Horseshoe Salinity Pattern in the Upper Indian Ocean and its Decadal Variability

Abstract

Salinity plays a crucial role in the global hydrological cycle and climate system by regulating upper ocean stratification and sea surface temperature (SST). Past studies have revealed the dynamics of salinity variation in the world’s major ocean basins and its linkage to global climate on interannual to decadal time scales. However, understanding of upper ocean salinity variability in the Indian Ocean, especially on decadal time scale and its global implications, remains limited. This study identifies a prominent Horseshoe Salinity Pattern (HSP), present in the Indian Ocean, extending from surface to 150m depth. It exhibits pronounced decadal variability and is closely associated with SST variability in the southeastern Indian Ocean (SEIO) with a lag of ∼18 months. Composite maps and a sea surface salinity budget analysis reveal that SST anomalies in the SEIO region modulate the local Walker and Hadley circulation anomaly causing the variation in the precipitation pattern. These changes drive upper ocean freshening or salinification, producing the observed HSP. Advection contributes regionally to salinity variability, particularly in the southern Indian Ocean, but does not play a dominant role in shaping the coherent HSP structure. Further, this pattern modulates upper-ocean stratification, leading to the development of SST anomalies in the western tropical Indian Ocean. These findings offer novel insight into the Indian Ocean salinity dynamics, highlighting the role of SST-salinity coupling and have the potential for improving predictions of decadal climate variability and its global implications.