Contributions of aerosol‐cloud interactions to mid‐Piacenzian seasonally sea ice‐free Arctic Ocean

Forcings and feedbacks controlling the seasonally sea ice‐free Arctic Ocean during the mid‐Piacenzian Warm period (3.264–3.025 Ma, MPWP), a period when CO2 level, geography, and topography were similar to present day, remain unclear given that many complex Earth System Models with comparatively higher skills at simulating twentieth century Arctic sea ice tend to produce perennial Arctic sea ice for this period. We demonstrate that explicitly simulating aerosol‐cloud interactions and the exclusion of industrial pollutants from model forcing conditions is key to simulating seasonally sea ice‐free Arctic Ocean of MPWP. The absence of industrial pollutants leads to fewer and larger cloud droplets over the high‐latitude Northern Europe and North Pacific, which allows greater absorption of solar radiation at the surface during the early summer. This enhanced absorption triggers the seasonally runaway sea ice surface albedo feedback that gives rise to September sea ice‐free Arctic Ocean and strongly amplified northern high‐latitude surface warmth.

在皮亚琴察暖期中期（326.4万 - 302.5万年前，MPWP），当时的二氧化碳水平、地理和地形与现今相似，但控制季节性无海冰北冰洋的强迫因子和反馈机制仍不清楚，因为许多在模拟20世纪北极海冰方面技能相对较高的复杂地球系统模型往往会得出该时期存在常年性北极海冰的结果。我们表明，明确模拟气溶胶 - 云相互作用以及在模型强迫条件中排除工业污染物是模拟皮亚琴察暖期中期季节性无海冰北冰洋的关键。没有工业污染物会导致北欧高纬度地区和北太平洋上空的云滴更少且更大，这使得初夏地表对太阳辐射的吸收更强。这种增强的吸收引发了季节性失控的海冰表面反照率反馈，导致9月北冰洋无海冰，并使北部高纬度地表暖化强烈增强。