北半球夏季高温变率的主模态及其海气背景

Leading modes and oceanic and atmospheric drivers of heat wave variability in the Northern Hemisphere

摘要:

极端高温热浪对全球人类栖息地构成持续且日益严重的威胁。目前,整个北半球高温热浪的主要空间模态及其相关的海洋和大气背景仍不清楚。本文采用百年半球尺度站点观测资料,研究了北半球夏季高温变率的主模态及与其相联系的海气背景。结果表明,北半球夏季高温(日最高气温≥35 ℃)频次的自然变率有3种主模态,解释方差占比52.6％。第一主模态表现为北半球高温频次异常的一致型变化模态。该模态与大西洋多年代际振荡(Atlantic Multi-decadal Oscillation,AMO)密切相关。与AMO相关的环流异常通过增强北半球多地的大范围高压异常,抑制云层形成并增加太阳辐射,从而促进大气增暖。第二主模态反映了欧亚大陆的纬向三极型异常分布和北美的经向偶极型异常分布。该模态与北大西洋涛动(North Atlantic Oscillation,NAO)相关的年际大气变率有关。NAO通过阻塞型高压和波列传输影响北美的关键区域高温异常。第三模态表现为欧亚大陆的经向三极型和北美的纬向偶极型异常分布,受太平洋年代际振荡(Pacific Decadal Oscillation,PDO)、厄尔尼诺-南方涛动(El Niño and Southern Oscillation,ENSO)和南印度洋(South Indian Ocean,SIO)海温异常的共同影响。PDO和ENSO可分别在年代际和年际尺度上影响北半球特别是亚欧大陆上空的位势高度异常,进而影响高温频次异常;位于南半球的SIO海温异常通过调节沃克环流和哈德莱环流异常进而影响北半球的高温异常分布。组合三个主模态的海气背景因子进行多元线性回归重建的效果评估进一步证明,多尺度海洋和大气信号的综合影响在北半球高温异常中具有重要作用。

Abstract:

Extreme heat events and heat waves pose an increasingly significant threat to human communities,affecting public health,agriculture,economic stability and fueling secondary disasters such as wildfires.In recent years,heat waves have become more frequent,intense,and prolonged,particularly in the densely populated Northern Hemisphere (NH).However,the primary spatial modes of heat waves across the NH,along with their associated oceanic and atmospheric conditions remain insufficiently understood.This study investigates the natural variability of NH heat waves during boreal summer over the past century.We identify three leading modes in the frequency of daily maximum temperatures exceeding 35 ℃,collectively accounting for 52.6％ of the explained variance.The first mode presents a uniform pattern of heat wave frequency anomalies across most of the NH.This interdecadal mode corelates with the Atlantic Multi-decadal Oscillation (AMO),which triggers atmospheric anticyclone anomalies in the upper troposphere,decreasing cloud cover and increasing surface diabatic heating.AMO-induced Rossby wave energy propagates from low to mid-latitudes,then eastward across Eurasia and North America,establishing wave-train anomalies linked to this primary mode via teleconnections.The second mode,showing a latitudinal tripole pattern across Eurasia and a meridional dipole over North America,reflects interannual atmospheric variability tied to the North Atlantic Oscillation (NAO).The NAO influences critical North American regions through high-pressure ridges and propagating wave trains.The third mode captures an Eurasian meridional tripole and North American latitudinal dipole pattern,shaped by the Pacific Decadal Oscillation (PDO),El Niño-Southern Oscillation (ENSO),and sea surface temperature anomalies of the South Indian Ocean (SIO).Both PDO and ENSO affect NH heatwave frequency anomalies through upper-to-lower level geopotential height variations over Eurasia on interdecadal and interannual timescales,respectively.ENSO's influence extends to NH heat wave patterns via the Pacific-North America (PNA) teleconnection and the Indian Ocean capacitor effect.The SIO modulates vertical atmospheric motion over regions such as East Asia and eastern North America via Walker and Hadley circulations,further affecting NH heat wave frequency anomalies.We develop a multiple linear regression model to reconstruct NH heat wave frequencies based on the air-sea background factors of these three leading modes,including their spatial distributions and variance contributions.The model aligns well with observed heat wave frequencies and extreme high temperature events,reinforcing the significant impact of multi-scale oceanic and atmospheric signal on NH heat wave anomalies.When the absolute temperature thresholds are increased to 37 ℃ and 40 ℃,the leading modes display similar spatial patterns,suggesting that the identified oceanic and atmospheric drivers remain influential.Analysis based on relative temperature thresholds show consistent results,although variability at high latitudes exhibits a distinct contribution.Additional natural variability components,potentially linked to sea ice,snow cover,and soil moisture,warrant further investigation.