Revisiting Coupled Instability Theory and the Initiation of ENSO

Abstract:

Understanding and predicting the initiation and subsequent growth of ENSO events has proven exceptionally challenging, particularly from the dynamical perspective. Often, the inherent complexities of the coupled system prove a substantial hurdle in determining why, on a mechanistic level, certain ENSO events are initiated and the associated uncertainty can be difficult to quantify. To isolate the initiation process, we present a state-of-the-art coupled climate model framework to isolate the unstable growth of ENSO events via dynamically coupled instabilities. Earlier studies show that sufficiently strong air-sea coupling can destabilize ocean waves and initiate ENSO events. The presented model framework allows for coupled instabilities to grow under a fully coupled climate model configuration, which modernizes earlier approaches implemented with highly simplified models. An ensemble of NCAR-CCSM4 experiments is integrated for various initialization months and results show that the experimental design successfully captures the growth of ENSO events from small, random perturbations that are not prescribed. Additionally, these particular events occur in the absence of a previous ENSO event, without a subsurface heat content precursor in the western Pacific, and without wind stress “trigger patterns” characterized by large deterministic components, all of which tend to bias the coupled state towards a particular ENSO phase. The occurrence and type of event may be sensitive to the initialized state and we demonstrate that the ENSO growth rate displays strong seasonal characteristics as well as dependence on the initialization month. We also quantify the skill saturation and show that there exists a well-defined seasonal limit to growth.