Impact of Internal Tides on Distributions and Variability of Chlorophyll-a and Nutrients in the Indonesian Seas

Abstract

Internal tides (ITs) in the Indonesian Seas were largely investigated and held responsible for strong water mass transformation and intense surface cooling. Here, we evaluate the ITs' impact on chlorophyll-a through a coupled INDESO ocean-biogeochemical model which is compared with in situ data and satellite products. The results show that explicit tides' inclusion within the model improves the representation of chlorophyll-a and nutrients. Previous studies highlighted that tides at spring-neap cycle cool the surface water by 0.2°C. Our current results show increases of chlorophyll-a by 0.2 up to 5 × 10⁻⁷ mg Chl m⁻³ (in log10) at ITs' generation sites (Sangihe, Ombai, Banda, and Halmahera Straits) and over the shallow Australian plateau and Java Sea, where barotropic tidal friction is high (Zaron et al., 2023, https://doi.org/10.5194/os-19-43-2023). In addition, maxima of chlorophyll-a concentration have a spring-neap tides pulse in good agreement with ocean color images. We use INDOMIX in situ vertical diffusivities in a 1D diffusion model to explain the biogeochemical tracers' transformation within the Halmahera Sea and to estimate the nutrients' turbulent flux. We find an associated increase in new production of ∼25% of the total and an increase in mean chlorophyll-a of ∼30%. These findings support the idea of enhanced surface mixing capable of providing cold and nutrient-rich water favorable for the phytoplankton growth. Hence, we confirm the key role of ITs in shaping vertical distribution and variability of chlorophyll-a, along with nutrients and oxygen, in the Indonesian archipelago at the hotspots of intensified mixing where strong ITs are found.