A Nonlinear Theory for El Niño Bursting

Abstract

A new mechanism is proposed that explains two key features of the observed El Niño-Southern Oscillation (ENSO) phenomenon-its irregularity and decadal amplitude changes. Using a low-order ENSO model, the authors show that the nonlinearities in the tropical heat budget can lead to bursting behavior characterized by decadal occurrences of strong El Niño events. La Niña events are not affected, a feature that is also seen in ENSO observations. One key result of this analysis is that decadal variability in the Tropics can be generated without invoking extratropical processes or stochastic forcing. The El Niño bursting behavior simulated by the low-order ENSO model can be understood in terms of the concept of homoclinic and heteroclinic connections. It is shown that this new model for ENSO amplitude modulations and irregularity, although difficult to prove, might explain some features of ENSO dynamics seen in more complex climate models and the observations.

Strong and moderate nonlinear El Niño regimes

“…Recent efforts have been dedicated to assess whether or not moderate EN events were driven by the recharge-discharge dynamics (Ren and Jin 2013;Mosquera-Vásquez et al 2013) or if nonlinear processes could explain why central Pacific EN are confined to the warm pool region (Xiang et al 2013;Mosquera-Vásquez et al 2014). Our results rather calls for extending previous studies Jin et al 2003) to explain the bursting behavior of ENSO, which shall convey insights on the conditions favoring a regime over the other. Such models would allow the exploration of the influence of mean state changes onto the regime selection mechanism and statistics, extending previous works dedicated to the investigation of the change in stability of ENSO due to mean state changes (Fedorov and Philander 2001;An and Jin 2001;Thual et al 2013).…”

“…In particular, the details of Kelvin wave propagation and the reflection of Rossby waves from the eastern boundary are apparently highly influential. Thus, although it has been previously parametrized in simple models of the recharge-discharge family (Jin et al 2003;Timmermann et al 2003;An and Jin 2004), we believe that the neglect of wave dynamics could qualitatively affect the results (e.g. Su et al 2010).…”

“…Next we explore the possible roles that nonlinearities in ocean advection (e.g. Jin et al 2003) and the atmospheric convective response to SST play in the origin of these regimes.…”

“…To understand the oceanic processes that control the SST changes in the two EN regimes, we analyze the role of temperature advection in the heat budget of a 50 m-deep surface ocean layer (Jin et al 2003;An and Jin 2004). The anomalous budget is formulated as:…”

“…respectively, while the "non-linear dynamical heating" (NDH; Jin et al 2003) is defined as: The bars and the primes indicate climatological values and anomalies, respectively. We should note that, in contrast to Jin et al (2003) and An and Jin (2004), our calculation of NDH explictly removes the seasonality of the nonlinear advection (last three terms in Eq.…”