Huw C. Davies & Mischa Croci-Maspoli Institute for Atmospheric and Climate Science,

1. A Characterization of Atmospheric Blocking Huw C. Davies & Mischa Croci-Maspoli Institute for Atmospheric and Climate Science, ETH Zurich, Switzerland & MeteoSwiss, Zurich, Switzerland

2. OUTLINE I Spatial Structure - Basis for the characterization II Temporal Features - Credibility of the characterization III Dynamics - Utility of the characterization . via consideration of :- block origin & resilience, quasi-stationarity & formation IV Relationship with other Phenomena

3. I: Spatial Structure Conventional Perspective • Notable features: • . • surface anticyclone, with • ridge aloft & local easterly flow • elevated tropopause & • jet bifurcation Tropopause SLP anomaly & 500hPa pattern Latitudinal cross-section of height anomaly

4. I : Spatial Structure An Alternative Characterization • Block also evident as : • . • a negative PV anomaly • on upper-level isentropes - anomaly located beneath an elevated tropopause PV=2 latitude [°N] - contiguous anomalies present at surface and upper-level

5. I : Spatial Structure Essence of Characterization ABLOCK constitutes . “a LENS of low PV located beneath an elevated tropopause”. Develop an "identification and tracking" tool that can catalogue every block (sic. negative PV lens) in terms of its: - amplitude, location, structure, movement and duration.

6. II: Temporal Features Some Salient Features • A Block / PV Lens • occurs in preferred geographical regions, • persists for supra-synoptic time scales, and • during its mature phase does NOT undergo significant : • . • - change of shape • despite being subject to large-scale deformation (sic. a structurally resilient system) • . • - translation • despite its location within band of zonal mean westerlies (sic. a quasi-stationary system)

7. II: Temporal Features Credibility of Characterization (B) Synoptic Simultaneity 1 2 3 4 5 6 7 8 9 10 • Lens Climatology • Comparable ! T&M P&H 13% 10% 5% 1% DJF TIME (days) 476 events -> 3.5 per month

8. Essence of a Block Quasi-stationarity

9. III: Dynamics Questions Questions prompted by “Lens” characterization of a Block: • Origin of the ‘Lens’ (i.e. the negative PV anomaly) ? • (B) Dynamics of system’s structuralresilience ? • (C) Dynamics of the system’s quasi-stationary ? • Establishment of the overall PV pattern ? • (- i.e. of the lens plus contiguous features)

10. III: Dynamics (A) Origin of Lens • NOTE: Two possible sources for anomalously low PV near tropopause : • - advection from low latitudes • - convection (- diabatic cross-isentropic flow) from the low troposphere. ASSESS relative contribution by - examining backward trajectories from the ‘Lens’ Indication that two major sources contribute to the ‘Lens’ - tropopause-level air from far-upstream, and - low level moist air-stream ascending after passing over warm SST anomaly

11. III: Dynamics (A) Origin of Lens QUERY : Is the LENS formation influenced by ascent of the coherent moist airstream ? NUMERICAL EXPERIMENT : Modify nature of airstream by changing the positive upstream anomalies in SST and land surface temperature TWO INFERENCES - Block formation sensitive to upstream surface conditions, - THE ULTIMATE TEST of a model’s cloud dynamics and microphysics is the delivery of ‘correct’ PV distribution aloft. Verifiying ECMWF Analysis Control Simulation

12. III: Dynamics (B) Resilience How does a “PV-Lens” retain its coherent structure ? (i) PV-lens in a horizontal uniformly sheared flow

13. III: Dynamics (C) Quasi-stationarity • What keeps a PV Lens quasi-stationary ? (i) PV-lens in a horizontal uniformly sheared westerly flow

14. III: Dynamics (C) Quasi-stationarity IMPLICATION: STATIONARITY requires a richer anomalous PV pattern - isolated LENS does not suffice High PV Low PV Consider the typical instantaneous PV distribution on an isentropic surface crossing the tropopause. North High PV Low PV

15. Dynamics An Example of a Block with a di-polar PV configuration

16. (C) Quasi-stationary: Schematic of possible alternative configurations III: Dynamics High PV Low PV

17. (C) Alternative quasi-stationary configurations III: Dynamics An Example of a Block with a tri-polar PV configuration

18. III: Dynamics (D) Establishment of overall PV-pattern BREAKING WAVE(s) SCENARIOS • TYPE C TYPE A High PV Low PV High PV Low PV High PV Low PV High PV Low PV High PV Low PV

19. III: Dynamics (D) Establishment of overall PV-pattern BREAKING WAVE(s) SCENARIOS High PV Low PV High PV Low PV

20. III: Dynamics (D) Establishment of overall PV-pattern EXAMPLE OF A BLOCK FORMATION Breaking wave (TYPE A) .. Secluded Lens Breaking wave (TYPE C) PVU PV on 320K

21. III: Dynamics (D) Establishment of overall PV-pattern HOVEMOELLER COMPOSITE (centred on Block) Meridional Velocity from Day-6 to DAY+6 • ATLANTIC PACIFIC

22. III: Dynamics (D) Establishment of overall PV-pattern COMPOSITE OF BREAKING WAVES • ATLANTIC • PACIFIC • TYPE A • TYPE C

23. IV: Related Phenomena Forcing, Patterns of Climate Variability (PCV) and BLOCKS CONVENTIONAL CAUSAL CHAIN Forcing PCV Character ofWeather Systems AN ALTERNATIVE CAUSAL CHAIN Forcing Weather Systems PCV

24. IV: Related Phenomena Forcing, Sudden Stratospheric Warmings and BLOCKS Troposphere - Stratosphere Linkage Baldwin and Dunkerton 2001

25. IV: Related Phenomena Sudden Stratospheric Warming & BLOCKS

26. IV: Related Phenomena Sudden Stratospheric Warmings & BLOCKS SSW rules OK !! ? A. Scaife Blocks rule OK !! ? Evolution of mean zonal wind at 600N between 1000 and 0.1 hPa

27. IV: Related Phenomena PCV, the NAO and BLOCKS Normalized time-traces of the Atlantic Blocking Frequency and the NAO - index for the three winter months Blocking Frequency NAO- r = -0.65

28. IV: Related Phenomena The NAO & BLOCKS Evolution of NAO index during a blocking event total tracks random random short tracks (< 10 days) short duration (< 10 days) long tracks (> 10 days) long duration (> 10 days) random random

29. SOME POSSIBLE INFERENCES What is a BLOCK ?? Requisite for representation of BLOCKS in models