An interdecadal decrease in extreme heat days in August over Northeast China around the early 1990s 1990年代初东北8月极端高温频次的年代际减少

Abstract

Northeast China (NEC) witnessed an interdecadal increase in summer extreme heat days (EHDs) around the mid-1990s. The current study reveals that this interdecadal increase only occurs in June and July, while August features a unique interdecadal decrease in EHDs around the early 1990s. Plausible reasons for the interdecadal decrease in EHDs in August are further investigated. Results show that the interdecadal decrease in EHDs in August is due to the deceased variability of daily maximum temperature (Tmax). Overall, the variation of the Tmax over NEC in August is modulated by the Eurasian teleconnection pattern, Silk Road pattern, and East Asia–Pacific pattern. However, the influence of the Silk Road pattern dramatically weakens after the early 1990s because the meridional wind variability along the westerly jet significantly decreases. The weakened influence of the Silk Road pattern contributes to the decreased Tmax variability over NEC. Meanwhile, the convection over the western North Pacific, which accompanies the East Asia–Pacific pattern, presents a significant decrease in variance after the early 1990s, further decreasing the Tmax variability over NEC.摘要东北夏季极端高温频次在1990年代中期出现年代际增多.本文指出该年代际增多只出现在6–7月, 而8月则呈现特殊性, 即在1990年代初出现年代际减少.进一步分析表明, 东北8月极端高温频次的年代际减少由日最高温度变率的年代际减小造成.东北日最高温度受到欧亚遥相关,丝绸之路遥相关和东亚-太平洋遥相关的共同调制.1990年代初之后, 西风急流上的经向风变率显著减小, 丝绸之路遥相关对下游的影响减弱, 导致东北日最高温度变率减小.同时, 西北太平洋热带对流的变率也在1990年代初出现年代际减小, 通过东亚-太平洋遥相关使东北日最高温度变率进一步减小.