Asymmetry variations in Arctic summer onset and ending: Role of sea-ice melting

Previous studies found that in Arctic regions with severe sea ice melting, summer ending (SE) variations are significantly larger than summer onset (SO) variations in the past few decades. Based on short-term observations, researchers preliminarily suggested that radiation variations caused by an earlier melting onset could be the possible reason for asymmetric Arctic SO/ending variations (AASV). Based on observations and National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) Reanalysis I dataset, here we quantitatively characterize AASV by calculating the difference between the 11 year sliding trend of Arctic SO and SE. The results show that AASV positively correlates with sea ice melting in summer. The increased summer sea ice melting increases the area to absorb short-wave radiation in summer and then release more long-wave radiation to heat the lower atmosphere and delay the peak time of long-wave radiation releasing. The variations in radiation lead to a significant delay of the Arctic SE, with no significant variations in SO. We introduce CMIP6 historical and future simulations of 15 models to verify further the relationship between AASV and summer sea ice melting. Historical run reproduces the observed asymmetry, and future simulations under various warming levels show that AASV will vanish with disappeared melting variations or be strengthened with increased melting. The latter could delay freeze-up and further exacerbate the following years' melting, which will enhance AASV. Furthermore, AASV will delay the onset and peak time of Arctic amplification.