ON THE RELATION BETWEEN EXPLOSIVE AFFECTING EUROPE AND THE NORTH ATLANTIC OSCILLATION

I. Gómara (a,b) B. Rodríguez-Fonseca (a,b) P. Zurita-Gotor (a,b) J.G. Pinto (c,d)

(a) Univ. Complutense Madrid, Spain (b) IGEO, CSIC, Spain (c) Univ. Reading, UK (d) Univ. Cologne, Germany

Gómara et al. [2014] Geophys. Res. Lett., 41, 2182-2190

Iñigo Gómara - [email protected] OUTLINE:

• Motivation and Introduction

• Data and Methodology:

• considerations

• NAO definition and classification

• Results

• Influence of the background field (NAO) over the cyclones

• Influence of the cyclones over the background field (NAO)

• Concluding remarks

Iñigo Gómara - [email protected] Tortosa 11/3/15 2/12 MOTIVATION - IMPACTS

Western Europe: Winter 2013/2014

Serial occurrence of European windstorms following a similar path:

Christian, Xaver, Dirk, Anne, Petra, Qumaira, Ruth, AEMET Stephanie, Tini… EFE

EUMETSAT Impacts:

• Flooding over southern UK

• Storm surges

• Strong wind gusts

UKMO 3/12 MOTIVATION - SCIENTIFIC

(1) INFLUENCE OF THE BACKGROUND FIELD (NAO) OVER NORTH ATLANTIC CYCLONES

• Analysis of anomalous mean flow fostering European windstorms (e.g., Raible, 2007; Pinto et al., 2009; Donat et al., 2010; Hanley and Caballero, 2012).

Is this relation sensitive to storm’s intensity?  We examine the NAO conditions fostering the occurrence of explosive vs. non explosive cyclones affecting Europe

(2) INFLUENCE OF THE CYCLONES OVER THE BACKGROUND FIELD (NAO)

• Intense cyclones can potentially modify the large-scale flow and thus drive the short- term NAO variabilty (e.g., Rivière and Orlanski, 2007; Michel et al., 2013).

Can explosive cyclones affecting Europe modify the large-scale flow? We examine the (1) feedback on the daily NAO Index

EXPLOSIVE MEAN FLOW THE TWO-WAY RELATION BETWEEN THE NAO CYCLONES AND EXPLOSIVE/NON EXPLOSIVE CYCLONES AFFECTING EUROPE IS EXAMINED (2)

Data and Methodology: Cyclone considerations

NCEP re-analysis: October-March 1950-2010 NoEC (1665) Automatic tracking algorithm (Murray and B = reversal of θ Simmonds, 1991; Pinto et al., 2005)  Laplacian meridional gradient of MSLP as proxy of relative geostrophic (PV2 surface) vorticity.

Normalized Deepening Rate (lag 0 – 24h of max. intensification start)

EC (424)

Iñigo Gómara - [email protected] 5/12 Data and Methodology: NAO definition and classification

North Atlantic Oscillation (NAO)

• Most prominent pattern influencing European weather variability. Storm activity strongly influenced by the NAO phase.

• EOF1 of monthly mean z500 anomalies: [25-80N, 80W-40E], Oct-Mar 1950-2010.

• Daily NAO Index (DNI): Projection of z500 daily anomalies onto EOF1.

5 NAO phases [Pinto et al., 2009]:

NAO++ (DNI ≥ +1.5) NAO+ (+0.5 ≤ DNI < +1.5) NAO 0 (-0.5 < DNI < +0.5) NAO- (-1.5 < DNI ≤ -0.5) NAO-- (DNI ≤ -1.5)

EOF1 (33.89%)

6/12 RESULTS: INFLUENCE OF THE BACKGROUND FIELD OVER THE CYCLONES

Composite anomalies of z500 and jet (99% Monte Carlo test)

Lag 0 = day in which 24 h of max. intensification start 7/12 RESULTS: ANALYSIS OF NAO CONDITIONS

Lag 0:

EC/EC95: NAO+ and NAO++ phases anomalously frequent

NoEC: opposite behaviour

Lags -12 to +12:

NAO+ (NAO-) cycles of 8 to 12 days associated with EC95/EC (NoEC)

EC95: Sharp growth and decay (2-6 days). Are Eddy momentum fluxes involved?

