Variability of the South Pacific Eastern Subtropical Mode Water and Associated Causal Mechanism During the Argo Period

Variability of the South Pacific eastern subtropical mode water (SPESTMW) is investigated using Argo data and ERA5 reanalysis products, with particular attention paid to its connection to the El Niño-Southern Oscillation (ENSO). Our analysis shows significant seasonal variation in the SPESTMW volume, reaching its maximum (minimum) in November (July). Changes in the mixed layer depth (MLD) dominated by shortwave radiation are the main cause of this seasonal variation. In contrast, latent heat flux is primarily responsible for the MLD and SPESTMW variability on interannual timescales. There is a considerable association between the SPESTMW volume and ENSO, decreasing (increasing) during El Niño (La Niña) years. Further analysis of ERA5 reanalysis products indicates that anomalous winds associated with ENSO modulate surface latent heat flux via evaporation, which further alters the MLD and the SPESTMW volume. Our analysis also reveals significant differences in the SPESTMW response to the central and eastern Pacific El Niño, implying potential importance of the SPESTMW in regional ocean and climate variability.