Modulation of the thermocline structure on asymmetry of ENSO amplitude in different regions of the equatorial Pacific

Based on the theoretical framework of subsurface temperature parameterization and the Zebiak-Cane (ZC) model, this study uses monthly reanalysis of Phase 3.3.1 of the Simple Ocean Data Assimilation provided by the University of Maryland and monthly sea surface temperature dataset from the Hadley Center over the period of 1980—2015 to investigate the role of subsurface temperature (T_sub) and its key controlling factors in the ENSO amplitude asymmetry with a focus on the contrasting mechanisms across the eastern and central-western equatorial Pacific. It is found that the asymmetry extends from the surface to the subsurface and exhibits a dipole structure, characterized by "positive in the east and negative in the west". This dipole primarily arises from asymmetric subsurface responses to zonal wind stress (τ), but the dominant mechanisms differ between the eastern and western-central equatorial Pacific. In the eastern equatorial Pacific, the positive asymmetry results from combined changes in thermocline depth (h) and thermocline sharpness (h^*). The stronger positive ${\tau }^{\prime }$ during the warm phase produces larger positive $h^{\prime }$ and negative ${h^{\ast }}^{\prime }$, leading to enhanced positive $T_{\mathrm{s}\mathrm{u}\mathrm{b}}^{\prime }$. In the central-western Pacific, the negative asymmetry is mainly controlled by h^*. The weaker negative ${\tau }^{\prime }$ during the cold phase induces stronger positive ${h^{\ast }}^{\prime }$, which generates stronger negative $T_{\mathrm{s}\mathrm{u}\mathrm{b}}^{\prime }$. However, $h^{\prime }$ contributes little in the western Pacific because the mean thermocline is much deeper than the mixed-layer depth. Overall, the asymmetric response of h^* to τ is the primary driver of the negative ENSO amplitude asymmetry in the central-western equatorial Pacific. By replacing the T_sub parameterization with a new scheme based on the h^* first-order approximation, the simulation of ENSO amplitude asymmetry in the ZC model has been significantly improved.