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

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

Conventional Perspective 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

An Alternative Characterization 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

Essence of Characterization Spatial Structure Essence of Characterization A BLOCK 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.

Some Salient Features II: Temporal 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)

Credibility of Characterization 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

Essence of a Block Quasi-stationarity

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

(A) Origin of Lens III: Dynamics 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

(A) Origin of Lens III: Dynamics TWO INFERENCES 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

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

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

(C) Quasi-stationarity 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

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

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

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

(D) Establishment of overall PV-pattern 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

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

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

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

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

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

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

Sudden Stratospheric Warming & BLOCKS IV: Related Phenomena Sudden Stratospheric Warming & BLOCKS

Sudden Stratospheric Warmings & BLOCKS 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

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

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

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