Prominent Precession Band Variance in ENSO Intensity Over the Last 300,000 Years

Three transient National Center for Atmospheric Research Community Climate System Model, version 3 model simulations were analyzed to study the responses of El Nino-Southern Oscillation (ENSO) and the equatorial Pacific annual cycle (AC) to external forcings over the last 300,000 years. The time-varying boundary conditions of insolation, greenhouse gases, and continental ice sheets, accelerated by a factor of 100, were sequentially added in these simulations. The simulated ENSO and AC amplitudes change in phase, and both have pronounced precession band variance (similar to 21,000 years). The precession-modulated slow (orbital time scales) ENSO evolution is dominated linearly by the change of the coupled ocean-atmosphere instability, notably the Ekman upwelling feedback and thermocline feedback. In contrast, the greenhouse gases and ice sheet forcings (similar to 100,000-year cycles) are opposed to each other as they influence ENSO variability through changes in AC amplitude via a common nonlinear frequency entrainment mechanism. The acceleration technique could dampen and delay the precession signals below the surface ocean associated with ENSO intensity. Plain Language Summary El Nino-Southern Oscillation in the equatorial Pacific Ocean is the largest oscillating year-to-year climate variability. We study the evolution of El Nino during the past 300,000 years using climate model simulations. How the slow time-varying changes in insolation, greenhouse gases concentration, and continental ice sheets could influence the behaviors of El Nino are taken into account. Our simulation results suggest that the evolution of the El Nino intensity is dominated by the insolation forcing on the precession time scale (similar to 21,000 years) and can be explained by the strength of coupled ocean-atmosphere feedbacks that control the growth of El Nino. On the other hand, the greenhouse gas and ice sheet forcings (in similar to 100,000-year cycles) tend to compensate each other as they slightly influence El Nifio-Southern Oscillation. The relatively long simulations which involve pronounced glacial-interglacial forcing effects gives us more confidence in understanding El Nino-Southern Oscillation forcing mechanisms, so they may shed light on El Nino dynamics and how El Nino will change in the future.