[A-CG45] Multi-scale ocean-atmosphere interaction in the tropical Indo-Pacific region

[ACG45-12] Origin of Equatorial Pacific Decadal Variability: Thermodynamic or Dynamic Coupling?

*Yu Zhang^1,2, Shang-Ping Xie³, Dillon J. Amaya^4,5, Qihua Peng⁶, Yu Kosaka⁷, Xiaopei Lin^1,2, Jun-Chao Yang^1,2, Sarah M. Larson⁸, Arthur J. Miller³ (1.Physical Oceanography Laboratory, Ocean University of China, 2.Qingdao National Laboratory for Marine Science and Technology, 3.Scripps Institution of Oceanography, University of California San Diego, 4.Cooperative Institute for Research in Environmental Sciences, University of Colorado Boulder, 5.Department of Atmospheric and Oceanic Sciences, University of Colorado Boulder, 6.State Key Laboratory of Tropical Oceanography, South China Sea Institute of Oceanology, Chinese Academy of Sciences, 7.Research Center for Advanced Science and Technology, The University of Tokyo, 8.Department of Marine, Earth, and Atmospheric Sciences, North Carolina State University)

Keywords:equatorial Pacific decadal variability, El Niño-Southern Oscillation, air-sea dynamic coupling, air-sea thermodynamic coupling, equatorial upwelling

In the equatorial Pacific, the El Niño-Southern Oscillation (ENSO) is the dominant mode of ocean-atmosphere coupled variability on interannual timescales. Additionally, the equatorial Pacific also exhibits significant decadal variability (EPDV hereafter). While it is well known that ENSO originates from air-sea coupling and wind-driven ocean dynamics, the origins of EPDV are still under debate. Several studies suggest that EPDV is similar to ENSO in that it originates from ocean-atmosphere dynamical coupling but is associated with lower frequency ocean dynamics. Other studies based on slab ocean model simulations suggest that EPDV is primarily driven by air-sea thermodynamic coupling independent of ocean dynamics. Here we test these hypotheses and demonstrate that EPDV originates from the dynamical coupling. We conduct a novel coupled model experiment, named Clim-τ, in which wind stress to the ocean is fixed to its climatology over the tropical Pacific. The simulation is thus free of anomalous wind-driven ocean dynamics but includes air-sea thermodynamic coupling and the effect of mean ocean circulation on ocean temperature anomalies, the latter of which is missing in slab ocean models. The Clim-τ experiment fills the gap between slab ocean models and fully coupled dynamical ocean models, allowing investigation of the origin of EPDV step by step. We find that the amplitude of EPDV is significantly reduced in Clim-τ compared to slab ocean models due to damping by the central-eastern equatorial climatological upwelling. In fully coupled models, EPDV re-emerges, suggesting an amplification through interactive ocean dynamics driven by anomalous wind stress. Additionally, anomalous surface net heat flux plays an important role in damping EPDV. With dynamical damping due to mean upwelling, EPDV is only attributed to air-sea dynamical coupling along with interactive ocean dynamics. We also find that the ocean dynamics for EPDV is distinct from ENSO in the eastern equatorial Pacific, the former of which shows much weaker zonal advection feedback and thermocline feedback. This difference implies timescale-dependent ocean dynamics in the eastern equatorial Pacific, which partly depends on the frequency of zonal wind stress forcing over the central equatorial Pacific as simulated in a shallow water model. Our study demonstrates that although results from slab ocean models are comparable to the fully coupled models, they might be misleading in the strong climatological upwelling regions where upper ocean temperature will be further influenced by dynamical damping. Our results may contribute to improving the long-term predictability of the equatorial Pacific coupled system.