Observed variability of intermediate water masses in tropical western Indian Ocean from a 2019-2025 subsurface mooring time series

Intermediate water masses in the tropical western Indian Ocean play a key role in subsurface thermohaline circulation by contributing to the redistribution of heat and salt, yet their variability on seasonal to interannual timescales remains poorly understood due to the historical scarcity of sustained in situ observations. We present an observational analysis of intermediate water mass variability based on a continuous subsurface mooring time series collected from 2019 to 2025 at the Seychelles-Chagos Thermocline Ridge (SCTR; 8°S, with the mooring located at 61°E during May 2019-June 2024 and relocated to 65°E thereafter). We focus on the intermediate layer spanning approximately 440-1190 m depth (corresponding to ~27.0-27.4/27.5 sigma-theta), using temperature, salinity, potential density, and spiciness. Pronounced changes in physical properties are observed between the earlier (2019-2021) and later (2022-2025) periods. Relative to the earlier years, the intermediate layer during later period exhibits freshening (34.82 to 34.79 PSU, -0.1%) and warming (7.06 to 7.19 °C, +1.8%), accompanied by a decrease in potential density (27.28 to 27.23 kgm^-3, -0.2%) and a concurrent increase in spiciness (0.64 to 0.66, +1.8%), suggesting potential changes in the relative contributions of intermediate water masses. To examine this possibility, we apply an optimal multiparameter (OMP) analysis to quantify the fractional contributions of Red Sea Overflow Water (RSOW), Indonesian Intermediate Water (IIW), and Antarctic Intermediate Water (AAIW). The OMP results show that RSOW has both the largest fractional contribution and the strongest interannual-scale variability among the three intermediate water masses at the SCTR accounting on average for ~0.59±0.05 of the intermediate layer indicating its dominant role in modulating intermediate-layer variability in the region. In comparison, IIW and AAIW contribute smaller mean fractions (~0.25±0.01 and ~0.13±0.03, respectively) and display comparatively weaker temporal variability. Notably, the mean RSOW fraction decreases during 2022-2025 from 0.62±0.04 to 0.57± 0.03 (-7.3%), whereas the contributions of IIW and AAIW increase from 0.24± 0.01 to 0.25± 0.01 (+7.2%) and from 0.11±0.03 to 0.15±0.02 (+34.6%), respectively. While RSOW remains the dominant intermediate water mass at the SCTR, the increased fractions of IIW and AAIW during the later years indicate an enhanced relative contributions of these water masses during the later period, consistent with the observed freshening and increase in spiciness in intermediate layer. By leveraging a rare continuous mooring time series, this study demonstrates the value of sustained in situ observations for resolving multi-year variability in intermediate water mass composition and properties at the SCTR region.