Nonlinear Controls on the Persistence of La Niña*

Abstract

A large fraction (35%-50%) of observed La Niña events last two years or longer, in contrast to the great majority of El Niño events, which last one year. Here, the authors explore the nonlinear processes responsible for the multiyear persistence of La Niña in the Community Climate System Model, version 4 (CCSM4), a coupled climate model that simulates the asymmetric duration of La Niña and El Niño events realistically. The authors develop a nonlinear delayed-oscillator (NDO) model of the El Niño-Southern Oscillation (ENSO) to explore the mechanisms governing the duration of La Niña. The NDO includes nonlinear and seasonally dependent feedbacks derived from the CCSM4 heat budget, which allow it to simulate key ENSO features in quantitative agreement with CCSM4.Sensitivity experiments with the NDO show that the nonlinearity in the delayed thermocline feedback is the sole process controlling the duration of La Niña events. The authors' results show that, as La Niña events become stronger, the delayed thermocline response does not increase proportionally. This nonlinearity arises from two processes: 1) the response of winds to sea surface temperature anomalies and 2) the ability of thermocline depth anomalies to influence temperatures at the base of the mixed layer. Thus, strong La Niña events require that the thermocline remains deeper for longer than 1 yr for sea surface temperatures to return to neutral. Ocean reanalysis data show evidence for this thermocline nonlinearity, suggesting that this process could be at work in nature.

Robust contribution of decadal anomalies to the frequency of central-Pacific El Niño

“…Different hypotheses have been proposed to explain the decadal/multi-decadal anomalies in the tropical Pacific202324252634353637383940414243. It has been argued that the El Niño-La Niña asymmetry may contribute to the decadal background change in the tropical Pacific25353637.…”

“…While the strong positive skewness in the eastern Pacific implies that warm decadal anomalies there may be partly ascribed to the EP El Niño itself, the weak negative skewness in the central Pacific indicates that decadal anomalies there are less influenced by the El Niño-La Niña asymmetry. It has also been argued that multi-year persisting La Niña anomalies induced by nonlinear processes and the rectification of El Niño-Southern Oscillation (ENSO) events may contribute to the Pacific decadal/multi-decadal anomalies3839. Previous studies have also suggested the important roles of internal ocean-atmosphere processes in generating the Pacific ENSO-like decadal/multi-decadal variations23244041.…”

The role of the southward wind shift in both, the seasonal synchronization and duration of ENSO events

“…It is generally accepted that there is an asymmetry in the duration of the two phases of ENSO events, with La Niña events lasting longer than El Niño events (Larkin and Harrison 2002;McPhaden and Zhang 2009;Obha and Ueda 2009;Okumura and Deser 2010;Okumura et al 2011;DiNezio and Deser 2014). Given that McGregor et al (2013) proposed that the asymmetries in the southward wind shift (e.g., El Niño event magnitude is strongly related to the extent of the meridional wind movement, while the meridional wind movement during La Niña events remains relatively small The boxplots in Fig.…”

The Annual-Cycle Modulation of Meridional Asymmetry in ENSO’s Atmospheric Response and Its Dependence on ENSO Zonal Structure

“…Many La Niña events persist through their second year and sometimes reemerge in the subsequent winter (e.g., Ohba and Ueda 2009;Ohba et al 2010;DiNezio and Deser 2014;Hu et al 2014). For example, the 1998/99 La Niña event starts at about July 1998 and lasts for nearly three consequent years.…”