Interannual Variability of Chlorophyll Concentrations in the Arabian Sea

Chlorophyll concentrations in the ocean exhibit significant spatial and temporal variability, driven by a complex interplay of physical, chemical, and biological factors. Seasonal changes are primarily influenced by variations in light availability, temperature, and nutrient supply, while interannual fluctuations are often linked to large-scale climate phenomena such as the El Niño-Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD). These variations have profound implications for oceanic primary productivity, marine ecosystems, and global biogeochemical cycles. The Arabian Sea, one of the most productive regions of the global ocean, experiences high biological productivity due to seasonal upwelling driven by the Southwest Monsoon and winter convective mixing. Upwelling brings nutrient-rich deep waters to the surface, stimulating phytoplankton growth and increasing chlorophyll concentrations, which in turn support a diverse marine ecosystem and economically significant fisheries. In our study, we analysed monthly and seasonal climatologies of chlorophyll(mg/m³) in the Arabian Sea, examining anomalies over a 24-year period (1998–2021). Our results revealed significant interannual variability, with notable peaks and dips in chlorophyll anomalies. For instance, anomalies exceeded a deviation from the climatological mean by 0.2 in 2005 but dropped as low as -0.2 in 2016. By analysing the standard deviation of log-transformed chlorophyll-a anomalies, we identified five regions exhibiting the highest variability. Further investigation into these regions revealed distinct patterns in upwelling dynamics across different years and seasons, emphasizing the diverse factors influencing upwelling processes in the Arabian Sea. Focusing on the western coast of India, we observed contrasting climatic behaviours between the northern and southern regions. In the northern part, wind anomalies did not directly correspond to chlorophyll anomalies, indicating a more complex interplay of factors. Conversely, in the southern region, a strong correlation between chlorophyll and wind anomalies suggests a dominant wind-driven upwelling mechanism. These findings enhance our understanding of the regional variability in upwelling processes and highlight the intricate interactions between oceanic and atmospheric drivers in this dynamic marine system. Our study provides valuable insights into the variability of chlorophyll concentrations in the Arabian Sea, offering a better understanding of its ecological and climatic significance. These findings contribute to improved modelling and prediction of primary productivity, which is crucial for both ecosystem management and climate studies.