El Niño pattern diversity and interactions with mean state trends

Recent research on the El Niño–Southern Oscillation (ENSO) phenomenon increasingly reveals the highly complex and diverse nature of ENSO variability. A method of quantifying ENSO spatial pattern uniqueness and diversity is presented and enables the field to move toward 1. formally distinguishing between unique and “canonical” El Niño events, 2. testing whether historical model simulations aptly capture ENSO diversity by comparing with instrumental observations, 3. predicting future ENSO diversity using future model simulations, 4. understanding the relationship between the tropical Pacific background state and ENSO diversity, and eventually 5. analyzing the associated diversity of ENSO–related atmospheric teleconnection patterns. We present a framework for measuring El Niño spatial SST pattern uniqueness and diversity for a given set of El Niño events using two indices, the El Niño Pattern Uniqueness (EPU) index and El Niño Pattern Diversity (EPD) index, respectively. By applying this framework to instrumental records, we independently confirm a recent regime shift in El Niño pattern diversity with an increase in unique El Niño event sea surface temperature patterns. We support recent work that asserts a link between the background cooling of the eastern tropical Pacific and changes in ENSO diversity towards Central Pacific (CP) type events. Using an atmospheric general circulation model (AGCM) we demonstrate a nonlinear weakening of Bjerknes feedback mechanisms during El Niño events when a strong CTM is present in the mean state. Other studies have shown a potential causal link between anthropogenic global warming and the relatively recent emergence of the CTM, which together imply that a strong CTM may continue to increase CP El Niño frequency. Finally, we investigate the effect of interbasin multi-decadal variability on modulating El Niño pattern diversity, testing the hypothesis that the climate oscillations like the Indian Ocean Dipole (IOD) and Pacific Decadal Oscillation (PDO) impact El Niño pattern diversity through mean state interaction.