Eddy-induced Chlorophyll-a Variations in the Northern Indian Ocean: A Study Using Multi-Source Satellite Data and Deep Learning

Abstract. Mesoscale eddies, including surface-intensified eddies (SEs) and subsurface-intensified eddies (SSEs), significantly influence phytoplankton distribution in the ocean. Nevertheless, due to the sparse in-situ data, it is still unclear in understanding the characteristics of SSEs and their influence on chlorophyll-a (Chl-a) concentration. Consequently, the study utilized a deep learning model to extract SEs and SSEs in the North Indian Ocean (NIO) from 2000 to 2015, using long time series of satellite-derived sea surface height (SSH) and sea surface temperature (SST) data. The analysis revealed that SSEs accounted for 44 % of the total eddies in the NIO, and their SST signatures exhibited an opposite behavior compared to SEs. Furthermore, by integrating ocean color remote sensing data, the study investigated the contrasting impacts of SEs and SSEs on Chl-a concentration in two basins of the NIO: the Arabian Sea (AS) and the Bay of Bengal (BoB), known for their disparate biological productivity. In the AS, SEs induced Chl-a anomalies that were two to three times higher than those caused by SSEs. Notably, there were no significant differences in Chl-a anomalies induced by the same type of eddies between summer and winter. In contrast, the BoB exhibited distinct seasonal variations, where SEs induced slightly higher Chl-a anomalies than SSEs during the summer, while substantial differences were observed during the winter. Specifically, subsurface-intensified anticyclonic eddies (SSAEs) led to positive Chl-a anomalies, contrasting the negative anomalies induced by surface-intensified anticyclonic eddies (SAEs) with comparable magnitudes. Moreover, while both subsurface-intensified cyclonic eddies (SSCEs) and surface-intensified cyclonic eddies (SCEs) resulted in positive Chl-a anomalies, the magnitude of SSCEs was only one-fourth of that induced by SCEs. The distinct Chl-a anomalies between SEs and SSEs can be attributed to the contrasting subsurface structures revealed by Argo profiles. The upward displacement of isopycnals within SSAEs and the downward displacement within SSCEs in the upper 30–50 meters lead to higher and lower Chl-a concentrations, respectively. The study provides a valuable approach to investigating subsurface eddies and contributes to a comprehensive understanding of their influence on chlorophyll concentration.