significant wetting trend has been observed in the Yangtze–Huai River Basin (YHRB) during winter, suggesting an increasing risk of seasonal-scale extremely pluvial winters under global warming. Unlike previous studies focusing on daily precipitation extremes, this study investigates future changes in the total precipitation amount, frequency, and intensity of extremely pluvial winters in the YHRB under the SSP2-4.5 and SSP5-8.5 scenarios, based on simulations from 29 CMIP6 models. The total precipitation, frequency, and intensity of extremely pluvial winters are projected to increase substantially under both scenarios, with larger increases under SSP5-8.5 and for more severe events defined by the 95th percentile threshold. Under SSP2-4.5, for events defined by the 90th percentile threshold, the total precipitation may increase by about 50%–160%, the frequency by 20%–200%, and the intensity by 15%–35% relative to the historical period. The intensification of extremely pluvial winters is primarily associated with enhanced net moisture transport into the YHRB, driven by circulation changes and increased humidity gradients between Southwest China and the Indochina Peninsula. Projection uncertainty in the intensity of extremely pluvial winters mainly reflects the inter-model spread associated with differences in simulated circulation responses, moisture transport processes, and vertical motion changes. In contrast, the uncertainty in frequency projections remains considerable. These findings highlight an increasing risk of severe pluvial winters in the YHRB under continued warming and emphasize the importance of better constraining associated dynamic and thermodynamic processes in future climate projections.
