Thermosteric dominance of sea level rise in the North Indian Ocean: sub- basin budget analysis (2003-2021)

This study investigates sea level trends in the North Indian Ocean (NIO), quantifying the relative contributions of thermosteric, halosteric, and ocean mass components using satellite altimetry, reanalysis, and GRACE gravimetry data over the 2003–2021 period. Over the NIO, sea level increased at a rate of 4.55 ± 0.61 mm/yr, a trend primarily driven by thermosteric expansion associated with ocean warming. Separation of NIO into six sub-basins reveal marked spatial heterogeneity in sea level trends: (1) Western Arabian Sea (4.10 ± 0.72 mm/yr); (2) Eastern Arabian Sea (4.62 ± 0.44 mm/yr); (3) Western Bay of Bengal (4.95 ± 0.83 mm/yr); (4) Eastern Bay of Bengal (5.15 ± 0.64 mm/yr); (5) Western Equatorial Indian Ocean (4.49 ± 0.47 mm/yr); and (6) Eastern Equatorial Indian Ocean (4.84 ± 0.54 mm/yr). Even though thermosteric changes dominate the basin-wide mean, the dominant drivers vary regionally. Halosteric effects exhibit a negative trend over the entire Arabian Sea, and is linked to inflows of saline water from the Red Sea and Persian Gulf, in contrast to a positive trend in the Bay of Bengal, influenced by substantial freshwater runoff from major rivers. In the Eastern Equatorial Indian Ocean, the mass component is predominant, likely influenced by crustal adjustments after the December 2004 Sumatra-Andaman earthquake. Interannual sea level variability closely follows steric changes, which are modulated by climate modes such as ENSO and the Indian Ocean Dipole, resulting in region-specific and often opposing phase relationships across the basin. Our results confirm that while the recent global and African sea level rise is predominantly mass-driven, the NIO remains distinctively steric-dominated, with a larger contribution from thermosteric changes.