New Metrics and Evidence Linking Arctic Amplification with a Wavier Jet Stream

Transcription

1 New Metrics and Evidence Linking Arctic Amplification with a Wavier Jet Stream Jennifer Francis Ins$tute of Marine and Coastal Sciences Rutgers University In collabora$on with Steve Vavrus, Jon Mar$n, and Fuyao Wang - - U. of Wisconsin Natasa Skific - - Rutgers

2 Chain of Events Linking Arctic Amplification (AA) with Increased Extreme Weather in Mid-Latitudes: a hypothesis Arc$c Amplifica$on The mechanisms are emerging More persistent weather pa=erns, extremes more likely Poleward temperature gradient weakening Larger waves progress eastward more slowly Upper- level westerly winds decreasing Amplified pa=erns more frequent Upper- level flow becoming more meridional Francis and Vavrus, 2012

3 Emerging mechanisms Ø Sea ice loss, AA over Barents/Kara creates ridge Ø Surface high => flow from Arc$c => trough Ø Wave energy transferred to stratosphere Ø Polar vortex disrupted Ø Wavier jet stream Cohen et al, 2014 Kim et al, 2014 Feldstein & Lee, 2014 Mori et al, 2014 Cohen et al, Nature Geosci., 2014

4 New Metrics and Evidence Ø Meridional Circulation Index (MCI) Ø Frequency of high-amplitude jet patterns Ø Sinuosity Ø Self-Organizing Maps (SOMs) analysis

5 Arctic Amplification Difference in near-surface air temperature anomalies Arctic (70 o N-90 o N) Mid-latitude (30 o N-60 o N) Winter Spring Summer Fall Annual

6 Arctic Amplification Not confined to surface! 250 Temperature changes 1995 to 2013 (annual) Height changes 1995 to 2013 Zonal wind changes 1995 to o N 80 o N 40 o N 80 o N 40 o N 80 o N but with large regional and seasonal variability

7 Francis and Vavrus, 2014, in review OND Changes in atmospheric thickness Poleward temperature gradient weakening Z T ( ) Δthick. gradient Zonal winds decreasing where gradient weakens ΔU500 U500

8 Meridional Circulation Index (MCI) MCI= v v / u 2 + v 2 v = north/south wind u = east/west wind u 1 V2 V1 v u 2 Francis and Vavrus, 2014, in review

9 OND Zonal winds decreasing where gradient weakens U hpa Δ MCI Upper- level flow becoming more meridional ΔU500 Less wavy More wavy Francis and Vavrus, 2014, in review

10 OND 500 hpa Upper- level flow becoming more meridional Less wavy More wavy Δ MCI Δthick. gradient Δ MCI ΔV500 Δthick. gradient ΔV500 Francis and Vavrus, 2014, in review

11 Amplified pa=erns more frequent High-Amplitude Patterns (HAPs) 500 hpa contour range > 35 o latitude Unprecedented Spain flooding Jan Polar Vortex

12 Change in contour height with time From 30-member CESM-CAM5 Large Ensemble November La$tude from F. Wang, pers. comm.

13 Are HAPs happening more often? Fall (Oct-Dec) Fall (Oct-Dec) Northern North Hemisphere Atlantic High-amplitude waves High-amplitude waves Upper-level westerly winds Upper-level westerly winds

14 Change (%) in HAP frequency versus Region JFM AMJ JAS OND Atlan$c E 19* 5 57** 47** North America E 18* 12 59** 23 Europe E Asia E * 65** Pacific E * * Northern Hemisphere < - 40% - 39 to 30% - 29 to 20% - 19 to 10% - 9 to 0% 0 to 9% 10 to 19% 20 to 29% 30 to 39% > 40% Francis and Vavrus, 2014, in review * > 1 SD, ** > 2 SD

15 Sinuosity DJF Sinuosity 0 Correla$on of S with AO index S = area inside contour area of equivalent la$tude circle 50/51 60/61 70/71 80/81 90/91 00/01 10/11 +S correlated with - AO Mar$n, Vavrus, Wang, and Francis, in prep

16 Sinuosity Annual cycle Mar$n, Vavrus, Wang, and Francis, in prep

17 Self-Organizing Maps (SOMs) La$tude (difference from daily mean) A neural-network approach to identify characteristic patterns in large data sets Longitude (0 to 360oE) 66 years of daily 500 hpa contours from NCEP reanalysis are used to create the matrix of clusters that represent dominant atmospheric patterns Francis, Skific, Cassano, and Cassano, in prep

18 Frequency By month Winter Longitude (0 to 360oE) Frequency Summer La$tude (difference from daily mean) Spring/Fall Month Francis et al, 2014, in prep

19 La$tude (difference from daily mean) Longitude (0 to 360 o E) days Frequency of Occurrence Francis et al, 2014, in prep

20 La$tude (difference from daily mean) Longitude (0 to 360 o E) o Lat Cluster- Mean Wave Amplitude Francis et al, 2014, in prep

21 FOC Amplitude Change in FOC Change in amplitude Changes from to Increased FOCs mainly for large-amp patterns. Low-amp patterns decreasing. Amplitudes of winter patterns increasing; summer amplitudes decreasing.

22 Summary Ø Several recent papers support robust winter mechanism linking B/K sea-ice loss with cold winters in Asia Ø New metrics, new evidence: MCI, Frequency of HAPs, Sinuosity, SOMs Ø MCI: link between AA and more meridional flow Ø HAPs: Increasing in seasons and regions where AA is strongest Ø Sinuosity: Robust negative correlation with AO. Recent increase in S consistent with recent decrease in AO. Ø SOMs: HAPs more frequent. Winter amplitudes increasing, summer decreasing. francis@imcs.rutgers.edu

23 Extras

24 Causes of Near-Surface Arctic Amplification Ice/snow-albedo feedback (e.g., Screen and Simmonds, 2010) Water vapor and clouds (e.g., Graversen and Wang, 2009) Non-linear IR emission (Pithan and Mauritsen, 2014) Differences in far-ir ε (Feldman et al, 2014)

25 JFM Poleward temperature gradient weakening 20 Z T ( ) 10 % change in 500 mb zonal winds in 4 x CO 2 run of CCSM4 Zonal winds decreasing where gradient weakens 0 Winter - 10 Fall Spring U500 Summer o N 45 o N 60 o N

26 JFM CCSM4 4 x CO2 Change in Meridional Wind Poleward temperature gradient weakening ZT( ) Upper- level flow becoming more meridional Winter Spring Summer Fall +33% +21% +21% +19%

27 Amplified pa=erns more frequent, blocking more likely Are blocking events occurring more frequently? Probability of blocking days per month (event x dura$on) 1968 to to 2012 by C. Reynolds, based on data from A. Lupe: h"p://solberg.snr.missouri.edu/gcc/

28 Emerging mechanisms AA era coincides with temperature changes over mid- la$tude land areas in winter: Coldest daily minimum temperature Ø Coldest daily min temps geang colder Ø Number of days below freezing is increasing Number of days < 0 o C Cohen et al, 2014

29 Emerging mechanisms Quasi-resonance (summer only) Coumou et al (2014), Petoukhov et al (2013) Ø Many summer extreme events (heat waves, floods, drought) caused by persistent, high- amplitude jet- stream waves Arc$c amplifica$on Resonance months Ø Waves trapped in wave guides created by split jet; become stagnant Ø Split jets more likely when west/east jet winds are weak Ø Arc$c amplifica$on causes weaker west/east winds Split jet stream during extreme European heat wave in July 2006

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