Masao Fukasawa

Impacts of the Fukushima Nuclear Power Plants on Marine Radioactivity

The impacts on the ocean of releases of radionuclides from the Fukushima Dai-ichi nuclear power plants remain unclear. However, information has been made public regarding the concentrations of radioactive isotopes of iodine and cesium in ocean water near the discharge point. These data allow us to draw some basic conclusions about the relative levels of radionuclides released which can be compared to prior ocean studies and be used to address dose consequences as discussed by Garnier-Laplace et al. in this journal. The data show peak ocean discharges in early April, one month after the earthquake and a factor of 1000 decrease in the month following. Interestingly, the concentrations through the end of July remain higher than expected implying continued releases from the reactors or other contaminated sources, such as groundwater or coastal sediments. By July, levels of (137)Cs are still more than 10,000 times higher than levels measured in 2010 in the coastal waters off Japan. Although some radionuclides are significantly elevated, dose calculations suggest minimal impact on marine biota or humans due to direct exposure in surrounding ocean waters, though considerations for biological uptake and consumption of seafood are discussed and further study is warranted.

Satellite altimeter monitoring the Kuroshio Transport south of Japan

In 1993–1995, we carried out observations of the Kuroshio south of Japan, including direct current measurements and repeated hydrographic surveys along a satellite track of the TOPEX/POSEIDON altimeter. The velocity field of the Kuroshio is determined by geostrophic calculation using the repeated hydrographic survey data, referenced to velocities observed at mid and abyssal depths. The volume transport of the Kuroshio is estimated from this velocity field. The estimated transports of the Kuroshio have a high correlation with sea-surface height differences across the Kuroshio. Having this relationship and using the altimeter data, we obtained a time series of the Kuroshio transport over seven years at ten-day intervals. The Kuroshio transport, excluding contributions by local recirculations, is estimated to be 42 × 106 m³/sec on average. The correlation between sea-surface height difference and transport provides a practical method of long-term monitoring of the Kuroshio transport using satellite altimetry.

The Generation of Large-Amplitude Unsteady Lee Waves by Subinertial K1 Tidal Flow: A Possible Vertical Mixing Mechanism in the Kuril Straits

Numerical experiments with a two-dimensional nonhydrostatic model are performed to investigate tidally generated internal waves in the Kuril Straits and their effect on vertical mixing. The results show that sill-scale internal waves at the K1 tidal frequency are confined to the sill slopes because the K1 tide is subinertial in the Kuril Straits. In contrast to previous theories, the authors show that intense short internal waves generated at the sill breaks by the subinertial K1 tidal current can propagate upstream as the tidal current slackens. Theoretical considerations identify these short waves as unsteady lee waves, which tend to be trapped at the generation region and grow into large-amplitude waves, eventually inducing vigorous mixing along their ray paths. In particular, superposition of a propagating unsteady lee wave and a newly generated lee wave over a sill causes significant wave breaking leading to a maximum vertical diffusivity of ;103 cm2 s21. This quite intense mixing reaches down to the density layer of the North Pacific Intermediate Water (NPIW). In contrast, the M2 tidal current does not cause such strong vertical mixing, because most of generated internal waves propagate away as first-mode internal tides and because the barotropic flow amplitude is small. The authors therefore suggest the possibility that generation of lee waves through interactions between the K1 current and the bottom topography of the Kuril Straits contributes to the observed modification of the Okhotsk Sea water required in the formation of the NPIW.

Bottom water warming in the North Pacific Ocean

Observations of changes in the properties of ocean waters have been restricted to surface or intermediate-depth waters, because the detection of change in bottom water is extremely difficult owing to the small magnitude of the expected signals. Nevertheless, temporal changes in the properties of such deep waters across an ocean basin are of particular interest, as they can be used to constrain the transport of water at the bottom of the ocean and to detect changes in the global thermohaline circulation. Here we present a comparison of a trans-Pacific survey completed in 1985 (refs 4, 5) and its repetition in 1999 (ref. 6). We find that the deepest waters of the North Pacific Ocean have warmed significantly across the entire width of the ocean basin. Our observations imply that changes in water properties are now detectable in water masses that have long been insulated from heat exchange with the atmosphere.

Simulated Rapid Warming of Abyssal North Pacific Waters

Warming the Deep The coldest ocean waters are located at the bottoms of the major ocean basins, and, because it takes a long time for water to sink from the surface to these regions, they are relatively isolated from the warming trends that are now occurring at shallower depths. However, warming in these deep waters has recently been observed, sooner than anticipated. Masuda et al. (p. 319, published online 24 June) performed computer simulations of ocean circulation and found that internal waves are able to transport heat rapidly from the surface waters around Antarctica to the bottom of the North Pacific, which can occur within four decades, rather than the centuries that conventional mechanisms have suggested. Computer simulations suggest a possible reason for the warming of North Pacific bottom water during the past four decades. Recent observational surveys have shown significant oceanic bottom-water warming. However, the mechanisms causing such warming remain poorly understood, and their time scales are uncertain. Here, we report computer simulations that reveal a fast teleconnection between changes in the surface air-sea heat flux off the Adélie Coast of Antarctica and the bottom-water warming in the North Pacific. In contrast to conventional estimates of a multicentennial time scale, this link is established over only four decades through the action of internal waves. Changes in the heat content of the deep ocean are thus far more sensitive to the air-sea thermal interchanges than previously considered. Our findings require a reassessment of the role of the Southern Ocean in determining the impact of atmospheric warming on deep oceanic waters.

