The nonlinear patterns of North American winter temperature and precipitation associated with ENSO

Nonlinear projections of the tropical Pacific sea surface temperature anomalies (SSTAs) onto North American winter (November-M arch) surface air temperature (SAT) and precipitation anomalies have been performed using neural networks. During El Nino, the linear SAT response has positive anomalies centered over Alaska and western Canada opposing weaker negative anomalies centered over the southeastern United States. In contrast, the nonlinear SAT response, which is excited during both strong El Nino and strong La Nina, has negative anomalies centered over Alaska and northwestern Canada and positive anomalies over much of the United States and southern Canada. For precipitation, the linear response during El Nino has a positive anomaly area stretching from the east coast to the southwest coast of the United States and another positive area in northern Canada, in opposition to the negative anomaly area over much of southern Canada and northern United States, and another negative area over Alaska. In contrast, the nonlinear precipitation response, which is excited during both strong El Nino and strong La Nina, displays positive anomalies over much of the United States and southern Canada, with the main center on the west coast at around 45 degrees N and a weak center along the southeast coast, and negative anomalies over northwestern Canada and Alaska. The nonlinear response accounts for about one-fourth and one-third as much variance as the linear response of the SAT and precipitation, respectively. A polynomial fit further verifies the nonlinear response of both the SAT and precipitation to be mainly a quadratic response to ENSO. Both the linear and nonlinear response patterns of the SAT and precipitation are basically consistent with the circulation anomalies (the 500-mb geopotential height anomalies), detected separately by nonlinear projection. A cross-validation test shows that including the nonlinear (quadratic) response can potentially contribute to additional forecast skill over North America.