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River freshwater effects on the ocean surface dynamics and plastic particles spreading in the Southeast Asian Seas: Insights from a high-resolution simulation using GloFAS
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Researchers simulated ocean currents in Southeast Asian seas using river flow data, then tracked virtual plastic particles released from Indonesian rivers over nine years. About 56% of particles ended up stranded on regional coasts, while about 43% drifted toward the Indian Ocean, showing river flow patterns affect where floating debris travels.
ABSTRACT River freshwater fluxes are fundamental runoff inputs for regional and global ocean models. Nonetheless, most models are currently forced by climatological runoff data, unable to represent seasonal and interannual variability and thus resulting in significant biases in the simulated ocean conditions. This study first evaluates daily river discharges at 1/10° of the Global Flood Awareness System (GloFAS) reanalysis over Southeast Asia (SEA) against observed records and climatology. The GloFAS product satisfactorily performs for the major SEA rivers, with an acceptable overestimation of their average discharge (mean relative bias of ~2.12) and a good representation of their seasonal cycle, but struggles to reproduce interannual variability (mean correlation of ~0.87 and ~0.37, respectively). We then use this high-frequency runoff for prescribing a 10-year numerical simulation of the SEA seas, assessed by comparison with surface salinity (SSS), temperature (SST) and elevation (SLA) data from satellite and reanalysis products over 2010-2018 (removing 2009 as spin-up). The latter indicates close similarity among the predicted and observed distributions (mean absolute bias of -0.1 for SSS, 0.48°C for SST and 0.09 m for SLA). Lastly, Lagrangian simulations of particles emitted from Indonesian rivers, assuming a linear relationship between daily discharge and the particles’ number, are performed based on this hydrodynamical simulation. Their statistical analysis suggests that most particles (~56% of the 9-year ensemble) strand on SEA coasts, but a significant portion (~43%) is advected towards the Indian Ocean. Hence, for a realistic reproduction of the SEA upper ocean dynamics and SSS patterns, we underline the importance of considering the seasonal changes of river discharges, although their interannual variability is less accurately represented. Indeed, along with other meteo-oceanographic processes, runoff contributes to shape SSS spatio-temporal behavior and affects marine debris’ surface dispersal with a significant impact on the stranding mechanism, especially within semi-enclosed SEA basins as the Indonesian Seas.
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