1) Moderate El Niño

(a) Rossby wave propagation

(b) Via stratosphere, weak polar vortex

(2) Stratosphere

(a) Equatorial cooling, polar warming (idealised models)

(b) Easterly QBO - Holton-Tan mechanism

- direct impact on troposphere?

(3) Blocking as the mechanism of negative NAO

Woollings et al (2008) Tim’s presentation

Negative AO, NAO – potential factors

(1) Moderate El Nino

(a) Rossby wave propagation

(b) Via stratosphere, weak polar vortex

Negative AO, NAO – potential factors

PMSL DJF 09/10

PMSL response to moderate El Nino, Jan-Feb 1950-2000

Toniazzo & Scaife (2006)

(2) Response to stratospheric heatingP10 Polar heating (10K)

5K0K

5K 0K

E5 Equatorial heating (5K) U5 Uniform heating (5K)

10K

Zonal wind responseE5 U5 P10

Idealised GCM

Haigh et al (2005)

See alsoLorenz & de Weaver (2007)

Control zonal wind

Equatorward jet shift for:

•Tropical cooling

•Polar heating

NAO/AO onset in December 2009

www.met.reading.ac.uk/Data/CurrentWeather/ using data from ECMWF operational analyses

Tropopause animation, 27 Nov – 4 Dec

Tropopause animation, 4-11 Dec

Tropopause animation, 11-18 Dec