Equatorial Origin of the Observed Tropical Pacific Quasi‐Decadal Variability From ENSO Nonlinearity.

Quasi‐decadal (QD) climate variability is detected in the tropical Pacific based on the recent 70 years of observations. This QD variability is identified in equatorial sea surface temperatures (SSTs), the pattern of which resembles the Central Pacific (CP) El Niño‐Southern Oscillation (ENSO) but extends further meridionally to the northeastern subtropical Pacific. Whereas equatorial upper‐ocean heat content and SSTs are in quadrature for ENSO, these two quantities are almost in phase on the QD timescale. Further analysis shows that nonlinear dynamical heating, primarily originating from strong El Niño events, tends to lead QD SSTs by a quarter of its dominant period (approximately 30 months) and shapes the dominant QD periodicity in observations. Our results suggest that the observed QD variability largely originates from ENSO nonlinearity and thus is fundamentally different from ENSO's oscillatory nature. Plain Language Summary: The climate variability of the tropical Pacific atmosphere‐ocean system profoundly influences the global weather and climate patterns on a wide range of temporal scales. The dynamical understanding of interannual climate variability dominated by the well‐known El Niño‐Southern Oscillation (ENSO) phenomena has established a promising way for the operational seasonal to interannual climate prediction. So far, nevertheless, there is still a lack of consensus for the understanding of tropical Pacific decadal climate variability despite the increasing desire for climatic information on longer timescales. In this article, we illustrate the evolution features and possible governing mechanisms of a prominent quasi‐decadal (QD) variability in the tropical Pacific with the utilization of an advanced spectrum approach and reliable observational data sets. The results show that this QD variability that makes up a substantial portion of tropical Pacific low‐frequency variance is largely equatorial originated from the ENSO nonlinearity, which is dynamically different from the oscillatory nature of the ENSO phenomenon. Our study has important implications for the decadal climate prediction and improving the representations of tropical Pacific decadal variability in climate models. Key Points: Observations show that CP‐ENSO‐like quasi‐decadal variability originates in the equatorial Pacific and extends to the off‐equatorial regionThe equatorial ocean heat content and sea surface temperature are in phase on quasi‐decadal timescales, whereas they are in quadrature for interannual ENSOEquatorial nonlinear dynamical heating is proposed as the dynamical origin of the quasi‐decadal variability [ABSTRACT FROM AUTHOR]