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

Abstract

Three transient National Center for Atmospheric Research Community Climate System Model, version 3 model simulations were analyzed to study the responses of El Niño–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 (~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 (~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.

Modulation of late Pleistocene ENSO strength by the tropical Pacific thermocline

“…In the case of a warmer / deeper thermocline, the result is a dampening of the dynamical response of the EEP, and the feedbacks from these processes further deepen the thermocline and reduce upwelling, weakening their effect on ENSO growth. The primacy of thermocline and upwelling feedbacks in determining ENSO strength over long time periods has been found in simulations of ENSO change over the past 21ky 6 , and over the past 300ky 7 , but long-term reconstructions of ENSO have been largely limited to the Holocene and LGM (e.g., 9, 10, 13, 16, 28), with only limited data at few discrete time points from the last 130ky 15 , 18 . Our findings demonstrate that these processes are the best predictor of El Niño amplitude across widely varying climatic boundary conditions.…”

“…However, precessional forcing of extra-tropical Southern Hemisphere equatorial Pacific source waters has been linked to upwelling variations at site 1240 in the EEP cold tongue 43 . Southern Hemisphere insolation forcing is also reflected in the strength of the upwelling and associated feedbacks in the EEP with subsequent impacts on ENSO strength in model simulations of the past 300,000 years 7 . Thus, both model results and palaeoceanographic data suggest that the tropical thermocline state is at least in part dependent on insolation signals transmitted to the EEP from the Southern Hemisphere via the source waters on millennial and glacial-interglacial time scales.…”

“…Multi-model intercomparison efforts have displayed widely divergent ENSO responses to future climate conditions 2 , 3 , largely due to differences in model expression of the feedback mechanisms that promote the growth and decay of the SST anomalies associated with ENSO 4 . The warmth and depth of the tropical thermocline influences the strength of many of these feedbacks by altering the surface temperature and vertical contrast of the eastern tropical Pacific where key ocean-atmosphere interactions occur 4 – 7 . These thermocline processes are dominant positive feedbacks in ENSO models 3 , 7 , 8 , and thermocline conditions have been linked to ENSO activity in the Holocene 9 and the Last Glacial Maximum (LGM) 10 .…”

“…The warmth and depth of the tropical thermocline influences the strength of many of these feedbacks by altering the surface temperature and vertical contrast of the eastern tropical Pacific where key ocean-atmosphere interactions occur 4 – 7 . These thermocline processes are dominant positive feedbacks in ENSO models 3 , 7 , 8 , and thermocline conditions have been linked to ENSO activity in the Holocene 9 and the Last Glacial Maximum (LGM) 10 . ENSO’s relationship with the background climate state, and therefore how it will change in the future, remains a central challenge in climate science.…”

“…For example, higher insolation during the growth phase of El Niño events in boreal late summer/early fall leads to unequal zonal warming that alters the tropical Pacific wind fields 19 , 20 , weakens the Walker circulation 21 – 23 , and weakens the upwelling feedback by warming and/or deepening the tropical Pacific thermocline 4 – 7 . The tropical thermocline influences the SST anomalies in the eastern equatorial Pacific (EEP), altering zonal E-W SST and atmospheric pressure gradients 5 , 7 , 24 , which in turn affect the wind fields that drive upwelling, setting up the ocean and atmospheric coupling important for ENSO 4 . These dynamical relationships may be altered during glacial states when lower sea level increases tropical land area and cooler SSTs reduce air-sea feedbacks 21 , 25 .…”