Sub-basin sea level budget analysis in the North Indian Ocean (2003-2024)

This study investigates sea level trends in the North Indian Ocean (NIO) from 2003 to 2024, quantifying the contributions of thermosteric, halosteric, and ocean mass components using satellite altimetry, observation-based gridded products, and GRACE gravimetry datasets. The NIO Sea level is rising at 4.83 ± 0.22 mm/yr, driven primarily by thermosteric sea level (2.21 ± 0.16 mm/yr) followed by ocean mass component (1.69 ± 0.09 mm/yr). Sub-basinal analysis reveals strong spatial heterogeneity, with the highest rates of sea level rise in the Western Bay of Bengal (5.19 ± 0.41 mm/yr) and the lowest in the Western Arabian Sea (4.29 ± 0.27 mm/yr). A distinct halosteric contrast exists, where freshening accelerates rise in the Bay of Bengal (up to 0.65 ± 0.08 mm/yr in the Eastern Bay), and increasing salinity suppresses it in the Arabian Sea (down to -1.12 ± 0.12 mm/yr in the western Arabian Sea). Furthermore, while western sub-basins are predominantly steric-driven, eastern sub-basins of Bay of Bengal and Equatorial Indian Ocean exhibit anomalously high GRACE-derived mass contributions (> 3 mm/yr), likely influenced by post-seismic crustal adjustments from the 2004 Sumatra-Andaman earthquake. Interannual variability closely tracks steric changes modulated by ENSO and the Indian Ocean Dipole (IOD), triggering basin-specific responses via wind anomalies and long-period waves. Further analysis of tropical SST indices reveals that the Western Tropical Indian Ocean SST index closely mirrors the spatial correlation structures of the Dipole Mode Index, and may serve as a useful indicator of regional sea level variability, particularly over the Equatorial Indian Ocean and Bay of Bengal. Ultimately, unlike the predominantly mass-driven global trends where both mass and steric contributions are of comparable magnitude, the NIO remains uniquely steric-dominated and the sea level budget leaves a residual of 0.98 mm/yr (~ 20% of the total trend), which, while comparable to uncertainties in individual components, highlights remaining challenges in fully closing the regional sea level budget.