Extratropical Climate Change During Periods Before and After an Arctic Ice‐Free Summer.

The temperature of a well‐mixed ice‐water mixture stays constant until the ice melts due to external heat. Whether the temperature over the Arctic Ocean exhibits an analogous stagewise evolution to reach an ice‐free point remains unclear. Therefore, this study explored the characteristics of extratropical climate change before and after a period during which the Arctic Ocean was ice‐free in summer using multimodel simulations. Here, we show that the seasonality of Arctic warming varies between the two periods separated by an ice‐free summer. The warming maximum in the cold season delayed for a month after becoming ice‐free than before. In addition, the warming maximum lagged behind the sea‐ice decline maximum before becoming ice‐free, whereas the maximums of the two became coordinated after becoming ice‐free. The closed cross‐season energy cycle demonstrated that the capacitor effect of the Arctic Ocean with delayed release of the energy taken up in spring and summer due to sea‐ice decline and seawater absorption is crucial for the seasonality observed in Arctic climate change. Moreover, we found that although Arctic amplification induced general weakening in high‐frequency weather variability in the mid‐high latitudes via decreased meridional temperature gradients, significant weakening was induced only after becoming ice‐free under high emission. Our findings suggest that the two stages of Arctic sea‐ice decline should be taken into consideration when dealing with global warming. Plain Language Summary: Taking the classic stagewise temperature evolution of ice‐water mixtures into consideration (Figure S1 in Supporting Information S1), this study investigated climate change in the Arctic before and after an ice‐free summer using multimodel simulations. Our results showed that the seasonality characteristics of Arctic warming varied between these two periods. The capacitor effect (charging and discharging in the warm and cold seasons, respectively) of the cross‐season energy cycle associated with sea‐ice decline on Arctic warming was observed. Moreover, our findings indicated that variability in the weather decreased in mid‐high latitudes due to Arctic amplification‐induced southward cold airflow from the Arctic not being as cold as before. This research highlights the importance of stagewise decline of Arctic sea‐ice. Key Points: Seasonality of Arctic warming will change after ice‐free relative to that before, under both the intermediate and high future emissionsCapacitor effect of the Arctic Ocean due to cross‐season energy cycle is crucial for seasonality in Arctic climate changeArctic amplification induces a decrease in weather variability in mid‐high latitudes, especially after ice‐free under high emission [ABSTRACT FROM AUTHOR]