Winter ENSO teleconnections in a warmer climate

Changes in the winter atmospheric response to sea surface temperature (SST) anomalies associated with the El Nino-Southern Oscillation (ENSO) in a warmer climate conditions are estimated from the two 20-member ensembles made by an atmospheric general circulation model of intermediate complexity. Warmer climate is simulated by a modification in the radiation parameterisation that corresponds to the doubled CO2 concentration, and SST forcing is represented by the same SST anomalies as in current climate (1855-2002) experiment superimposed on the climatological SST that was obtained from a complex atmosphere-ocean general circulation model forced with the doubled CO2. SST anomalies in the Nino3.4 region, categorised into five classes, enabled a composite analysis of changes in the Northern Hemisphere tropical/extratropical teleconnections. The main features of the tropical-extratropical teleconnections are maintained in both experiments; for example, irrespective of the sign of SST anomalies, the amplitude of the atmospheric response is positively correlated with the intensity of ENSO event and the El Nino impact is stronger than that of La Nina of the same intensity. The strongest extratropical signal in the warmer climate, particularly significant for strong warm events, is found over the Pacific/North American region; however, this extratropical teleconnections is reduced in a warmer climate relative to the current climate. Over the North Atlantic/European region, a detectable signal linked to ENSO is found; this model response is significantly strengthened in the experiment with the doubled CO2 concentration. Such an atmospheric response in a warmer climate is found to be associated with changes in the mean state followed as well as in the jet waveguiding effect and stationary wave activity.