Regime shift increase in East Asia's summer extreme hot day frequency across the late 1990s

Here we analysed the long-term change in extreme hot days (EHDs) in East Asia during boreal summer (June-July-August) since 1979, where EHDs was defined as days exceeding or equalling the 90th percentile threshold of the cli-matological (1991-2020) daily T-max and T-min. EHDs frequency occurrence in East Asia during summer showed not only an increasing trend but also a dis-tinct regime shift increase since the late 1990s. Based on this regime shift, we divided these years into two periods, P1 (1979-1998) and P2 (1999-2021), and found that different physical processes operated for each period's EHDs vari-ability. P2's EHDs was related to the stationary wave originating from both the North Atlantic Ocean and the Indo-Pacific warm pool, but these influences did not appear in P1. To investigate whether the observed regime shift increase was caused by natural variability or greenhouse gas concentration increases, we conducted a CO2 quadrupling experiment as well as a present-day experi-ment with a fixed CO2 concentration using the Community Earth System Model with 28 ensemble members. We demonstrated that the regime shift increase of East Asian EHDs occurrences was due to increasing greenhouse gas concentrations. We further discussed the influence of Arctic sea ice reduc-tion due to global warming on EHDs occurrences in East Asia.