Roles of the Okhotsk Sea and Gulf of Alaska in forming the North Pacific Intermediate Water

Recently obtained World Ocean Circulation Experiment (WOCE) sections and pre-WOCE hydrography are used to study the water-mass structure and formation and transformation of North Pacific Intermediate Water (NPIW). Five neutral density surfaces are selected and mapped, encompassing NPIW from 400 to 900 m in the subtropical latitudes with a distance of ∼100 m between a pair of surfaces. NPIW is defined as a subtropical gyre salinity minimum which is well followed by a neutral density surface σN = 26.9. Formation and transformation of NPIW is examined by the mapped Turner angle on neutral density surfaces. Apparent diffusive double diffusion is found in the Alaskan gyre on σN = 26.5 neutral surface, in the northwest subpolar gyre and the Okhotsk Sea on σN = 26.9 neutral surface, and mainly in the Okhotsk Sea on the two deep neutral surfaces σN =27.2 and σN = 27.4. These diffusive regions indicate transformation sources for NPIW. Along with additional information of potential vorticity and stream function, it is found that there are two different NPIW formation sources: one in the Gulf of Alaska characterized by high potential vorticity and the other in the Okhotsk Sea characterized by low potential vorticity. The former lies shallower at σN =26.2–26.5, but its effect deepens to NPIW core density level at σN = 26.8 on the basis of potential vorticity distribution. The latter includes the influence of the northwest subpolar gyre and extends much deeper to σN = 27.4. We call them Gulf of Alaska Intermediate Water (GAIW) and Okhotsk Intermediate Water (OIW), respectively. GAIW contributes to NPIW in the eastern part of the subtropical gyre east of date line, whilst OIW dominates in the west and entire lower part of NPIW. Seasonal flow stream function mapped on neutral surfaces shows that the contribution of GAIW to NPIW occurs mainly in the wintertime, because in winter a significant northward shift of zero wind stress curl makes the Gulf of Alaska an additional source for NPIW.

In Situ Calibration of Optode-Based Oxygen Sensors

Eleven optode-based oxygen sensors were used for shipboard hydrographic casts in the North Pacific. Oxygen data from the optode sensors were compared with high-quality oxygen data obtained with discrete water samples, and the performance of the sensors was evaluated. The response of the sensing foil of the optode decreases with increasing ambient pressure, and this pressure effect was found to decrease the response by 3.2% (1000 dbar) 1 . A new calibration equation for the optode sensors was proposed. On the basis of oxygen data from water samples, the optode sensors were calibrated so that the reproducibility was less than 1%. High-quality oxygen profiles from the optode were obtained for fast-profiling conductivity– temperature–depth (CTD) observations, by compensating for the temperature-dependent delay in the optode data due to the slow response time of the optode.

Distribution of total carbonate and related properties in the North Pacific along 30°N

Cross sections of the total carbonate (TC), titration alkalinity (TA), and pH were observed during the WOCE/JGOFS-P2 cruise (30°N, 130°E –122°W). The increases in TC in the years since 1973 are calculated by the comparison of TC data observed during P2 and those observed during the Geochemical Ocean Section Study. At stations east of 170°W, TC has increased proportionally to the atmospheric increase of CO2 in water shallower than the σθ = 26.20 density surface, while at stations west of 180°E TC had increased proportionally to the atmospheric increase only in water shallower than the σθ = 25.40 density surface. Increase in the column inventory of TC is 143±32 gC/m2 on average and does not significantly differ with longitude.

In Situ Calibration of the SeaBird 9plus CTD Thermometer

Abstract A Sea-Bird Electronics (SBE 35) deep ocean reference thermometer is used with the SBE 9plus CTD system to calibrate the SBE 3 ocean thermometers of the CTD. The SBE 35 is standardized in water-triple-point and gallium-melting-point cells. The SBE 3 is calibrated with the SBE 35 under the assumption that discrepancies between SBE 3 and SBE 35 data are due to pressure sensitivity, the viscous heating effect, and time drift of the SBE 3. Based on the results of an in situ calibration, the pressure sensitivity and the viscous heating effect were evaluated for 11 SBE 3 thermometers. Three SBE 3s showed little pressure sensitivity, and eight had pressure sensitivities of 1–2 mK at 6000 dbar. The average viscous heating effect on the standard SBE 3 measurements was 0.5 mK. Both the accuracy and precision of the in situ calibrated SBE 3 data at depths greater than 2000 dbar were 0.4 mK relative to the SBE 35 reference.

Decadal Changes in Bomb-Produced Radiocarbon in the Pacific Ocean from the 1990s to 2000s

In the 2000s, radiocarbon in dissolved inorganic carbon was measured during 7 revisit cruises along the lines of the World Ocean Circulation Experiment in the Pacific Ocean. Comparison of 14C data along these lines from the 1990s and 2000s revealed decadal changes of 14C concentration in the thermocline, most of which were due to temporal changes in the bomb-produced 14C. Vertical profiles and vertical-integrated inventories of the bomb 14C in the subarctic and equatorial regions did not change appreciably. In the southern subtropical region, 14C decreased in the upper thermocline from the surface to ~500 m depth. In contrast, 14C increased in the lower thermocline below ~500 m depth. The opposing directions in 14C change resulted in small temporal changes in the total inventory of bomb 14C. On the other hand, the water-column inventory significantly decreased in the northwestern subtropical region due to the 14C decrease in the upper thermocline. These decadal changes in bomb 14C indicate that the turnover time of thermocline circulation in the northwestern subtropical region is faster than that in the southern subtropical region, and imply an interbasin transport of bomb 14C from the North Pacific to other basins.