Decadal Variability in Salinity of the Indian Ocean Subtropical Underwater During the Argo Period

Salinity of the Indian Ocean Subtropical Underwater (IOSTUW) showed robust decadal variability during the Argo period, with a freshening trend between 2005 and 2013 and a salinification trend between 2014 and 2019. The salinity variability of the IOSTUW originated from both the sea surface salinity maximum region in the subtropical South Indian Ocean and the southeastern tropical Indian Ocean. The mixed layer salinity variabilities in these two regions are dominated by horizontal advection, which is mainly influenced by anomalous local surface currents and salinity anomalies from the western tropical Pacific and Maritime Continent. Both of these two factors have a close relationship with the El Niño‐Southern Oscillation (ENSO) signal. As a result, the mixed layer salinity in those two regions changed dramatically during the strong and long‐lasting ENSO events and thus led to the decadal variability in salinity of the IOSTUW. Plain Language Summary The Indian Ocean Subtropical Underwater (IOSTUW) is identified as a shallow subsurface salinity maximum within the upper 400 m in the subtropical South Indian Ocean. It is an essential component of freshwater exchanges between the western Pacific, the Indian Ocean, and the South Atlantic. The variability in salinity of the IOSTUW has a close relationship with the global hydrological cycle and is of crucial significance to regional sea‐level change. Here we provide the first observation‐based estimate of the decadal variability in salinity of the IOSTUW and the relevant influencing factors. Our results suggest that the IOSTUW shows robust decadal variability with a freshening trend during 2005–2013 and a salinification trend during 2014–2019. This signal originates from the sea surface salinity maximum region in the subtropical south Indian Ocean and the southeast tropical Indian Ocean. It is mainly modulated by horizontal advection, associated with the advection of salinity anomalies from the western tropical Pacific and Maritime Continent and anomalous local surface currents, both having a close relationship with the El Niño‐Southern Oscillation (ENSO) signal. Key Points Salinity of the Indian Ocean Subtropical Underwater showed robust decadal variability during 2005–2019 This signal resulted from dramatic changes of mixed layer salinity in southeast Indian Ocean during strong and long‐lasting ENSO events Contributing factors include anomalous local surface currents and anomalies from the western tropical Pacifi