Evaluating 21st century projections of anticyclonic weather frequency and persistence over the British Isle

Christian Franzke (7193108)

Colin Harpham (7193105)

Gregor C. Leckebusch (7191341)

John K. Hillier (3147690)

Paolo De Luca (7191575)

Robert Wilby (1255929)

English

Loughborough University Institutional Repository

Evaluating 21st century projections of anticyclonic weather frequency andpersistence over the British IslesPaolo De Luca1, Colin Harpham2, Rob Wilby1, Christian Franzke3, Gregor Leckebusch4, JohnHillier11Geography Department, Loughborough University, UK2Climatic Research Unit (CRU), School of Environmental Sciences, University of East Anglia, UK3Meteorological Institute and Center for Earth System Research and Sustainability (CEN), University of Hamburg, Germany4School of Geography Earth and Environmental Sciences, University of Birmingham, UKHydro-meteorological hazards such as heatwaves, droughts and flooding are often linked to persistentweather patterns[1]. In this study we investigated the ability of a CMIP5 multi-model ensemble (MME)of 10 GCMs at reproducing seasonal frequencies and lag-1 persistence[2] of anticyclonic (A) weatherpatterns[3, 4] over the British Isles (BI). Historical runs, along with 21st century projections of A-typeoccurrence and persistence under RCP8.5, provide a basis for evaluating climate models and inferredimpacts on society.A-type frequency is projected to increase significantly (p<0.01) under RCP8.5 (2006-2100) duringsummer (June-August). A-type persistence shows large variability within the MME and is on average un-derestimated in winter (December-November), but within other seasons there is better agreement whencompared with reanalyses (i.e. ERA-20CR and NCEP-NCAR, during the historical period 1971-2000).Changes in A-type persistence between the MME historical and 2020s, 2050s and 2080s within RCP8.5are not statistically significant according to the Mann-Whitney-Wilcoxon two-tailed test. However, whentesting the statistical significance of each GCM within the MME using boot-strapping (n=1,000 simula-tions), all RCP8.5 model projections fall outside the 95% confidence intervals of historical persistence.Our results show that A-type persistence increases during summer. Greater A-type frequency and persis-tence during summer implies increased likelihood of blocking episodes that could translate into higherrisk of droughts, heatwaves and episodes of severe air pollution.Assessing changes in the frequency and persistence of the A-type is useful for diagnosing GCMs real-ism and also provides a basis for investigating future variations and narratives for hydro-meteorologicalhazards. CMIP5 models lack the ability in reproducing A-type activity at mid-latitudes, an issue thathopefully will be addressed by the next generation of high-resolution models that will be used in CMIP6.Our pragmatic approach is readily applicable to weather types linked to flooding (e.g. cyclonic and west-erly), as well to other mid-latitude regions with objective weather classification schemes in Europe andNorth America.References[1] Munich Re., Natural catastrophes 2014: Analyses, assessments, positions, Topics Geo (2015)[2] Wilby, R. L. Stochastic weather type simulation for regional climate change impact assessment,Water Resour. Res. 30, 3395-3403 (1994)[3] Lamb, H. H. British Isles Weather types and a register of daily sequence of circulation patterns,1861-1971, Geophysical Memoir 116, London, HMSO (1972)[4] Jones, P. D., Hulme, M. & Briffa, K. R. A comparison of Lamb circulation types with an objectiveclassification scheme. 13, 655–663 Int. J. Climatol. 13, 655-663 (1993)1

Evaluating 21st century projections of anticyclonic weather frequency and persistence over the British Isles

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Evaluating 21st century projections of anticyclonic weather frequency and persistence over the British Isles

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