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 P MSL DJF 09/10 P MSL response to moderate El Nino, Jan-Feb Toniazzo & Scaife (2006)

(2) Response to stratospheric heating P10 Polar heating (10K) 5K 0K 5K 0K E5 Equatorial heating (5K)U5 Uniform heating (5K) 10K Zonal wind response E5U5P10 Idealised GCM Haigh et al (2005) See also Lorenz & de Weaver (2007) Control zonal wind Equatorward jet shift for: Tropical cooling Polar heating

(2) Annular response to localised heating Lorenz & De Weaver (2007) Equatorward jet shift for: Tropical cooling, inc. troposphere Polar stratospheric heating Zonal wind response to localised heating 150hPa deep, 20° wide latitude

NAO/AO onset in December using data from ECMWF operational analyses Tropopause animation, 27 Nov – 4 Dec Tropopause animation, 4-11 Dec Tropopause animation, Dec