Observed Winds Crucial for September Arctic Sea Ice Loss

Arctic sea ice has declined strikingly over the past 40 years under climate change. The physical processes driving these changes and the relative roles of external forcing versus natural variability remain under debate, partly due to lack of confidence in the simulation of Arctic sea ice by coupled climate models. Here, we nudge a widely-used CMIP5-class model (CESM1) toward observed winds above the boundary layer. These simulations reproduce observed Arctic trend patterns and Arctic-average interannual fluctuations in near-surface temperature. We find that atmospheric circulation plays a crucial role in September Arctic sea ice loss and sea ice interannual variability. Trends in the observed atmospheric circulation that are not part of the modeled forced response cause approximately 20%-25% of the modeled September sea ice decline, which is in close agreement with the observed trend. However, our simulations do not capture sea ice loss in other seasons, pointing to important model biases in winter and spring.