In April–May 2011, just after the accident, the 134Cs activity was as high as 1000 Bq m−3 in the surface waters of the transition area and just to the south of the KE (30°N–40°N) along approximately 145°E–152°E, more than 500 km from the FNPP1 (Figure 2). In April 2011, 134Cs activity was also observed at stations in the subarctic and subtropical regions, more than 1000 km distant from the plant26,28. The wide dispersal of Fukushima-derived 134Cs in the western North Pacific within about two months of the accident is consistent with patterns of atmospheric deposition of 134Cs simulated by atmospheric models13,25,38. A low-pressure system traveling across Japan from 14–15 March 2011 was found to be effective in lifting particles containing 134Cs from the surface layer to the altitude of the westerly jet stream, which carried the particles across the North Pacific within 3–4 days39.

In the transition area between 35°N and 40°N, the 134Cs activities in surface waters during June–August 2011 were significantly higher than in April–May 2011 (Figure 2), which implied that contaminated waters discharged from the FNPP1 had been transported by the eastward-flowing North Pacific Current (Figure 5). The radiocesium activities in surface seawater collected by commercial cruise ships revealed an eastward propagation of the main plume of the directly discharged 134Cs. The zonal speed of the plume was estimated to be about 200 km month−1, a speed that was consistent with trajectories of Argo floats launched near the FNPP128. Therefore, arrival of the directly discharged 134Cs water in June–August 2011 was delayed by about two months relative to the atmospheric deposition in April–May 2011. The activity decrease in September–December 2011 indicated that the main body of the plume had passed to the east between April–May and September–December 2011. The radiocesium, however, also had spread vertically and penetrated deeper in the winter of 2012 (a depth of about 300 m) compared to June 2011 (a depth of about 200 m).

Figure 5 A schematic view of formation and subduction of mode waters in the North Pacific. Yellow and yellow-shaded ellipses indicate spreading and formation areas, respectively, of STMW (25.0–25.6 σ θ ). Green and green-shaded areas indicate spreading and formation areas, respectively, of CMW (26.0–26.6 σ θ ), which is denser than STMW. Thick broken and solid arrows show spreading directions of STMW and CMW, respectively. Blue and red dotted lines are surface water currents of the subarctic and subtropical gyres, respectively. The broken line denotes our observational line at 149°E in the winter of 2012. SAF, KEF and STF indicate the subarctic, Kuroshio Extension and subtropical fronts along the observational line, respectively. The map in this figure were drawn using Ocean Data View54 and this figure has been modified from one in the literature55. Full size image

The 134Cs activity in the subarctic region was lower than in the transition area throughout the observational period; its pattern of temporal change, however, was similar to that in the transition area (Figure 2). Whether there were intrusions of directly discharged 134Cs from the transition area to the subarctic region is unclear, because the transitory increase in June–August 2011 was obscure in the subarctic region. Off the Kuril Islands, the activities in the surface waters of the Oyashio Current, which flows into the subarctic region (Figure 5), were less than a few Bq m−3 in April 201127. If the supply of directly discharged 134Cs to the subarctic region had been blocked by the subarctic front, the surface activity in the subarctic region would have dropped more sharply because of the inflow of Oyashio Current water, the 134Cs activity of which was low. In fact, the low activity at the northernmost station in the winter of 2012 implies an intrusion of Oyashio Current water (Figure 3a). Therefore, it is likely that the directly discharged 134Cs was transported into the subarctic region through water exchanges between the transition area and the subarctic region. The gradual decrease of surface 134Cs in the subarctic region indicates that the directly discharged 134Cs was transported eastward and diffused vertically over time, as was also the case in the transition area.

