Intermediate Water Salinification Counteracts Thermosteric Sea‐Level Rise in the Tropical Southwest Indian Ocean

Abstract Sea level in the tropical Southwest Indian Ocean has risen rapidly during the Argo era, yet whether its mid‐depth halosteric component reflects along‐isopycnal change or isopycnal displacement remains unresolved. Using gridded Argo data for 2004–2023, we decompose steric sea level into thermosteric and halosteric components and partition temperature and salinity trends into heave (isopycnal displacement) and spice (along‐isopycnal change). Thermosteric expansion is comparable between the upper 300 m and the mid‐depth layer, 300–2,000 m (0.95 ± 0.72 and 1.08 ± 0.17 cm decade −1 ), though the upper‐layer estimate is less certain. Depth‐integrated halosteric contraction is larger at mid‐depth (−0.20 ± 0.13 and −0.63 ± 0.07 cm decade −1 ), compensating 58 ± 10% of mid‐depth thermosteric expansion. The mid‐depth salinity trend is overwhelmingly spice‐driven (3:97), so the compensation is primarily along‐isopycnal; the temperature partition (25:75) is weakly constrained. This is consistent with, but does not establish, salinification within the Indian‐sector AAIW‐influenced density range.