10:45 AM - 11:05 AM
[ACG29-07] Radiocesium as a tracer for the mode waters in the North Pacific Ocean
★Invited Papers
Keywords:North Pacific Ocean, mode water, radiocesium
In the mid-latitudes of the western North Pacific, due to large heat loss in winter, the surface water becomes denser and a surface mixed-layer depth reaches several hundred meters. Then, the surface mixed-layer water is transported horizontally to the south along with a subsurface isopycnal layer of the base of the surface mixed-layer, which results in a subsurface minimum of potential vorticity, named a "mode water". In general, the lighter North Pacific Subtropical Mode Water (NPSTMW, 25.0 - 25.6 σθ approx.) and denser North Pacific Central Mode Water (NPCMW, 26.0 - 26.6 σθ approx.) are formed in the Kuroshio recirculation area south of the Kuroshio front and the mixed area north of the front, respectively. These mode waters sink into the subsurface layers (subduction) with water temperature and salinity anomalies and re-emerge on the sea surface (obduction), where it can give feedback to the atmosphere, within several years. The subduction and obduction of the mode waters probably affect the circulation of nutrients and biological production in the thermocline. Quantitative analysis of the three-dimensional circulation of the mode waters is essential for understanding the atmospheric-ocean interaction system and ecosystem in the mid-latitudes of the North Pacific Ocean. A tracer that dissolved in and moves with seawater is useful for this purpose. Radiocesium released by atmospheric nuclear weapons tests and deposited in the North Pacific mainly in the 1950s and 1960s is one of the most useful tracers. This bomb-produced radiocesium deposited in the North Pacific Ocean was rapidly dissolved in seawater and conveyed into the ocean interior with the subduction of NPSTMW and NPCMW. About half a century after the deposition, it was transported from the North Pacific Ocean to the South Pacific Ocean. In addition, radiocesium was also released into the North Pacific Ocean due to the accident of the Fukushima Dai-ichi Nuclear Power Plants in 2011. Tracing the Fukushima-derived radiocesium has elucidated the transport of NPSTMW over the past 10 years. These results of the radiocesium measurements in the mid-latitudes will be introduced in my presentation. One of the most notable results is a detection of the Fukushima-derived radiocesium at a depth of about 300 m at 20°N about 10 months after the Fukushima accident. If it is transported by the southward subduction of NPSTMW, it cannot be explained by the advection rate of NPSTMW that was derived from conventional geostrophic analysis. Recently, spatiotemporally high-density observations by Argo floats, satellite observations, and high-resolution model simulations have suggested that the transport of dissolved substances in seawater is governed by not only mesoscale eddies (100 to 300 km) but also sub-mesoscale (1 to 50 km) eddies and stripes. The rapid southward transport of the Fukushima-derived radiocesium supports this hypothesis. Due to the radioactive decay, the activity concentration of the Fukushima-derived radiocesium has decreased to less than 1/30 of the initial concentration just after the accident. As a result, it has become difficult to obtain new observational data of the Fukushima-derived radiocesium in the open ocean. In the future, it will be necessary to make efficient observational plans in selected areas that are suggested by the synthesis of the observation data obtained in the past and high-resolution model simulations. This work was partially supported by a Grant-in-Aid for Scientific Research on Innovative Areas from the Ministry of Education, Culture, Sports, Science and Technology of Japan (KAKENHI), JP20H05173.