A fresh perspective on ENSO nonlinearity: the ENSO pattern continuum metric

Abstract The El Niño-Southern Oscillation (ENSO) is characterized by a zonal (longitudinal) displacement of the background Walker circulation across the equatorial Pacific, or spatial shifting. The warm pool edge position (WPEP) commonly measures the intensity of this shifting. Hereby organizing sea surface temperature (SST) maps according to WPEP quantiles, we construct a synthetic, continuous sequence of patterns—termed the pattern continuum—that captures the SST evolution along the movement, namely the gradual transition from broad Central Pacific La Niña cooling to Central then Eastern Pacific El Niño warming. This pattern continuum can also be approximated by a “shifted-mean” framework, wherein a fixed spatial structure shifts zonally. The present diagnosis provides insight into key dynamical features of the ENSO, such as its spatial diversity, asymmetry, and nonlinearity, including the quadratic relationship between SST principal components. It may also be useful for practical model evaluation and intercomparison.