Alison M. Macdonald

Fukushima-derived radionuclides in the ocean and biota off Japan

The Tōhoku earthquake and tsunami of March 11, 2011, resulted in unprecedented radioactivity releases from the Fukushima Dai-ichi nuclear power plants to the Northwest Pacific Ocean. Results are presented here from an international study of radionuclide contaminants in surface and subsurface waters, as well as in zooplankton and fish, off Japan in June 2011. A major finding is detection of Fukushima-derived 134Cs and 137Cs throughout waters 30–600 km offshore, with the highest activities associated with near-shore eddies and the Kuroshio Current acting as a southern boundary for transport. Fukushima-derived Cs isotopes were also detected in zooplankton and mesopelagic fish, and unique to this study we also find 110mAg in zooplankton. Vertical profiles are used to calculate a total inventory of ∼2 PBq 137Cs in an ocean area of 150,000 km2. Our results can only be understood in the context of our drifter data and an oceanographic model that shows rapid advection of contaminants further out in the Pacific. Importantly, our data are consistent with higher estimates of the magnitude of Fukushima fallout and direct releases [Stohl et al. (2011) Atmos Chem Phys Discuss 11:28319–28394; Bailly du Bois et al. (2011) J Environ Radioact, 10.1016/j.jenvrad.2011.11.015]. We address risks to public health and marine biota by showing that though Cs isotopes are elevated 10–1,000× over prior levels in waters off Japan, radiation risks due to these radionuclides are below those generally considered harmful to marine animals and human consumers, and even below those from naturally occurring radionuclides.

Property fluxes at 30°S and their implications for the Pacific‐Indian throughflow and the global heat budget

Six hydrographic basinwide sections, two in each of the three major ocean basins, are employed in a set of inverse calculations to determine the extent of exchange between the Pacific and Indian Oceans through the Indonesian Archipelago and the net global oceanic heat flux. All existing estimates of Indonesian Passage throughflow, including the largest (20 Sv), are consistent with the model constraints which combine data from the southern Pacific, Indian and Atlantic Oceans. The models and data are unable to limit the extent of the exchange, i.e., both smaller and larger throughflows produce physically reasonable circulation patterns. Seasonal and interannual variations, which have been found by other investigators and which are not resolved, suggest that in the long-term mean an estimate of about 10 Sv for the throughflow is most reasonable. Globally, at 30°S, the estimated net oceanic heat flux is −0.7 ± 0.1 PW (1 PW = 1015 W), dominated by a large (> 1 PW), southward flux in the Indian Ocean. Large equatorward heat flux values, O(0.8 PW), in the South Atlantic Basin are not consistent with the data. Therefore although the data are consistent with some water following the “warm water” return path for North Atlantic Deep Water (NADW), the “cold water” path must play the dominant role in the maintenance of the global thermohaline cell associated with the formation process of NADW.

Changes in Ocean Heat, Carbon Content, and Ventilation: A Review of the First Decade of GO-SHIP Global Repeat Hydrography.

Global ship-based programs, with highly accurate, full water column physical and biogeochemical observations repeated decadally since the 1970s, provide a crucial resource for documenting ocean change. The ocean, a central component of Earths climate system, is taking up most of Earths excess anthropogenic heat, with about 19% of this excess in the abyssal ocean beneath 2,000 m, dominated by Southern Ocean warming. The ocean also has taken up about 27% of anthropogenic carbon, resulting in acidification of the upper ocean. Increased stratification has resulted in a decline in oxygen and increase in nutrients in the Northern Hemisphere thermocline and an expansion of tropical oxygen minimum zones. Southern Hemisphere thermocline oxygen increased in the 2000s owing to stronger wind forcing and ventilation. The most recent decade of global hydrography has mapped dissolved organic carbon, a large, bioactive reservoir, for the first time and quantified its contribution to export production (∼20%) and deep-ocean oxygen utilization. Ship-based measurements also show that vertical diffusivity increases from a minimum in the thermocline to a maximum within the bottom 1,500 m, shifting our physical paradigm of the oceans overturning circulation.

