Variability of the South Pacific Western Subtropical Mode Water and Its Relationship With ENSO During the Argo Period

This study investigates variability of the South Pacific western subtropical mode water (SPWSTMW), its physical processes, and relationship with El Nino-Southern Oscillation (ENSO), using a gridded Argo data product from January 2004 to September 2019. On seasonal timescale, the SPWSTMW volume shows a significant variability, which involves three periods: the formation period (June-October), the isolation period (November-February), and the dissipation period (March-May). This seasonal variability is related to seasonal fluctuation of the mixed layer depth. During the Argo period from 2004 to 2019, interannual variability of the SPWSTMW volume is tightly linked to the ENSO, increasing during El Nino periods and decreasing during La Nina periods. Further analyses indicate that ENSO-related anomalous winds are primarily responsible for interannual variability of the SPWSTMW volume. The anomalous winds first influence the surface heat flux through evaporation and then the mixed layer depth through convection, leaving an imprint of ENSO on the SPWSTMW. This study also shows that the SPWSTMW responds differently to the central Pacific (CP) El Nino and eastern Pacific (EP) El Nino. Plain Language Summary Using recently available Argo data set and atmospheric reanalysis products, this study investigates the impact of El Nino-Southern Oscillation (ENSO) on the volume of the South Pacific western subtropical mode water (SPWSTMW) and its underlying mechanisms. The results show that interannual variability of the SPWSTMW volume has a strong ENSO signal during the Argo period from 2004 to 2019, which is mainly forced by ENSO-induced wind anomalies. During El Nino years, surface winds in the SPWSTMW formation region are stronger than during normal years, resulting in a decrease in sea surface temperature (SST) through the wind-evaporation-SST (WES) feedback. The reduced SST can decrease the stratification and deepens the mixed layer depth (MLD), which enhances the horizontal MLD gradients and consequently the lateral induction and subduction of the SPWSTMW, directly contributing to the increase of the SPWSTMW volume. This study provides new insights into our understanding of regional oceanography and climate change in the South Pacific.