The role of the stratosphere in the European climate response to El Niño

Winter 2015/16: A Turning Point in ENSO-Based Seasonal Forecasts

“…The high autumn snow cover and low Arctic sea ice were eventually followed by an abnormally weak polar vortex, a sudden stratospheric warming (SSW) event, a negative AO, and cold temperatures across parts of the Northern Hemisphere mid-latitudes during mid-winter, all of which are consistent with the mechanism outlined by Cohen et al (2014b). A weak polar vortex and negative AO were also found to be favored during El Niño winters (Ineson and Scaife, 2009). The atmospheric behavior in winter 2015/16 over Eurasia is also consistent with the response expected with diminished sea ice conditions, namely a strengthening and northwestward expansion of the Siberian high.…”

The Stratospheric Pathway of La Niña

“…The stratospheric signal during the warm ENSO phase (El Nino) has been extensively documented (Garcia-Herrera et al 2006;Manzini et al 2006;Fletcher and Kushner 2011); during El Nino winters, upward wave activity toward the stratosphere is enhanced through constructive interference between the El Nino anomalies and the climatological eddies, resulting in a warmer and a weaker polar vortex. Related zonal mean zonal wind anomalies in the stratosphere propagate downward into the troposphere, having an impact on the North Atlantic and European (NAE) region (Cagnazzo and Manzini 2009;Ineson and Scaife 2009;Bell et al 2009). It has been indeed shown that El Nino has the potential to improve seasonal predictability over Europe (Domeisen et al 2015).…”

Factors influencing Northern Hemisphere winter mean atmospheric circulation anomalies during the period 1960/61 to 2001/02

“…Overall, we can say on the basis of these results that, on interannual time-scales, the Tropics do indeed influence the NAO in the model, (although weakly), that perfect knowledge of extratropical SST and sea-ice adds to that influence, and that perfect knowledge of the stratosphere has about the same level of influence. Of course, it is possible that some of the influence from the stratosphere is in reality initiated in the Tropics and that, similarly, influence from the Tropics is carried to the North Atlantic sector via the stratosphere (Ineson and Scaife, 2009, give an example), an issue for further study. Nevertheless, since the average correlation between individual model realisations and the observed NAO index is at best 0.3 in these experiments, none of the influences considered in our model experiments can account for more than about 10% of the observed interannual variance in the NAO index during the 42 winters being studied here.…”

“…Fraedrich andMüller (1992) provide evidence for a tendency towards the negative (positive) NAO during El Niño (La Niña) years whereas Greatbatch et al (2004) indicate non-stationary behaviour (also Greatbatch and Jung, 2007) with a very different impact from the Tropics over the Euro-Atlantic sector before and after the 1976/77 Pacific climate shift (Trenberth et al, 2002). Toniazzo and Scaife (2006) argue that the influence of El Niño over Europe depends on the strength of the event in the tropical Pacific and Ineson and Scaife (2009) have presented evidence that the stratosphere can act as a bridge between the tropical Pacific and Europe. The mechanism involves interference between the forced response and the seasonally evolving climatological stationary waves, an issue that has been explored for forcing from different parts of the Tropics by Fletcher and Kushner (2011).…”