Abstract
The drivers of El Niño–Southern Oscillation (ENSO) intensity change during the mid-Holocene (MH) are investigated through employing a coupled model that exhibits excellent performance in simulating the present-day ENSO behaviors. The model shows a 28% decrease in ENSO intensity in the MH simulation compared to the pre-industrial (PI) simulation, showing an agreement with the ranges indicated by the paleo-proxies. Based on quantitative analyses, we reveal that the changes in the oceanic dynamic processes (including the thermocline, zonal-advection, and Ekman feedback terms, in the order from being most to least important) were the major drivers of the reduced ENSO intensity in the MH. Further diagnosis analyses show that the weakening in all three oceanic dynamic terms could be traced back to the weakened thermocline response to zonal wind stress anomaly in the MH compared to that in the PI. Such weakened thermocline response was due to the relatively flattened meridional structure of ENSO-related interannual anomaly field (e.g., zonal wind stress anomaly field) in the MH, which arose from the strengthening of the mean meridional overturning circulation, namely, the mean Pacific subtropical cell (STC). The process-oriented analyses throughout this study suggest a critical linkage between ENSO intensity change and mean STC change in the MH, through documenting how the mean STC change altered the oceanic dynamic processes and thus drove the ENSO intensity change.
Highlights
The El Niño–Southern Oscillation (ENSO) is one of the most marked interannual variabilities in the climate system (Philander 1990)
A coupled general circulation models (CGCMs) of FGOALS-g2 that exhibits excellent skill in simulating the present-day ENSO was used to investigate the drivers of the reduced ENSO variability in the MH compared to that in the PI
Through quantifying ENSO-related dynamic and thermodynamic coupling processes in both PI and MH simulations, we demonstrated that the reduced ENSO variance during the MH arose from the weakening in the oceanic dynamic terms, while the thermodynamic term change had a negative contribution to the reduced ENSO intensity
Summary
The El Niño–Southern Oscillation (ENSO) is one of the most marked interannual variabilities in the climate system (Philander 1990). In addition to the paleoclimate records, plenty of studies (e.g., Clement et al 2000; Liu et al 2000; Kitoh and Murakami 2002; Otto-Bliesner et al 2003; Brown et al 2008; Zheng et al 2008; Chiang et al 2009; Braconnot et al 2012; Luan et al 2012, 2015; Tian and Jiang 2013; An and Choi 2014; Roberts et al 2014; Emile-Geay et al 2016; Tian et al 2017, 2018) turned to coupled ocean–atmosphere models for help They found a weakening in ENSO intensity emerged from a wide range of model simulations, the extent of the reduction in ENSO intensity varied across these modeling results.
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