Westerly Wind Bursts, ENSO Asymmetry, and Tropical Pacific Decadal Variability

Abstract:

Over the past half century, epochs of enhanced and suppressed ENSO activity have coincided with changes in mean trade wind strength (tropical Pacific mean state) and westerly wind burst variability. Previous research has considered this westerly wind burst variability as state-dependent noise forcing for ENSO. In agreement with the observations, we find that in the current generation of coupled climate models, the westerly wind burst variability is highly correlated with decadal and multi-decadal scale Pacific mean state variability as well as ENSO variability. However, precisely how concurrent changes to ENSO, the tropical Pacific mean state, and westerly wind burst variability is still an open question. Our previous research has shown that there is a relationship between the magnitude of the state-dependence of the noise forcing and ENSO amplitude asymmetry (i.e. that the El Nino events have a larger temperature anomaly than the La Nina events) . In turn, ENSO amplitude asymmetry can produce a signal on the tropical Pacific mean state on multi-decadal timescales. Here, we develop a systematic method to investigate the hypothesis that the inherent randomness of the state-dependent noise forcing of ENSO can explain the coincident changes in ENSO variability and the tropical Pacific mean state, using a hierarchy of coupled models, from conceptual to intermediate to coupled GCMs.