Between 30°N and 35°N in the subtropical region, the 134Cs derived from atmospheric deposition during April–May 2011 was apparently swept out in June–August 2011 (Figure 2). In May 2011, Fukushima-derived 134Cs was not detected in surface waters just south of Japan28, where the Kuroshio Current (the upper stream of the KE) flows northeastward (Figure 5). This low 134Cs activity in the Kuroshio Current region suggests that a new and relatively “clean” KE current from the west probably flushed out the 134Cs in the surface water between 30°N and 35°N. This process was also clearly demonstrated in ocean model simulations12,13 and suggests that an exchange of surface seawater between the transition area and the subtropical region was restrained by the KE front. The 134Cs activity in the surface mixed layer between 25°N and 35°N was low but detectable in the winter of 2012 (Figure 3a). The 134Cs derived from atmospheric deposition just after the accident probably recirculated within the western subtropical region (Figure 5). Alternatively, the 134Cs in the mixed layer could be explained by entrainment of 134Cs from the subsurface maximum just below the mixed layer. To the south of 20°N, the 134Cs was detected only in surface waters collected with a bucket. Although the cause of those surface activities is not sure, a little contamination on the bucket is possible.

In the subtropical region between 20°N and 35°N, we found a subsurface 134Cs maximum just below the surface mixed layer in the winter of 2012 (Figure 3a). This tongue-shaped subsurface plume appeared on a pycnostad between 25.0 and 25.6 σ θ (Figure 3d) that resulted in a subsurface minimum of potential vorticity in the corresponding layers (Figure 3e). We conclude that the 134Cs subsurface maximum was derived from formation and subduction of Subtropical Mode Water (STMW)40. To the south of the KE between approximately 30°N and 35°N, STMW is formed and penetrates to a depth of about 400 m (25.6 σ θ ) in late winter. This STMW then spreads to nearly the subtropical front35 through advection over the Kuroshio recirculation region41,42 (Figure 5). Atmospheric deposition of the Fukushima-derived 134Cs in the North Pacific Ocean occurred mainly in March 2011, when STMW was just being formed. Therefore, the 134Cs deposited just to the south of the KE was probably mixed vertically to depths of 300–400 m immediately. The high activities in the 134Cs subsurface plume at 32°N and 34°N (10–20 Bq m−3) were nearly identical with those in the surface waters between 30°N and 35°N in April–May 2011 (Figure 2). One could argue that the high subsurface activities in the winter of 2012 were remnants of the 134Cs that penetrated deeply during March 2011. The 134Cs in newly formed STMW then started to spread to around 20°N along subsurface isopycnals (25.0–25.6 σ θ ). In June–August 2011, the 134Cs in the surface mixed layer between 30°N and 35°N may have been flushed out and the subsurface plume appeared between 20°N and 35°N (Figure 3a). The subsurface maximum observed at 36°N to the south of the KE in June 201117 is consistent with the immediate subduction of the Fukushima-derived 134Cs.

The deeper penetration of 134Cs to depths of about 600 m (26.6 σ θ ) between 32°N and 35°N (Figure 3a) cannot be explained by formation of STMW, the deepest convection of which is to about 400 m (25.6 σ θ ). The penetration of the 134Cs to 26.0–26.6 σ θ is reminiscent of ventilation of another, denser mode water in the North Pacific, the Central Mode Water (CMW)43. The formation area of CMW is situated in the transition area in the central North Pacific. The CMW spreads eastward along the North Pacific Current, turns southward and then turns westward (Figure 5). Despite its similar water density anomaly (26.0–26.6 σ θ ), the path of the CMW as it spreads is likely to be to the south of approximately 30°N, along 149°E. In addition, a transit time as short as about 10 months (between March 2011 and January 2012) from the formation area to 149°E longitude is not plausible, because the renewal time of CMW is more than 20 years44.

