POSTER Session 1

Shifting optical states, ocean color and heating rates in the coastal waters of eastern Arabian Sea

Our study investigates trends in chlorophyll-a in the coastal waters of the eastern Arabian Sea (< 200 m) using remote sensing reflectance, in situ optical observations, and diagnostics of optical complexity and heating rates. Analysis of 4 km monthly averaged Ocean Colour Climate Change Initiative data (1998-2022) revealed distinct regions with statistically significant positive and negative trends in chlorophyll-a anomalies. Further analysis of in-situ measurements of inherent and apparent optical properties collected between 2008-2017 and 2019-2021 helped attribute these trends to patterns in water constituent concentrations. A k-means clustering and apparent visible wavelength analysis of in-situ hyperspectral reflectance revealed the presence of nine optical water types, five with positive trends and four with negative trends. An absorption budget showed these positive and negative trends were dominated by particulate (chlorophyll-a and total suspended matter) and dissolved (colored dissolved organic matter, CDOM) matter, respectively. The manifestation of these optical states influenced the vertical transmission of solar energy. In-situ measurements revealed that about 90% of the water constituent-induced surface radiant heating penetrated to 2 m depths at locations dominated by dissolved matter (negative trends), while at locations dominated by particulate matter (positive trends), radiant heating penetrated deeper, to 3 m depth. Euphotic depth integrated heat content was 7% higher for negative trends. Applying a sequential increase in CDOM to a three-stream light model configured with in-situ measured irradiances and initial optical conditions representing positive trends, resulted in a shift in optical state corresponding to the negative trends of chlorophyll-a.

Arjun Adhikari, Goa University, India, https://orcid.org/0000-0001-5007-6634

Bronwyn Cahill, Leibniz Institute for Baltic Sea Research, Germany

Harilal Menon, Goa University, India

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