Seasonal Prediction of Extreme High-Temperature Days in Southwestern China Based on the Physical Precursors

Abstract

Extreme high temperatures frequently occur in southwestern China, significantly impacting the local ecological system and economic development. However, accurate prediction of extreme high-temperature days (EHDs) in this region is still an unresolved challenge. Based on the spatiotemporal characteristics of EHDs over China, a domain-averaged EHD index over southwestern China (SWC-EHDs) during April–May is defined. The simultaneous dynamic and thermodynamic fields associated with the increased SWC-EHDs are a local upper-level anticyclonic (high-pressure) anomaly and wavy geopotential height anomaly patterns over Eurasia. In tracing the origins of the lower boundary anomalies, two physically meaningful precursors are detected for SWC-EHDs. They are the tripolar SST change tendency from December–January to February–March in the northern Atlantic and the February–March mean snow depth in central Asia. Using these two selected predictors, a physics-based empirical model prediction was applied to the training period of 1961–2005 to obtain a skillful prediction of the EHDs index, attaining a correlation coefficient of 0.76 in the independent prediction period (2006–19), suggesting that 58% of the total SWC-EHDs variability is predictable. This study provides an estimate for the lower bound of the seasonal predictability of EHDs as well as for the hydrological drought over southwestern China.

摘 要

近年来,中国西南地区极端高温事件频发,对当地生态系统和经济发展产生严重影响。然而,准确预测该地区极端高温日数年际变化仍具挑战。基于中国极端高温日数时空分布特征,本文定义了4-5月中国西南区域平均极端高温日数指数(SWC-EHDs)。从SWC-EHDs相关同期环流场上发现西南地区极端高温日数的增加伴随着局地高空反气旋(高压)异常和欧亚大陆上空呈波列形势的位势高度异常。通过诊断SWC-EHDs相关的前期下垫面异常,挑选出两个具有物理意义的前兆信号。第一个信号是12-1月至2-3月的北大西洋海温三级子变化倾向;第二个信号是中亚地区2-3月平均的积雪深度。利用1961-2005年训练期的前兆信号建立基于物理机制的SWC-EHDs季节预报模型,该模型在独立预测时期(2006-2019年)预报的SWC-EHDs相关系数可达0.76,表明其变率的58%是可预测的。本研究可为西南地区极端高温日数以及气象水文干旱的季节预测提供参考。

Electronic Supplementary Material to

Seasonal Prediction of Extreme High-Temperature Days in Southwestern China Based on the Physical Precursors