Land Components Control the ENSO Representation in the Earth System Model MIROC-ES2L

Terrestrial vegetation affects the atmosphere through both biogeochemical and physical processes. Radiative forcing is affected by carbon uptake and release, while albedo, evapotranspiration, and surface roughness also depend on vegetation. In Earth system models (ESMs), vegetation growth is often represented by prognostic simulations of the leaf area index (LAI). MIROC-ES2L, an ESM version of the MIROC climate model, simulates a larger El Niño-Southern Oscillation (ENSO) amplitude compared to simulations that prescribe observed LAI.

To investigate the cause of this enhanced ENSO amplitude in MIROC-ES2L, we compared the feedback processes contributing to El Niño growth in two experiments: one with observed LAI and the other with model-prognosed LAI. The prognosed LAI experiment showed stronger zonal advection, Ekman, and meridional advection feedbacks, associated with warmer sea surface temperatures (SSTs) in the eastern equatorial Pacific.

Sensitivity experiments were conducted to identify the regions where LAI contributes most significantly to SST changes. These experiments constrained LAI to observed values in specific regions, while using model-prognosed values elsewhere. The results show that the ENSO amplitude is particularly sensitive to LAI over South America, where the model overestimates LAI along the west coast.

We conclude that the underlying mechanism is as follows: increased LAI over South America induces surface cooling due to enhanced latent heat release. This modifies the tropospheric circulation, weakening the local Walker circulation over the Andes and consistently altering the SST distributions. The resulting SST warming off the coast of Peru enhances the convective response to SST anomalies, strengthening the effective Bjerknes feedback and amplifying ENSO.