Lag 0 = day in which 24 h of max. intesification start RESULTS: INFLUENCE OF THE CYCLONES ON THE BACKGROUND FIELD

ECP (103)

Histogram: DNI variation after cyclone peak intensity (DNIlag+4 - DNIlag0) ECN (169) EC95/EC: bimodal shape, abrupt DNI variations.

NoEC: unimodal shape

¿WHY? 9/12 RESULTS: INFLUENCE OF THE CYCLONES ON THE BACKGROUND FIELD

EDDY MOMENTUM FLUXES (EMF)

EMF = u’v’ (250 hPa, Lanczos filter 1-5 days) Climatology Zonal momentum equation:

Direction of Momentum Jet Induced NAO breaking fluxes displacement state

Anticyclonic Positive Poleward NAO+

Cyclonic Negative Equatorward NAO-

EC NoEC NoEC

10/12 RESULTS: INFLUENCE OF THE CYCLONES ON THE BACKGROUND FIELD

ECN (169) ECP (103)

z500 z500

EMF EMF

Negative peak (towards NAO-): Positive peak (towards NAO+): Downstream propagation of z500 anomalies Positive momentum fluxes involved CONCLUDING REMARKS

INFLUENCE OF THE BACKGROUND FIELD (NAO) OVER THE CYCLONES

• NAO conditions fostering explosive/non explosive cyclones are different

• Explosive: NAO+ and an accelerated jet over Western Europe (upper level forcing + waveguide)

• Non Explosive: negative NAO (lower level forcing + Greenland )

Predictions of cyclones impacting Europe may be improved (lags -4/-3)

INFLUENCE OF THE CYCLONES OVER THE BACKGROUND FIELD (NAO)

• Important differences are found on the NAO dynamics after peak intensity of explosive and non explosive cyclones

• Explosive cyclones are associated with stronger DNI variations (2 predominant mechanisms)

• Downstream propagation of z500 anomalies (toward NAO-)

• Positive eddy momentum fluxes (toward NAO+)

Predicatibility on short-term NAO-like variability could be enhanced 12/12 THANKS FOR YOUR ATTENTION!

tropa.fis.ucm.es EXTRAS RESULTS: INFLUENCE OF THE CYCLONES ON THE BACKGROUND FIELD RESULTS: INFLUENCE OF THE CYCLONES ON THE BACKGROUND FIELD RESULTS: INFLUENCE OF THE CYCLONES ON THE BACKGROUND FIELD RESULTS: INFLUENCE OF THE BACKGROUND FIELD ON THE CYCLONES RESULTS: INFLUENCE OF THE BACKGROUND FIELD ON THE CYCLONES Data and Methods: Cyclone tracking

Cyclone tracking after Murray and Simmonds, 1991; Pinto et al, 2005

“Common” developping cyclones Bombs

1) d/dt(∇2P) >=0.3 deg.lat-1day-1 1) -4) as left and 2) LT>1day 3) minP<1000 hPa 5) NDR > 1 Bergeron 4) ∇2P > 0.6 deg.lat-2 Case Study: TOP 7 Event Eastern NA (30 Jan - 4 February 1994)

Lag 0 Lag -1

Lag +1

θ (PV2 surface) and jet 250hPa (m/s)

Two-sided RWB  Explosive Lag +2 over Eastern North Atlantic

Cyclonic RWB over NW Europe Top50E - RWB and Jet Anomalies

/1 m/s

Lag -2 Lag 0

Lag 0 Lag +2

Iñigo Gómara - [email protected] 9/18 Storm clustering over Western Europe & RWB

Jan 2007 18.01.07 - Kyrill

Two-sided RWB  Steered & intensified jet towards Western Europe  Storm families over EU (explosive)

Pinto et al. [2014]

Iñigo Gómara - [email protected] 17/18