Intensity dependence of El Niño and La Niña evolution and mixed‐layer heat‐budget processes

Abstract El Niño and La Niña events exhibit distinct spatial, temporal, and dynamical behaviors that vary with their intensity, yet these variations remain underexplored. Using European Centre for Medium‐Range Weather Forecasts (ECMWF) Reanalysis 5th Generation (ERA5) data (1940–2024), we examine the onset, peak, decay, duration, and spatial evolution of sea‐surface temperature (SST) and precipitation anomalies for weak, moderate, strong, and very strong El Niño and La Niña events. Composite analyses reveal pronounced intensity‐dependent behavior. La Niña events maintain consistent locations of peak SST and rainfall anomalies regardless of intensity, whereas El Niño events exhibit a clear east‐to‐west shift in the location of maximum SST with decreasing intensity. Event duration increases with intensity: stronger events start earlier and persist longer. Strong El Niños last up to two months longer than strong La Niñas, whereas weak La Niñas outlast weak El Niños by about one month. To diagnose the mechanisms behind these intensity‐dependent behaviors, a mixed‐layer heat budget is applied using the Ocean Reanalysis System 5 (ORAS5; 1979–2018). Thermocline and zonal advective terms strengthen, expand eastward, and emerge earlier with increasing intensity for both El Niño and La Niña. In contrast, the Ekman contribution shows opposite intensity dependence for El Niño: it is pronounced in the eastern equatorial Pacific during weak events but weaker and more spatially diffuse during stronger El Niño events, whereas stronger La Niña events exhibit more coherent Ekman cooling. The residual term provides strong, coherent damping during weak El Niño but is weaker and less organized in stronger events. For La Niña, residual positive anomalies scale with SST magnitude, being larger and more coherent for stronger events. These results demonstrate that the intensity of El Niño and La Niña shapes the spatial and temporal evolution of SST and rainfall anomalies strongly and modulates the contributions of dynamical and thermodynamic processes driving event growth and decay.