Contribution of changes in atmospheric circulation patterns to extreme temperature trends

Abstract

Surface weather conditions are closely governed by the large-scale 1 circulation of the atmosphere. Recent increases in the occurrence of some extreme 2 weather phenomena 1,2 have led to multiple mechanistic hypotheses linking changes 3 in atmospheric circulation to increasing extreme event probability 3-5 . However, 4 observed evidence of long-term change in atmospheric circulation remains 5 inconclusive 6-8 . Here we identify statistically significant trends in the occurrence of 6 mid-atmospheric circulation patterns, which partially explain observed trends in 7 surface temperature extremes over seven mid-latitude regions of the Northern 8Hemisphere. Utilizing self-organizing map (SOM) cluster analysis 9-12 , we detect 9 robust pattern trends in a subset of these regions during both the satellite 10 observation era Although most land regions show robust warming over the past century 13 , the 21 pattern of change has not been spatially uniform 14 . This heterogeneity results from 22 regional differences in the response of the climate system to increasing radiative forcing, 23and from the background noise of climate variability. Together, these factors 24 substantially increase the challenge of climate change detection, attribution, and 25 projection at regional and local scales 14,16 . 26The spatial pattern of changes in extreme weather events has generated arguments 27 that global warming has caused dynamic and/or thermodynamic changes that have 28 differentially altered extreme event probabilities 1,17 . Thermodynamic arguments are well 29 3 understood and observed. For example, the accumulation of heat in the atmosphere has 30 resulted in upward trends in hot extremes, downward trends in the majority of cold 31 extremes, and more intense hydroclimatic events 1,2 . Dynamic arguments have greater 32 uncertainties [15][16][17][18][19] . Changes in the large-scale atmospheric circulation -for instance, an 33 increase in the occurrence or persistence of high-amplitude wave patterns -could alter 34 the likelihood of extreme events 20 . Recent extremes in the Northern Hemisphere mid-35 latitudes 1,2,17 have motivated hypotheses of a dynamic linkage between "Arctic 36 Amplification", altered atmospheric circulation patterns, and changes in the probability of 37 mid-latitude extremes e.g., [3][4][5]17 . Despite divergent views on the causal direction of this 38 linkage 17 , altered atmospheric dynamics are consistently invoked. Although trends in 39 mean-seasonal mid-atmospheric geopotential height anomalies have been identified (Fig. 40 2.36 ref. 21; Fig. 1), evidence of changes in the occurrence of sub-seasonal atmospheric 41 patterns remains equivocal, as does their contribution to extreme event probabilities [6][7][8] . 42Previous efforts to detect trends in atmospheric circulation may have been 43 hampered by narrowly-defined, spatially-sensitive, and/or non-standardized metrics 3,[6][7][8]17 . 44We therefore employ a large-scale spatial characterization approach -Self-Organizing 45Map ("SOM") cluster an...

The dynamics of temperature extremes

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“…Unless progress is made on these fronts, we will continue to be surprised by deviations from the thermodynamic expectations of global warming. ■ Theodore G. Shepherd 2 report that this heatwave was part of a long-term trend towards more anticyclonic summertime circulation regimes over Europe.…”

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“…Model results from several studies by Screen and others this past year [7][8][9] show little support for future seasonal average midlatitude cooling trends or increases in the number of extreme weather events over the Northern Hemisphere. Other results suggest that if the Arctic does affect severe weather to the south, its influence is regional in extent and episodic, even within seasons.…”

The Impact of Climate Change on Biodiversity in Nepal: Current Knowledge, Lacunae, and Opportunities

“…Numerous studies agree that the intensity of temperature extremes will increase [59][60][61][62][63][64][65][66][67][68][69][70]. Several studies have projected significant increases in mean precipitation and in monsoon variability [38], due to a several anticipated factors, including an increased moisture availability that parallels increases in surface temperature [35] and intensification of the thermal low over northern India [35,38,[71][72][73][74].…”