Expansion of low-chlorophyll regions in the tropical Indian Ocean: spatial patterns and physical–biogeochemical associations

Context The Indian Ocean has been warming at a faster rate than the global average; hence, evaluating decadal (2015–24) shifts in key oceanographic parameters across the Tropical Indian Ocean is crucial to understand how this warming may be contributing to the expansion of low-productivity regions and the onset of ocean desertification. Aims To evaluate trends in satellite-derived chlorophyll-a (Chl-a), sea-surface temperature and sea-surface salinity, alongside complementary ocean reanalysis data for nutrients, dissolved oxygen, partial pressure of carbon dioxide and pH to evaluate the physical and biogeochemical drivers of expanding low-productivity zones. Methods Using satellite observations and ocean reanalysis products from the Copernicus Marine Environment Monitoring Service (CMEMS), we analysed monthly and seasonal climatologies. Trends were evaluated using autocorrelation-corrected Mann–Kendall tests, Theil–Sen slopes and climate event departure analysis (El Niño–Southern Oscillation, ENSO, and Indian Ocean Dipole, IOD). Key results Oligotrophic waters (Chl-a < 0.1 mg m−3) expanded northward (15°S to 15°N), strongly driven by surface warming (r = −0.72). Reanalysis showed a diagnostic nutrient decoupling (stable pools, reduced biological uptake), persistent oxygen depletion and widespread ocean acidification (pH declining ~0.002 year−1). Inter-annual climate modes (ENSO and IOD) heavily modulated these decadal trends. Conclusions Surface warming drives oligotrophic expansion across the Tropical Indian Ocean, suppressing biological productivity despite available nutrient pools and accelerating basin-wide acidification. Implications Expanding low-productivity waters and concurrent acidification threaten marine food webs, underscoring the urgent need for integrated basin-wide observational networks.