Observed Extreme Freshening in the Central Andaman Sea Induced by Strong Positive Indian Ocean Dipole

The Indian Ocean Dipole (IOD) has been extensively studied for its significant impact on salinity distribution in tropical regions. However, its off-equatorial influence in the northeastern Indian Ocean (IO) has received limited documentation thus far. In October 2019, a buoy in the central Andaman Sea (AS) observed an extreme freshening event that resulted in intensified upper-ocean stratification and increased internal wave activities. The salt budget evaluation revealed the dominant role of horizontal advection. Interannual variability in autumn and winter sea surface salinity (SSS) showed a significant correlation with the IOD. Extreme freshening was observed exclusively during strong positive IOD (pIOD) years. This freshening primarily resulted from the outflow of low-salinity water from the northeastern coast of the AS, driven by an anomalous anti-cyclonic coastal-trapped circulation exclusive to strong pIODs in the northeastern IO. This circulation tends to hinder southward freshwater transport in the western region while enhancing it in the eastern region. This circulation pattern is primarily influenced by Kelvin wave forcing, which is triggered by robust equatorial easterly anomalies that are typically more pronounced during strong pIOD events and weaker during weak pIOD events. The anticipated increase in the frequency of extreme freshening events due to the greenhouse warming has the potential to significantly modify the salinity distribution and freshwater transport in the future. Salinity variability in the northeastern Indian Ocean (IO) is widely concerned owing to the substantial freshwater influx and its significant impact on oceanic and climatic dynamics. While the impact of the Indian Ocean Dipole (IOD) on salinity distribution in the tropical IO has been well-studied, we have limited information about its influence outside of the equatorial region in the northeastern IO. A buoy deployed in the central AS in 2019 recorded a sharp freshening event. This event caused stronger salinity differences between different depths of the ocean and increased stability in the upper layers. Consequently, it resulted in increased internal wave activities near the ocean surface. By studying the mechanism behind the salinity variations, researchers found that this event was caused by a unique clockwise pattern of coastal currents during strong pIOD years. These currents bring freshwater from the northeastern AS toward the open ocean. This circulation pattern, likely driven by long waves originating in the equatorial region, has the potential to alter freshwater transport in the northeastern IO. It is anticipated that such extreme freshening events will occur more frequently under future projections of increased strong pIOD occurrences, warranting further investigation into the associated climatic, chemical, and ecological impacts. The first in-situ observation captured the extreme freshening event in the central Andaman SeaThe observation unveiled unique coastal-trapped, anti-cyclonic circulation anomalies in the northeastern IO exclusive to strong pIODThe contrasting impacts of strong and weak pIODs on the circulation and salinity distribution in the northeastern IO were firstly revealed