ENSO-Induced Variability in Vertical Mixing in the Central Equatorial Pacific

Abstract

The El Niño Southern Oscillation (ENSO) sets an important global atmospheric state for climate and weather, impacting agriculture, population health, and other sectors. Evaluating the factors that influence El Niño and La Niña conditions is invaluable for predicting them. Vertical mixing is one important positive feedback in sea surface temperature (SST), responsible for the oscillations of ENSO. During an El Niño, as trade wind velocity decreases, Equatorial Undercurrent (EUC) velocity decreases, reducing subsurface shear and mixing, which increases SST, reducing wind velocity further. Mixing can be parametrized with such metrics as Richardson numbers 𝑅𝑖, Thorpe Scales 𝐿𝑇, and turbulent dissipation rates . Data to evaluate these metrics were obtained with Acoustic Doppler Current Profilers (ADCP) and Conductivity, Temperature, and Depth (CTD) profiles from warming La Niña and peak warming El Niño cruises without Tropical Instability Wave (TIW) presence, demonstrating the ‘background state’ of mixing. The integrated probability density of normalized 𝑅𝑖 distributions associated with high mixing potentials from EUC-induced shear was 0.33 for La Niña 2023 and 0.17 for El Niño 2024. These results demonstrate increased mixing potential during La Niña. There is also the possibility of fast density-inversion smoothing (no large 𝐿𝑇) both years. These ‘background state’ results were qualitatively compared with three studies utilizing TAO data, ARGO float data, and a numerical model with general agreement. Various approaches to the ENSO equatorial vertical mixing problem yielded consistent results across different time and distance scales, important to understanding the broader impacts of ENSO on mixing and climate.