The Salience of Nonlinearities in the Boreal Winter Response to ENSO

A series of simulations using the NASA Goddard Earth Observing System Chemistry-Climate Model are analyzed in order to assess the prominence of nonlinearities in the response to the El Nino - Southern Oscillation (ENSO). In the Central North Pacific region where the sea level pressure response to ENSO peaks, nonlinearities are relatively muted. In contrast,
changes to the east of this region (i.e. the far-Northeastern Pacific) and to the north of this region (over Alaska) in response to different ENSO phases are more clearly nonlinear, and become statistically robust after more than 15 events are considered. The relative prominence of these nonlinearities is related to the zonal wavenumber of the tropical precipitation response. Associated with these nonlinearities over the far-Northeastern Pacific are nonlinearities in precipitation over Western United States and surface temperature
over Northwest North America and Midwestern United States. In all regions at least 15 events of each type are necessary before nonlinearities can be identified as statistically significant at the 95% confidence level due to the presence of internal atmospheric variability. As there have only been a similar number of ENSO events to the total needed for significance since 1920, it is not surprising that it has been difficult to establish statistically significant nonlinearities using observational data