Unprecedented...lobal Warming_Chunzai WANG.pdf

Unprecedented Heatwave in Western North America during Late Juneof 2021:Roles of Atmospheric Circulation and Global WarmingChunzai WANG*1,2,3,Jiayu ZHENG*1,2,3,5,Wei LIN2,4,and Yuqing WANG2,41Southern Marine Science and Engineering Guangdong Laboratory(Guangzhou),Guangzhou 511458,China2State Key Laboratory of Tropical Oceanography,South China Sea Institute of Oceanology,Chinese Academy of Sciences,Guangzhou 510301,China3Innovation Academy of South China Sea Ecology and Environmental Engineering,Chinese Academy of Sciences,Guangzhou 510301,China4University of Chinese Academy of Sciences,Beijing 100049,China5State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics,Institute of Atmospheric Physics,Chinese Academy of Sciences,Beijing 100029,China(Received 18 March 2022;revised 12 June 2022;accepted 30 June 2022)ABSTRACTAn extraordinary and unprecedented heatwave swept across western North America(i.e.,the Pacific Northwest)in lateJune of 2021,resulting in hundreds of deaths,a massive die-off of sea creatures off the coast,and horrific wildfires.Here,we use observational data to find the atmospheric circulation variabilities of the North Pacific and Arctic-Pacific-Canadapatterns that co-occurred with the development and mature phases of the heatwave,as well as the North America pattern,which coincided with the decaying and eastward movement of the heatwave.Climate models from the Coupled ModelIntercomparison Project(Phase 6)are not designed to simulate a particular heatwave event like this one.Still,models showthat greenhouse gases are the main reason for the long-term increase of average daily maximum temperature in westernNorth America in the past and future.Key words:heatwave,climate change,atmospheric circulation pattern,Pacific NorthwestCitation:Wang,C.Z.,J.Y.Zheng,W.Lin,and Y.Q.Wang,2023:Unprecedented heatwave in western North Americaduring late June of 2021:Roles of atmospheric circulation and global warming.Adv.Atmos.Sci.,40(1),1428,https:/doi.org/10.1007/s00376-022-2078-2.Article Highlights:The North Pacific and Arctic-Pacific-Canada patterns co-occur with the development and mature phases of the heatwavein western North America.The North America pattern coincides with the decaying and eastward movement of the heatwave in western NorthAmerica.Greenhouse gases are a main reason for the average daily maximum temperature increase in the past and future.1.IntroductionWestern North America(i.e.,the Pacific Northwest ofthe United States and Canada),known for its temperateweather in June,normally has average high temperatures inthe comfortable 18C to 24C range.However,westernNorth America experienced an extreme heatwave in lateJune and early July of 2021,resulting in record temperaturesin many places.In particular,Lytton,British Columbiabroke Canadas all-time record on 30 June when the tempera-ture topped 49.5C.This heatwave is also responsible for hun-dreds of deaths,a massive die-off of sea creatures off thecoast,and a spate of horrific wildfires(https:/en.wikipedia.org/wiki/2021_Western_North_America_heat_wave;Over-land,2021).Investigating and understanding the causes ofthis heatwave are both scientifically and socially important.Some studies indicate that increasing trends in the fre-quency,duration,and intensity of heatwaves have acceleratedon global and regional scales under global warming(Perkins et al.,2012;Perkins-Kirkpatrick and Lewis,2020;Wang et al.,2020).There is increasing evidence that anthro-*Corresponding authors:Chunzai WANG,Jiayu ZHENGEmails:,ADVANCES IN ATMOSPHERIC SCIENCES,VOL.40,JANUARY 2023,1428 Original Paper The Author(s)2023.This article is published with open access at .pogenic forcing,such as the forcing of rising greenhousegases,is having an impact on heatwaves (IPCC,2013,2021;Wang et al.,2020;Seong et al.,2021).In addition toexternal forcing,internal variability may also affect the chang-ing nature of heatwaves.Changes in temperature and heat-waves are obviously related to climate modes and atmo-spheric circulations (Lau and Nath,2012;Li and Ruan,2018;Li et al.,2019).Zheng and Wang(2019)identifiedseven atmospheric circulation patterns and combined thesepatterns along with global warming to explain summer sur-face air temperature variations in the Northern Hemisphere.Horton et al.(2015)showed that although a substantial portionof the observed change in extreme temperature occurrencehas resulted from thermodynamic changes,extreme tempera-ture trends have also been altered by recent changes in atmo-spheric circulation patterns.Some studies have also shownthat heatwaves can be modulated by sea surface temperature(SST)anomalies.SST anomalies can remotely influence sum-mer heatwave variabilities over land by generating severe con-vection and triggering atmospheric teleconnections(Wu etal.,2012;Wang et al.,2017;Chen et al.,2019;Deng et al.,2019).This paper uses data from observations and climate mod-els to study physical processes that may be res