Potential Link Between Ice Nucleation and Climate Model Spread in Arctic Amplification.

Arctic amplification (AA) is simulated by all global climate models, however the spread in the degree of projected warming is large and the underlying mechanisms driving it are poorly understood. The impact of the temperature dependence of immersion freezing on cloud feedbacks and AA is studied using NASA's GEOS‐5 model. Parameterizations that exhibit low ice‐nucleating particle (INP) concentrations in the high Arctic during summer are found to weaken the cloud‐phase feedback. This allows sunlight to readily melt sea‐ice in the summer, which decreases the stability of the lower troposphere, causing a decrease in wintertime cloud fraction over open ocean. Arctic amplification was found to span from ∼1.4 to >2.6, which spans 30% of the spread in AA in the coupled model intercomparison project models, depending on the temperature dependence of immersion freezing. These results suggest that summertime INP concentrations may provide an observational constraint on AA. Plain Language Summary: The Arctic is warming at more than twice the pace as the rest of the globe. Global climate models also exhibit the largest spread in surface warming in the Arctic region, where low‐level stratiform clouds are poorly represented. These clouds frequently consist of mixtures of liquid droplets and ice crystals, the latter which form with the aid of ice‐nucleating particles (INPs). Here, we show that the sensitivity of the activation of INPs in stratiform clouds to temperature results in a difference in Arctic amplification (AA) that spans over 30% of the spread in the latest generation of global climate models. The apparent warming mechanism is triggered by the extent of thermodynamic phase changes in Arctic mixed‐phase clouds during the summer and their subsequent influence of sea‐ice melt. Accurate measurements of collocated in‐cloud INP concentrations and temperature are critical for improved constraints on AA. Key Points: Ice‐nucleating particles (INPs) may trigger summertime cloud‐phase feedback interactions with sea ice that strongly impact Arctic amplification (AA)AA is highly sensitive to temperature‐dependent ice nucleation schemesSummertime measurements of (INPs) are lacking but important for constraining Arctic cloud feedbacks [ABSTRACT FROM AUTHOR]