Role of ocean dynamics in equatorial Pacific decadal variability

The tropical Pacific exhibits decadal El Nino-Southern Oscillation (ENSO)-like variability, characterized by meridionally broad sea surface temperature anomalies in the eastern Pacific. In this study, we focus on the variability in the equatorial Pacific band (5 degrees S-5 degrees N), termed equatorial Pacific decadal variability (EPDV). While it is known that ocean dynamics plays an essential role in EPDV, the simulations by air-sea thermodynamically coupled slab ocean models (SOM) obscure the nature of the role of ocean dynamics. To confront this issue, we use a mechanically decoupled simulation, which isolates the effects of thermodynamic coupling processes and mean ocean circulation on EPDV. Thus, by comparing the simulation to a SOM, we investigate the role of mean ocean circulation and show that it plays a role in damping EPDV, primarily through mean equatorial Pacific upwelling. By comparing the simulation to a fully coupled dynamic ocean model (DOM), we examine the role of anomalous wind-driven ocean circulation and demonstrate that it plays a role in amplifying EPDV. Further, this amplification strength overwhelms the upwelling damping effect, resulting in the anomalous wind-driven ocean circulation forcing EPDV. Finally, we examine the origin of EPDV in the DOM and show that it originates from a zonal dipole mode in the tropical Pacific, which is strongly associated with decadal modulation of ENSO amplitude. Taking EPDV as an example, our study advances the understanding of the two distinct dynamical systems (SOM and DOM), benefiting the physical interpretation of other climate variabilities.