Climatic Modulation of the Euphotic Zone in the Arabian Sea: Multi-Sensor Satellite Evidence from 1998–2023

The Euphotic Zone Depth (Zeu) refers to the depth in the ocean where only 1% of surface light remains available for photosynthesis. It serves as a crucial indicator of water clarity, water quality, and the primary productivity of marine ecosystems. This study presents a detailed examination of the spatial and temporal dynamics of the Zeu over the Arabian Sea from 1998 to 2023, utilizing satellite-derived observations. Results reveal that Zeu exhibits pronounced variability on interannual, intra-annual, and decadal timescales. The analysis incorporates three key sub-regions — the Northern Arabian Sea (NAS), South Eastern Arabian Sea (SEAS), and South Western Arabian Sea (SWAS). The basin-wide annual mean Zeu ranges between 6 m and 80 m, indicating marked spatial differences in water transparency. Regionally, Zeu values vary from approximately 10–62 m in the NAS, 10–74 m in the SEAS, and 12–72 m in the SWAS. Empirical Orthogonal Function (EOF) analysis indicates that interannual variations explain 21.57% of the total variance, predominantly influenced by the Indian Ocean Dipole (IOD) and, to a lesser extent, the El Niño–Southern Oscillation (ENSO), particularly within the SEAS and SWAS. In the NAS, Zeu demonstrates a notable lagged relationship, with the first principal component (PC1) lagging the Dipole Mode Index (DMI) and ENSO by roughly 10 and 8 months, respectively. A significant positive trend in Zeu is observed across the Arabian Sea and within each sub region, with the NAS showing the greatest increase. Seasonal trend analysis further reveals consistent increases in Zeu across all seasons, with the most pronounced rise (0.47 m yr⁻¹) occurring in the NAS during the October–November (ON) season. A clear inverse relationship between Zeu and chlorophyll-a is observed across all regions indicating that variations in phytoplankton biomass are a primary control on light penetration and water clarity. Overall, this study provides the first comprehensive insight into the multi-decadal variability of Zeu across the Arabian Sea based on multi-sensor satellite observations. These findings carry direct implications for the Blue Economy, particularly in sectors such as fisheries, aquaculture, and marine biodiversity conservation. A deepening Zeu may signal ecosystem shifts that can affect fishery potential, primary production, and carbon cycling, highlighting the need for region-specific marine spatial planning and climate-resilient strategies. Monitoring Zeu variability can thus serve as a valuable indicator for assessing ocean health, in alignment with the Sustainable Development Goals (SDGs) 13: Climate Action and 14: Life Below Water.