Deep Ocean Changes near the Western Boundary of the South Pacific Ocean

Repeated occupations of two hydrographic sections in the southwest Pacific basin from the 1990s to 2000s track property changes of Antarctic Bottom Water (AABW). The largest property changes—warming, freshening, increase in total carbon, and decrease in oxygen—are found near the basin’s deep western boundary between 508 and 208S. The magnitude of the property changes decreases with increasing distance from the western boundary. At the deep western boundary, analysis of the relative importance of AABW (g n .28.1kgm 23 )freshening,heating,orisopycnalheavesuggeststhatthedeepoceanstratificationchangeisthe result of both warming and freshening processes. The consistent deep ocean changes near the western boundary of the southwest Pacific basin dispel the notion that the deep ocean is quiescent. High-latitude climate variability is being directly transmitted into the deep southwest Pacific basin and the global deep ocean through dynamic deep western boundary currents.

Observed eastward progression of the Fukushima 134Cs signal across the North Pacific

Radionuclide samples taken as part of hydrographic surveys at 30°N in the North Pacific reveal that the easternmost edge of Fukushima-derived 134Cs observed at 174.3°W in 2012 had progressed eastward across the basin to 160.6°W by 2013. The 2013 30°N observations indicate surface 134Cs concentrations of 3–5 Bq/m3 between 160°E and 160°W, slightly lower concentrations west of 160°E and no detectable signal east of 160.6°W. Profile samples show 134Cs penetration to 500 m west of 180° with shoaling penetration depth toward to the east. The near-uniform vertical distribution of 137Cs between 152°W and 121.3°W in the top 500 m is indicative of trace amounts of radionuclides remaining from weapons testing. The physical processes responsible for the deep 134Cs penetration in the western Pacific appear to be related to distinct water mass subduction pathways; however, the timing and rapidity of deep penetration over the broad scales observed has yet to be clarified.

Drifter-based estimate of the 5 year dispersal of Fukushima-derived radionuclides

Employing some 40 years of North Pacific drifter-track observations from the Global Drifter Program database, statistics defining the horizontal spread of radionuclides from Fukushima nuclear power plant into the Pacific Ocean are investigated over a time scale of 5 years. A novel two-iteration method is employed to make the best use of the available drifter data. Drifter-based predictions of the temporal progression of the leading edge of the radionuclide distribution are compared to observed radionuclide concentrations from research surveys occupied in 2012 and 2013. Good agreement between the drifter-based predictions and the observations is found.

Ocean Heat Transport

Abstract The ability to ascertain the implications of a changing climate are based first and foremost on our understanding the fundamental balances comprising earth’s climate. The global energy budget is integral to this understanding, and the poleward transport of heat by the oceans is an intrinsic component of the energy balance. In this chapter, we look to describe what is presently known about ocean heat transport and its role in the climate system. At the end of the last century, as part of the World Ocean Circulation Experiment (WOCE) and the Joint Global Ocean Flux Study (JGOFS), the ocean-going research community completed a decade-long international effort to explore the ocean through the acquisition of a single consistent and comparable set of high-quality, full-depth observations. This global-scale field campaign was designed to determine the mean steady-state basin-scale circulation and meridional ocean heat transport. At the time, it was recognized that this mean was unlikely to be stationary. It was also understood that patterns of variability that had yet to be measured would influence our eventual understanding of these one-time “synoptic” observations. A decade later, time series of ocean heat transport (from moorings, repeat XBT and CTD lines and profiling floats), although still quite short and either spatially and/or temporally sparse, are now becoming available. The WOCE/JGOFS programs of the 1990s have been extended through large international efforts first by the Climate Variability and Predictability program and now by the Global Ocean Ship-based Hydrographic Investigations Program. Through these newly acquired time series, improvements in analysis techniques, and recent modeling advances, we are beginning to describe ocean heat transport variability and to understand its possible response to and role in climate change. In this chapter, we focus on what has been learnt about ocean heat transport through in situ observations. We include some history, a detailed description of ocean heat transport computation and decomposition, and a discussion of the present state of the science, seeking to measure ocean heat transport variability.

