EL-NINO, LA NINA, AND THE NONLINEARITY OF THEIR TELECONNECTIONS

The paradigm of an atmospheric system varying linearly with respect to extreme phases of the EI Nino-Southern Oscillation is questioned. It is argued that the global response to tropical Pacific sea surface tem perature forcing will be inherently nonlinear. A physical basis far th is intrinsic nonlinearity is the thermodynamic control on deep convect ion. Climate statistics for warm and cold events of the tropical Pacif ic are analyzed separately for the northern winter periods during 1950 -96. Composite analysis of 500-mb heights reveal planetary-scale telec onnection patterns, as noted in earlier studies. A new result is the e vidence for an appreciable 35 degrees longitude phase shift between th e warm and cold event circulation composites, and the two wave trains appear to have different tropical origins. A large nonlinear component in North American surface climate anomalies is also found, which is c onsistent with such a phase shift in teleconnections. In the Tropics, rainfall anomalies also show evidence of nonlinear behavior. The maxim um rain anomalies along the equator are located east of the date line during warm events, but west of the date line during cold events. The interpretation of this behavior is complicated, however, by the fact t hat composite warm event SST anomalies are not the exact inverse of th eir cold event counterparts. Idealized atmospheric general circulation model (AGCM) experiments are performed in order to test the question of whether the observed nonlinearity is an intrinsic property of the a tmospheric system. The model is forced with a composite SST anomaly th at undergoes a realistic seasonally varying ENSO life cycle, as descri bed by E. Rasmusson and T. Carpenter. Both positive and negative phase s of the SST anomaly are used, and a 40-member ensemble of warm and co ld event model simulations is conducted. A nonlinear climate response in the AGCM is found that closely resembles the observed composites, i ncluding a shift in the equatorial positions of the maxmium rain respo nses and a phase shift of teleconnection patterns in the upper troposp here. Barotropic model experiments indicate that the inherent nonlinea rity in the tropical rain response may itself be responsible for the p hase shift in the extratropical teleconnection patterns.