[A-CG33] Multi-scale ocean-atmosphere interaction in the tropics

[ACG33-09] More frequent hidden El Niño in a warming climate

*Lianyi Zhang¹, Yan Du¹, Wenju Cai^2,3, Yuhong Zhang¹, Jin-Yi Yu⁴ (1.South China Sea Institute of Oceanology, Chinese Academy of Sciences, 2.Pilot National Laboratory for Marine Science and Technology (Qingdao), Qingdao, China. , 3.Centre for Southern Hemisphere Oceans Research (CSHOR), CSIRO Oceans and Atmosphere, Hobart, TAS, Australia., 4.Department of Earth System Science, University of California, Irvine, CA, USA)

Keywords:El Niño, Thermocline Dynamics, Climate Warming

El Niño-Southern Oscillation (ENSO) features strong diversity in anomaly patterns causing different influences over the globe. The diversity manifests as two basic sea surface temperature (SST) modes: the Eastern Pacific (EP) pattern, which is dynamically dominated by the equatorial thermocline dynamics (T-dynamics), known as the Thermocline-SST-Wind feedback; and the Central-Pacific (CP) pattern, which evolves from the northeastern Pacific via mixed-layer dynamics (M-dynamics), associated with the Wind-Evaporative-SST feedback. Both are measured by a general Niño3.4 index (mean SST anomalies over 170°-120°W, 5°S-5°N). However, during the ENSO peak season in 1990/91, 1977/78, and 2019/20, Niño3.4 exhibited weak amplitude, like "El Niño hiding itself", yet the associated SST pattern generated El Niño-like impacts. The dynamics of such "hidden" El Niño and its potential responses to a warming climate are unknown. Here, we show that hidden El Niño results from an out-of-phase combination of T-dynamics and M-dynamics, and find an increased frequency by more than 50% in hidden El Niño occurrences under both emission scenarios of Shared Socioeconomic Pathways 3-7.0 and 5-8.5, supported by a strong inter-experiment consensus (80%) in climate model large ensembles and an inter-model agreement (71%) in Phase 6 of the Coupled Model Intercomparison Project, respectively. The projected warming of the EP SST surpasses that of other Pacific regions, lowering SST skewness and boosting a thermocline-SST coupling. Consequently, T-dynamics cooling events almost double, which, in conjunction with frequent M-dynamics warming events, contribute to an increase of hidden El Niño occurrences. Our findings reveal the potential impacts of the seemingly weak climate events and project an increased frequency of the associated climate extremes under greenhouse warming.