Interannual Variability of Coastal Upwelling in the Lakshadweep Sea Using a Two-Step Data-Assimilative Ocean Model and a New Strength of Upwelling Index

This study examines interannual variability of coastal upwelling in the Lakshadweep Sea (2015–2020) using a high-resolution (1/20°) NEMO model within a novel two-step data assimilation framework. Pre-monsoon conditions show limited interannual differences, whereas the mature monsoon phase reveals substantial variability in the spatial extent and persistence of cold, dense upwelled waters within the euphotic zone. The strongest upwelling occurred in 2016 and 2018, with weaker conditions in 2015, 2017, 2019, and 2020. The new Strength of Upwelling (SoU) index, defined as the temporal integral of the area occupied by waters colder than a specific threshold (26 °C in this case) within a coastal band, quantifies upwelling intensity and duration directly from modelled hydrographic fields. Subsurface metrics at 14 m depth provide clearer interannual discrimination than surface indicators alone. Latitudinal analysis reveals spatial heterogeneity in upwelling intensity, suggesting that local processes modulate large-scale monsoon forcing. Ultimately, the SoU index offers a new physically based tool for assessing upwelling variability relevant to ecosystem dynamics and fisheries management.