Abstract
The mechanisms of El Niño–Southern Oscillation (ENSO) variability change during the mid-Holocene (MH) were investigated through analyzing the model simulations from the Paleoclimate Modelling Intercomparison Project Phases (PMIP) phase-2 and phase-3. The majority of PMIP2 and PMIP3 model simulations show a lower level of ENSO activity in the MH simulation compared to the pre-industrial (PI) simulation, which is qualitatively consistent with that inferred from the paleoclimate proxies. Through employing the Bjerknes stability index, we quantified the dynamic and thermodynamic air-sea feedbacks in the PI and MH simulations. The quantitative analyses showed that the reduced ENSO variability in MH arose from the weakening in the thermocline (TH), zonal-advection (ZA) and Ekman (EK) feedback terms. We found that all the weakened TH, ZA, and EK terms are associated with the reduction in the response of anomalous thermocline depth (Dʹ) to the zonal wind stress anomaly ({tau ^{prime}_x}^{{}}) in MH compared to PI. The reduced Dʹ response is attributed to the flattened meridional structure of ENSO-related {tau ^{prime}_x}^{{}} field in MH, which is linked to the enhanced surface poleward mean meridional current in MH. Among many aspects of the mean state changes under the MH orbital forcing, this study identified that the surface mean meridional current change might be a key factor behind the suppressed ENSO variability in MH. Lastly, through comparing our findings with the ENSO future projection studies, we found that the wind-thermocline feedback is susceptible in a changing climate, which implies that minimizing the uncertainty in the wind-thermocline feedback change may help constrain future ENSO response.
Highlights
As the prominent interannual mode in the Earth climate system, the El Niño–Southern Oscillation (ENSO) exerts great influence on the climate and weather across the globe (e.g., Wallace and Gutzler 1981; Rasmusson and Carpenter 1982; Philander 1990; Feng and Li 2011, 2013)
In the present modeling study, we investigated the physical cause of the ENSO intensity change in MH climate compared to present-day climate, through analyzing a suite of simulations from PMIP2 and PMIP3 models
The majority of PMIP2 and PMIP3 models show a decrease in ENSO intensity in MH compared to PI, the degree of the decrease varies across the models
Summary
As the prominent interannual mode in the Earth climate system, the El Niño–Southern Oscillation (ENSO) exerts great influence on the climate and weather across the globe (e.g., Wallace and Gutzler 1981; Rasmusson and Carpenter 1982; Philander 1990; Feng and Li 2011, 2013). Through conducting idealized simulations with an intermediate coupled model, Chiang et al (2009) found that the aforementioned factors (e.g., the tropical mean state changes associated with the strengthened Asian monsoon, and the frequency entrainment’s effect) did not appear to be operating Rather, they suggested that the MH ENSO variability reduction was induced by the suppression of the atmospheric activity in the North Pacific extratropic region. Luan et al (2012) found that the reduced ENSO intensity in MH simulated by a CGCM developed by IPSL arose from the counteractive effect due to the change in the seasonality of the equatorial thermocline depth, and they further linked the altered seasonality of thermocline depth to the enhanced Asian summer monsoon and intensified trade wind These studies have not fully pointed out the key physical processes behind the strengthened equatorial easterly, i.e., how the strengthening in the background mean easterly modulated the interannual variability.
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