Characteristic blocking events over Ural-Siberia in Boreal Winter under Present and Future Climate Conditions Wen Zhou & Hoffman Cheung Guy Carpenter Asia-Pacific Climate Impact Center School of Energy and Environment, City University of Hong Kong Page 1 Aug 20-23, 2017 Chengjiang, China
Recent Arctic amplification and East Asian winter climate Cohen et al. (2014, NGEO) Sea ice reduction Increasing moisture flux towards Siberia Increasing snow cover over Siberia Persistence of snow cover Mori et al. (2014, NGEO) Page 2 Aug 20-23, 2017 Chengjiang, China
2008 South China snowstorm * persistent blocking high over Ural-Siberia *enhanced moisture advection from BOB * deep inversion layer Page 3 Aug 20-23, 2017 Chengjiang, China Zhou W. et al. (2009) MWR
2016 Extreme Cold Midwinter Intense cold surge HK hit by coldest temperatures in nearly 60 years A Strong Phase Reversal of the Arctic Oscillation in Midwinter 2015/16: Role of the Stratospheric Polar Vortex and Tropospheric Blocking Cheung, Zhou, et al. (2016) JGR Page 4 Aug 20-23, 2017 Chengjiang, China
1-D blocking detection Zonal index equations at lon l: [e.g. Tibaldi and Molteni 1990 Tellus; Barriopedro et al. 2006 JC] ZGS (m/deg) ZGN (m/deg) Extension: at least 12.5 degrees Persistence: at least 4 days 500-hPa height and Zonal index equations on 27 Mar 2008. Page 5 Aug 20-23, 2017 Chengjiang, China 5
wintertime atmospheric blocking over Urals Atmospheric blocking: the persistence of a quasi-stationary high pressure system over the extratropics Ural blocking enhances the southward cold advection downstream and reinforces the Siberian high Page 6 Aug 20-23, 2017 Chengjiang, China
Thermodynamic Diagnostic Importance of latent heat release in ascending air streams for atmospheric blocking S. Pfahl, C. Schwierz, M. Croci-Maspoli, C. M. Grams and H. Wernli. Nature Geoscience, 2015 Establishment Decay a: temperature tendency b: horizontal temp advection c: adiabatic heating due to vertical motion d:diabatic heating Horizontal temperature advection tropos phere Stratos phere Diabatic cooling + Warm advection in stratosphere Page 7 Aug 20-23, 2017 Chengjiang, China
Diagnostic: QG vorticity equation Establishment Decay Horizontal advection of absolute vorticity anticyclonic cyclonic a: geostrophic vorticity tendency b: horizontal vorticity advection c: tilting d: ageostrophic vorticity tendency e: divergence f: vertical advection Page 8 Aug 20-23, 2017 Chengjiang, China
Blocking Intensity & Blocking Extension Correlation Intensity Extension Duration Intensity 0.670 0.262 (>99.9%) (96.9%) Extension 0.670 0.233 (>99.9%) (94.4%) Duration 0.262 (96.9%) 0.233 (94.4%) Characteristic events Character Minimum q 0.25 Median q 0.75 Maximum Intensity (no unit) 0.9 3.0 4.3 5.6 10.1 Percentile Min 25th 50th 75th Weak Strong Max Page 9 Aug 20-23, 2017 Chengjiang, China
Cyclogenesis Forcing Mediterranean Sea Thermal & Height Structure Cyclogenesis upstream of a developing USB Thickness height tendency Height anomaly Temperature anomaly Page 10 Aug 20-23, 2017 Chengjiang, China
Cyclogenesis upstream of Ural-Siberia The developing USB undergo the strongest intensification Stronger positive temperature tendency & height tendency Weaker in dt/dt and dz/dt The cyclogenesis over the Mediterranean sea is not clear The strength of cyclogenesis upstream contribute to USB intensity Thickness height tendency Thickness air temp tendency Page 11 Aug 20-23, 2017 Chengjiang, China
An open ridge over Europe Z500 anom (shading), Z500 (contour) For a Strong USB: A deepened trough over NA & a ridge extends northeastward, Characterized by stronger amplification of a Rossby wavetrain Page 12 Aug 20-23, 2017 Chengjiang, China
Pre-existing cold anomalies over Siberia Above-normal SAT for strong USB Dipole pattern (N-S direction) Below-normal SAT for weak USB Dipole pattern (NW-SE) Enhance anticyclonic & warm eddies imported from upstream Favorable for USB formation without upstream cyclogenesis Page 13 Aug 20-23, 2017 Chengjiang, China
Different precursors for strong and weak Ural Blockings Strong Weak Pre-existing Ridge Tyrlis and Hoskins 2008 JAS + Intense cyclogenesis Colucci 1985 JAS; Tsou and Smith 1990 Tellus Surface cold anomalies over western Siberia Takaya and Nakamura 2005 JAS + Surface warm anomalies in the vicinity of the Urals Stronger amplification of eastward propagating Rossby wave Nakamura et al. 1997 Mon Wea Rev Interaction between Siberian high and upper tropospheric wave packet Takaya and Nakamura 2005 JAS Page 14 Aug 20-23, 2017 Chengjiang, China
Urals Winter Blocking Frequency in Future Climate Atlantic The mean blocking frequency over Atlantic and Pacific decreases systematically in the twenty-first century, and such a decrease is more pronounced in the Pacific sector and in the RCP8.5 run Page 15 Aug 20-23, 2017 Chengjiang, China Cheung HN, Zhou W (2015) JC
250-hPa zonal wind NCEP (contour), MME (shading) NCEP-MME (contour), 95% confidence (shading) Eastward displacement of the Atlantic jet stream Eastward shift of the Atlantic storm tracks Stronger zonal flow over Europe More transient eddies are transported toward the Urals Underestimation of European blocking frequency No significant change of Ural blocking frequency Page 16 Aug 20-23, 2017 Chengjiang, China
Summary Establishment: Horizontal advections of temperature and absolute vorticity via two different pathways. Strong: an open ridge and a cyclogenesis upstream of Ural-Siberia Weak: surface cold anomalies over Siberia Establishment Buildup of the Siberian high Decay: Diabatic cooling along the vertical column, warm advection in the stratosphere; cyclonic vorticity advection Breakdown of Siberian high cold air outbreak in East Asia for both strong and weak events Page 17 Aug 20-23, 2017 Chengjiang, China
Page 18 Aug 20-23, 2017 Chengjiang, China Thank You!