Either IOD leading or ENSO leading triggers extreme thermohaline events in the central tropical Indian Ocean

Based on 12 years (2007-2018) of salinity data from the Array for Real-Time Geostrophic Oceanography (Argo) dataset, we found significant positive salinity anomalies in the upper layer of the central tropical Indian Ocean from autumn 2010 to spring 2011 and from autumn 2016 to spring 2017. We used wind, precipitation, outgoing longwave radiation and ocean current data from satellites and reanalysis datasets to analyze the atmospheric conditions, ocean dynamic processes and salinity budget associated with these high salinity events. Our results suggest that surface buoyancy fluxes are not the dominant factor affecting the positive salinity anomalies and that ocean dynamic processes have a more important role. Under the influence of the La Nina events and strong negative Indian Ocean dipole in 2010 and 2016, positive salinity anomalies appeared in the eastern Indian Ocean at the end of 2010 and 2016 as a result of strong westerlies and positive zonal currents. However, because the La Nina event in 2010 was stronger than that in 2016, the salinity anomalies in 2010 were also stronger and the decrease in the following year was both stronger and lasted longer, meaning that the salinity anomalies weakened only gradually. The maximum value of the salinity anomalies in 2011 therefore appeared in January, whereas in 2017 the salinity anomalies first decreased and then increased, with the largest anomalies in March. Salinity budget analyses showed that ocean advection was the main factor leading to the variations in the salinity anomaly during these two periods and that the changes in the zonal velocity in the zonal advection anomalies had the greatest impact. Zonal advection was positive and strongest at the end of 2010 and negative in early 2011, but weakly positive at the end of 2016. In early 2017, the zonal advection was first negative, then became positive and strengthened in spring, so the salinity anomalies in spring 2017 were higher than those in 2011. The freshwater flux had a small, negative effect on the positive salinity anomalies for these two events. The mutual effects of the horizontal advection and the freshwater flux led to high salinity anomalies. The high salinity anomalies reflect the response of the upper ocean to climate events and may influence regional air-sea interactions and large-scale processes.