Effects of equatorial ocean currents bias on El Niño pattern in CMIP6 models

Abstract This study utilized the Coupled Model Intercomparison Project Phase 6 (CMIP6) models to examine the simulations of equatorial ocean currents and explore their substantial influences on the systematic bias of westward–extended sea surface temperature anomalies (SSTA) pattern during El Niño. The results show that models simulate an excessive westerly ocean current over the equatorial central Pacific in the climatology field. It tends to suppress the equatorial eastward ocean current anomalies with their maximum centering over the equatorial western Pacific in the El Niño developing phase. As a consequence, there exists an overestimated zonal advective feedback toward the maritime continent, subsequently inducing the biased westward extension of SSTA pattern. Our analyses illustrate that the mean–state performance of equatorial ocean currents control to a large degree the climatic interannual variability related to El Niño events in CMIP6 models. Plain Language Summary El Niño is the warm phase of El Niño–Southern Oscillation, knowns as a coupled air–sea phenomenon with a considerable interannual variability in the tropics. In the recent decades, many climate models have been developed to examine the capability of modeling El Niño to help understand the potential dynamics effects. This study emphasizes the role of dynamical process related to ocean currents in affecting the behavior of El Niño pattern in a series of latest released CMIP6 climate models. We find that models have a relatively large bias in simulating the equatorial ocean currents in Pacific. With an extremely strong equatorial zonal current in mean–state field, the maximum of zonal current anomalies tends to shift toward the equatorial western Pacific and enhances the corresponding oceanic feedback mechanism, which substantially contributes to the overestimated sea surface temperature anomalies during El Niño events. 1. Introduction El Niño–Southern Oscillation (ENSO) phenomenon is a salient interannual variabilities of tropical atmosphere–ocean interaction. Its warm and cold phase, respectively known as El Niño and La Niña, has great influences on global climate system (Rasmusson & Carpenter, 1982; Hoerling et al., 1997; Larkin & Harrison, 2005; Yang et al., 2018). With the development of climate models, skills of the coupled general circulation models (CGCMs) from the Coupled Model Intercomparison Project (CMIP) have been examined to better understand the dynamics of ENSO diversity (e.g., Zhang et al., 2013; Grose et al., 2014; Taschetto, et al., 2014; Li et al., 2018; Freund et al., 2020; McKenna et al., 2020). However, a westward extension of ENSO sea surface temperature (SST) anomalies still commonly exists in the CGCMs (Wittenberg et al., 2006; Taschetto et al., 2014; Li et al., 2018; Jiang et al., 2021). This spatial systematic error exerts profound impacts on the modeled behaviors in climate characteristics. For instance, accompanied by the ENSO SST anomalies (SSTA) pattern expanding much toward the warm pool, the surface westerly wind anomalies, as well as the positive precipitation anomalies, shifts extremely west over the equatorial western Pacific (EWP) (Kug et al., 2012; Ham & Kug, 2014; Wang et al., 2021). The biased eastward upwelling and downwelling Kelvin waves can also be induced by the westward migration of wind stress anomalies, and favors an unrealistic ENSO evolution (Taschetto et al., 2014). On top of that, models tend to simulate a weaker ENSO asymmetry because the warm subsurface temperature anomalies are underestimated in the equatorial eastern Pacific (EEP), which is mainly associated with the maximum warm SSTA shifting too far to the west (Zhang & Sun, 2014). In the recent decades, numbers of studies have been committed to investigating the sources of the discrepancies related to ENSO characteristics in climate models. It was demonstrated that ENSO behaviors are greatly rely on the simulated climatological state (Kug et al., 2012; Sun et al., 2013; Bellenger et al., 2014; Ham & Kug, 2014; Kim et al., 2014; Zhang & Sun, 2014; Li et al., 2018; Li et al., 2019). In particular, the excessive Pacific cold tongue is one of the prevalent mean–state biases in CGCMs (Li & Xie, 2012, 2014; Zhang & Sun, 2014; Li et al., 2015, 2016). This relatively cold SST is linked with the too westward Walker circulation, and hinders the simulation of atmospheric processes for modifying ENSO. For example, the amplifying wind–SST feedback and damping heat flux–SST feedback, both dependent on the location of Walker circulation ascending zone, are underestimated in a variety of CGCMs (Lloyd et al., 2009; Bellenger et al., 2014; Kim et al., 2014). These two feedback biases may cause unreasonable ENSO phase locking and weak ENSO asymmetry; in addition, the corresponding westward migration of convective response is the main factor that contributes to the westward extension of ENSO SSTA pattern (Bayr et al., 2018, 2019). However, limited attention has been made in the relations between the performance of modeled equatorial ocean currents and ENSO SSTA, even though the oceanic factors also play a role in controlling the ENSO diversity. The velocities of equatorial zonal currents (e.g., North Equatorial Countercurrent, South Equatorial Current and Equatorial Undercurrent) reflect largely on the location of maximum ENSO SSTA (Johnson et al., 2002; Wang & Wu, 2013). The direction of equatorial upper layer currents can be a predictor of the extreme El Niño events as the climatological zonal currents all change to eastward flows in April–May (Kim & Cai, 2014). For dynamical mechanisms, the thermocline feedback (TH) and zonal advective feedback (ZA) and are act as the two dominant feedbacks in the recharge/discharge oscillator that conduce to the growth and phase transition of ENSO (Jin & An, 1999; Zhang et al., 2007; Ren & Jin; 2013). In this study, we used the 14 CMIP phase 6 (CMIP6) models to investigate the behaviors of simulated equatorial ocean currents and the effects of their oceanic feedbacks on El Niño SSTA. Robust biases in the zonal currents over the equatorial Pacific basin are responsible for the misestimated ZA, which to some extent reduce the ability for simulating the El Niño structure. Our intention is to provide an aspect that good simulations in surface ocean currents cannot be discounted for modeling a realistic ENSO.