Subtropical Mode Water south of Japan impacts typhoon intensity.

Subtropical Mode Water (STMW), characterized by vertically uniform temperature of ~17°C, is distributed horizontally over 5000 kilometers at the 100- to 500-meter depths in the subtropical North Pacific Ocean. Its formation and spreading fluctuate in relation to the Pacific Decadal Oscillation and the Kuroshio path variation, but the feedback from STMW on the sea surface temperature (SST) and the overlying atmosphere remains unclear. Using Argo profiling float data, we show that STMW south of Japan, whose thickness varies decadally, modulates the overlying thermal structure throughout the year by increasing isotherm uplift with increasing thickness. The STMW-induced decadal temperature change has a magnitude of up to ~1°C and is large in the warm season in the presence of the seasonal thermocline. Furthermore, 50-year observations, together with numerical simulation, show that SST, upper ocean heat content, and typhoon intensification rate have been significantly lower in years with thicker STMW and higher in years with thinner STMW.

Time evolution of Fukushima-derived radiocesium in the western subtropical gyre of the North Pacific Ocean by 2017.

In 2015-2017, we measured activity concentration of radiocesium in the western subtropical gyre of the North Pacific Ocean and revealed the time evolution of radiocesium derived from the Fukushima Dai-ichi Nuclear Power Plant (FNPP1) accident between 2011 and 2017. The FNPP1-derived radiocesium deposited on the area south of the Kuroshio/Kuroshio Extension Currents in March 2011 was transported southward and westward through subsurface layers due to subduction of the subtropical mode water. In 2014, the radiocesium in the subsurface layers returned to the north and circulated within the Kuroshio recirculation area. Then in 2015-2017, the radiocesium re-circulated with the area.

Radiocaesium derived from the TEPCO Fukushima accident in the North Pacific Ocean: Surface transport processes until 2017.

We report temporal variations of 137Cs activity concentrations in surface waters of six regions of the western and central North Pacific Ocean during 2011-2017 using a combination of 1264 previously published data and 42 new data. In the western and central North Pacific Ocean at latitudes of 30-42°N and longitudes of 140°E to 160°W, eastward transport of radiocaesium was clearly apparent. 137Cs activity concentrations in surface water decreased rapidly to ∼2-3 Bq m-3 in 2015/2016, still a bit higher than 137Cs activity concentrations before the FNPP1 accident (1.5-2 Bq m-3). 134Cs/137Cs activity ratios decay-corrected to 11 March 2011 were ∼0.5-0.8. To the south of 30°N and between 130°E and 160°W in the western and central Pacific Ocean, 137Cs activity concentrations were around 1-7 Bq m-3 in 2011/2012 but then stabilized at a few Bq m-3 up to 2017.134Cs activity concentrations were detected at levels of 0.1-0.9 Bq m-3, and 134Cs/137Cs activity ratios decay-corrected to 11 March 2011 were ∼0.3-0.5. Temporal variations of model-simulated 137Cs activity concentrations in surface water in the region of interest showed good agreement with observations, except in the southwestern North Pacific Ocean.

Fukushima-derived radiocesium in the western North Pacific in 2014.

In 2014, we measured activity concentration of radiocesium in the western North Pacific Ocean. In the north of Kuroshio Front high activity concentration of Fukushima-derived radiocesium in surface mixed layer in 2012 had been transported eastward by 2014. In the south of the front we found a radiocesium subsurface maximum in 200-600 m depth, which was similar to that observed in 2012. The subsurface maximum spread southward from 18°N to 15°N between 2012 and 2014, which suggests spreading of Fukushima-derived radiocesium into the whole western subtropical area by 2014 due to formation and subduction of the subtropical mode water.

Recirculation of FNPP1-derived radiocaesium observed in 2015/2016 in coastal regions of Japan.

We conducted enhanced surface water sampling at more than 80 stations in coastal regions on both the Japan Sea and Pacific Ocean sides of Japan in winter 2015/2016 to examine the recirculation behaviour of FNPP1-derived radiocaesium in the surface layer 5 years after the 2011 FNPP1 accident. We found that a small part of the FNPP1-derived radiocaesium had already recirculated in the surface layer and reached the Japanese coast.