Two decades of Pacific anthropogenic carbon storage and ocean acidification along Global Ocean Ship-based Hydrographic Investigations Program sections P16 and P02

A modified version of the extended multiple linear regression (eMLR) method is used to estimate anthropogenic carbon concentration (Canth) changes along the Pacific P02 and P16 hydrographic sections over the past two decades. P02 is a zonal section crossing the North Pacific at 30°N, and P16 is a meridional section crossing the North and South Pacific at ~150°W. The eMLR modifications allow the uncertainties associated with choices of regression parameters to be both resolved and reduced. Canth is found to have increased throughout the water column from the surface to ~1000 m depth along both lines in both decades. Mean column Canth inventory increased consistently during the earlier (1990s–2000s) and recent (2000s–2010s) decades along P02, at rates of 0.53 ± 0.11 and 0.46 ± 0.11 mol C m−2 a−1, respectively. By contrast, Canth storage accelerated from 0.29 ± 0.10 to 0.45 ± 0.11 mol C m−2 a−1 along P16. Shifts in water mass distributions are ruled out as a potential cause of this increase, which is instead attributed to recent increases in the ventilation of the South Pacific Subtropical Cell. Decadal changes along P16 are extrapolated across the gyre to estimate a Pacific Basin average storage between 60°S and 60°N of 6.1 ± 1.5 PgC decade−1 in the earlier decade and 8.8 ± 2.2 PgC decade−1 in the recent decade. This storage estimate is large despite the shallow Pacific Canth penetration due to the large volume of the Pacific Ocean. By 2014, Canth storage had changed Pacific surface seawater pH by −0.08 to −0.14 and aragonite saturation state by −0.57 to −0.82.A modified version of the extended multiple linear regression (eMLR) method is used to estimate anthropogenic carbon concentration (Canth) changes along the Pacific P02 and P16 hydrographic sections over the past two decades. P02 is a zonal section crossing the North Pacific at 30°N and P16 is a meridional section crossing the North and South Pacific at ~150°W. The eMLR modifications allow the uncertainties associated with choices of regression parameters to be both resolved and reduced. Canth is found to have increased throughout the water column from the surface to ~1000 m depth along both lines in both decades. Mean column Canth inventory increased consistently during the earlier (1990s-2000s) and recent (2000s-2010s) decades along P02, at rates of 0.53 ± 0.11 and 0.46 ± 0.11 mol C m-2 a˗1, respectively. By contrast, Canth storage accelerated from 0.29 ± 0.10 to 0.45 ± 0.11 mol C m˗2 a˗1 along P16. Shifts in water mass distributions are ruled out as a potential cause of this increase, which is instead attributed to recent increases in the ventilation of the South Pacific Subtropical Cell. Decadal changes along P16 are extrapolated across the gyre to estimate a Pacific Basin average storage between 60°S and 60°N of 6.1 ± 1.5 PgC decade˗1 in the earlier decade and 8.8 ± 2.2 PgC decade˗1 in the recent decade. This storage estimate is large despite the shallow Pacific Canth penetration due to the large volume of the Pacific Ocean. By 2014, Canth storage had changed Pacific surface seawater pH by ˗0.08 to ˗0.14 and aragonite saturation state by ˗0.57 to ˗0.82.

Accelerated freshening of Antarctic Bottom Water over the last decade in the Southern Indian Ocean

Accelerated freshening reduced warming of Antarctic Bottom Water from 2007 to 2016 in the Australian-Antarctic Basin. Southern Ocean abyssal waters, in contact with the atmosphere at their formation sites around Antarctica, not only bring signals of a changing climate with them as they move around the globe but also contribute to that change through heat uptake and sea level rise. A repeat hydrographic line in the Indian sector of the Southern Ocean, occupied three times in the last two decades (1994, 2007, and, most recently, 2016), reveals that Antarctic Bottom Water (AABW) continues to become fresher (0.004 ± 0.001 kg/g decade−1), warmer (0.06° ± 0.01°C decade−1), and less dense (0.011 ± 0.002 kg/m3 decade−1). The most recent observations in the Australian-Antarctic Basin show a particularly striking acceleration in AABW freshening between 2007 and 2016 (0.008 ± 0.001 kg/g decade−1) compared to the 0.002 ± 0.001 kg/g decade−1 seen between 1994 and 2007. Freshening is, in part, responsible for an overall shift of the mean temperature-salinity curve toward lower densities. The marked freshening may be linked to an abrupt iceberg-glacier collision and calving event that occurred in 2010 on the George V/Adélie Land Coast, the main source region of bottom waters for the Australian-Antarctic Basin. Because AABW is a key component of the global overturning circulation, the persistent decrease in bottom water density and the associated increase in steric height that result from continued warming and freshening have important consequences beyond the Southern Indian Ocean.

Meridional overturning circulation and heat transport observations in the Atlantic Ocean

Special supplement to the Bulletin of the American Meteorological Society vol.94, No. 8, August 2013