Robustness of the nonlinear climate response to ENSO's extreme phases

Analysis of a suite of atmospheric GCM experiments for 1950-94 shows that b oth the tropical and the extratropical wintertime climate respond nonlinear ly with respect to opposite phases of ENSO. Such behavior is found to be re producible among four different GCMs studied and confirms that several obse rved asymmetries in wintertime anomalies with respect to ENSO phases are sy mptomatic of nonlinearity rather than sampling error. Nonlinearity in the tropical Pacific rainfall response is related to an SST threshold for convection that leads to saturation at modestly cold SST for cing but a linear increase for warmer SST forcing. A spatial shift in the r ainfall response is also a feature of the various GCMs' nonlinear behavior, that is, accentuated by the large zonal gradient of climatological SSTs ac ross the equatorial Pacific and the fact that convection responds to the to tal rather than the anomalous SST. Regarding upper-tropospheric teleconnection responses over the Pacific-Nort h American region, nonlinearity exists in both the strength of the midlatit ude response and its spatial phase. The four GCMs are found to be unanimous in having a 500-mb height response whose amplitude is roughly double for e xtreme warm tropical Pacific SSTs as compared with extreme cold SST forcing . The longitudinal phase of the GCMs' teleconnections is also shifted eastw ard during warm events as compared with cold events, though this displaceme nt is smaller than that observed. Further analysis of model simulations reveals that nonlinearity in climate responses emerges mainly for stronger ENSO events, and a predominantly line ar response is found for weaker tropical Pacific SST forcing. In particular , climate simulations using both realistic and idealized SSTs indicate that tropical Pacific SST anomalies greater than one standard deviation of the interannual variation are required for initiating an appreciable nonlinear climate response.