A collaboration between PML and researchers in Japan, recently published in the Journal of Hydrology, has enabled the evaluation of how tropical cyclones – known as typhoons - can affect extreme river runoff in the ocean. By combining river and ocean models scientists have now tracked and monitored radioactive particles in the rivers and coastal region of Fukushima, Japan, and discovered the role typhoons can play.
When a magnitude 9.0 earthquake and tsunami hit the eastern coast of Japan in March 2011, it caused major damage to the Fukushima Daiichi Nuclear Power Plant, and led to the release of radioactivity into the surrounding river system and onwards to the ocean. During a nine-month monitoring period following the event, an extreme typhoon occurred in September 2011, and mobilised more than 60% of the total load of radiocaesium – the radioactive isotope released in the Fukushima disaster – delivered by the river runoff to the coastal waters. This means that a single typhoon event had a significant influence on the amount of radioactive contaminants reaching coastal waters.
With this in mind, a team, led by the Graduate School of Advanced Integrated Studies in Human Survivability (GSAIS) and Disaster Prevention Research Institute (DPRI) at Kyoto University, modelled hourly peak river water discharge during extreme typhoon events for a targeted set of rivers in the coastal zone of north-eastern Japan. The results were coupled with high-resolution ocean model operated by Japan Agency for Marine-Earth Science and Technology (JAMSTEC) to simulate changes in sea surface salinity distribution following the extreme precipitations and river water runoff caused by typhoons in 2002 and 2011. Then, in collaboration with PML, they observed striking similarities in the distributions of sea surface salinity from the model, and ocean colour from satellite, before and after the passage of the typhoons.
“Ocean colour is used here as complementary method to verify the dispersion of the modelled river runoff,” said PML’s Dr Marie-Fanny Racault. “When river runoff reaches the coastal zone, it brings small particles and nutrients – a source of food for the surface floating microscopic flora, phytoplankton. These phytoplankton and small particles alter the colour of the ocean, which is seen from space, and is shown to match closely the modelled river discharge as indicated by the low surface salinity in coastal waters.”
With the advances in weather forecasting systems, and specific availability of forecasts of typhoon trajectory and intensity, the team hopes to be able to predict river water spill impacts on the coastal marine environment before typhoon events occur, and thus contribute greatly to the management and protection of the coastal zone.
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