ENSO, stratospheric warmings and recent trends in winter climate

Observations suggest that most of the trend in high latitude climate in the Northern Hemisphere over the past 40 years occurs during El Niño years. Observations also show that midwinter stratospheric warmings occur more frequently during El Niño years. The NCAR WACCM model shows a strong tendency to produce more planetary wave flux and stratospheric warmings during El Niño years. When observed ozone depletions are introduced into the WACCM model, the response of the polar climate in the model to the ozone depletion is much stronger for El Niño conditions than for La Niña conditions. Both observations and model results suggest that annular mode trends are more prominent for El Niño conditions. It is hypothesized that the thermal forcing of the stratosphere suppresses stratosphere warmings, but only for those conditions when they would otherwise have occurred, which is predominantly during El Niño years.