Another possible explanation for the deeper penetration is conveyance of 134Cs from the transition area across the KE. The satellite image of SSH indicates that stations at 32°N and 34°N were located near a cyclonic eddy centered at 33°N, 151°E (B in Figure 1b). This cyclonic eddy originated in a southward meander of the KE front around 158°E and pinched off southward from the meander in September 2011. Then the eddy moved westward and reached 151°E in January 2012. Similar to the relatively high activity at the station located near the center of the southward meander of the KE at 148°E (A in Figure 1b), the cyclonic eddy probably consisted of denser waters with a higher activity of 134Cs, because the surface 134Cs activity in the source area (the transition area) was more than 50 Bq m−3 in October 201129. A model simulation has indicated that a cyclonic eddy detached from the KE front holds the transition area water in it, while small leakage occurs from layers denser than 26.0 σ θ 45. Although the vertical profiles of temperature and salinity do not indicate the presence of a cyclonic eddy between 32°N and 34°N (Figs. 3b and 3c), a small amount of leakage of 134Cs from such an eddy could explain the deeper penetration of the 134Cs (Figure 3a). Alternatively, the deeper penetration can be attributed to direct advection along subsurface isopycnals from the transition area. A salinity minimum observed just south of the KE has been explained by intrusion of Oyashio low-salinity water in the transition area; this intrusion was associated with the frontal wave structure of the KE46,47. The deeper 134Cs penetration just south of the KE (Figure 3a) implies that a similar subsurface intrusion occurred in the winter of 2012.

In the winter of 2012 the areal inventory of 134Cs (decay-corrected to the date of the accident) in the subtropical region (20°N–35°N) was estimated to be 1.6 ± 0.1 kBq m−2, which is about one-third of the areal inventory in the transition area (35°N–40°N), 4.6 ± 0.3 kBq m−2 (Figure 4). The integral of the areal inventory along the meridian in the subtropical region, however, was 2.7 ± 0.1 GBq m−1, which was about twice the value of the integral in the transition area, 1.4 ± 0.1 GBq m−1. The large inventory in the subtropical region suggests that the 134Cs released from the FNPP1 had been transported not only eastward but also southward. The average activity of the decay-corrected 134Cs in the STMW was 5.6 ± 0.4 Bq m−3. We here assumed that this average activity could be regarded as the mean activity of the whole STMW in the North Pacific, because our observational line was located near the center of the area of STMW (Figure 5). An estimation of the total volume of STMW (about 1 × 106 km3)44 implies that the STMW contained about 6 PBq of 134Cs. Estimates of the total 134Cs released to the North Pacific Ocean ranged from 10 PBq (direct discharge of 4 PBq + atmospheric deposition 6 PBq) to 46 PBq (16 + 30 PBq). Thus, the 6 PBq inventory accounts for 10–60% of the total release. However, the total inventory in the subtropical region derived from the activity in STMW may be underestimated, because CMW probably carried the radiocesium into the subtropical region, too (Figure 5).

In this study we reconstructed the temporal change in Fukushima-derived radiocesium in surface water of the western North Pacific during about one year and a half after the accident. In April–May 2011 the 134Cs activity between 30°N and 40°N arose from the atmospheric deposition (Figure 2). In the north of the KE front, the transition area and subarctic region the discharged 134Cs was added while in the south of the KE front the atmospheric-deposited 134Cs was flushed out by the KE current during the following period. We found the subsurface maximum of 134Cs in the subtropical region about 10 months after the accident. The radiocesium that entered the ocean just south of the KE front via atmospheric deposition was subducted southward immediately because of formation of STMW. This process is reminiscent of the southward spreading of radiocesium derived from the nuclear bomb testing in the North Pacific via STMW formation48. In addition, there is an indication that the Fukushima-derived radiocesium in the transition area was conveyed southward across the KE by cyclonic eddies that detached from the KE and by subsurface intrusion under the KE. The rapid southward spreading of the 134Cs through subsurface layers seems to not have been simulated well in ocean models13,15,16,32,33, probably because of problems associated with the simulation of processes responsible for formation/subduction of STMW in these models. The estimated inventory in the subtropical region (6 PBq or 10–60% of the total inventory) is probably a lower limit of estimation because contribution of CMW was not counted. The results in this study clearly suggest that radiocesium released from FNPP1 into the North Pacific Ocean had been transported not only eastward along with the surface currents but also southward due to formation/subduction of STMW within about 10 months after the accident.