Macdonald Alison M.

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Macdonald

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Alison M.

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Interannual variation in summer N2O concentration in the hypoxic region of the northern Gulf of Mexico, 1985–2007

(Copernicus Publications on behalf of the European Geosciences Union, 2013-11-01) Kim, I.-N. ; Lee, K. ; Bange, Hermann W. ; Macdonald, Alison M.

Microbial nitrous oxide (N2O) production in the ocean is enhanced under low-oxygen (O2) conditions. This is especially important in the context of increasing hypoxia (i.e., oceanic zones with extremely reduced O2 concentrations). Here, we present a study on the interannual variation in summertime nitrous oxide (N2O) concentrations in the bottom waters of the northern Gulf of Mexico (nGOM), which is well-known as the site of the second largest seasonally occurring hypoxic zone worldwide. To this end we developed a simple model that computes bottom-water N2O concentrations with a tri-linear ΔN2O/O2 relationship based on water-column O2 concentrations, derived from summer (July) Texas–Louisiana shelf-wide hydrographic data between 1985 and 2007. ΔN2O (i.e., excess N2O) was computed including nitrification and denitrification as the major microbial production and consumption pathways of N2O. The mean modeled bottom-water N2O concentration for July in the nGOM was 14.5 ± 2.3 nmol L−1 (min: 11.0 ± 4.5 nmol L−1 in 2000 and max: 20.6 ± 11.3 nmol L−1 in 2002). The mean bottom-water N2O concentrations were significantly correlated with the areal extent of hypoxia in the nGOM. Our modeling analysis indicates that the nGOM is a persistent summer source of N2O, and nitrification is dominating N2O production in this region. Based on the ongoing increase in the areal extent of hypoxia in the nGOM, we conclude that N2O production (and its subsequent emissions) from this environmentally stressed region will probably continue to increase into the future.
Deep ocean changes near the Western Boundary of the South Pacific Ocean

(American Meteorological Society, 2013-10) Sloyan, Bernadette M. ; Wijffels, Susan E. ; Tilbrook, Bronte ; Katsumata, Katsuro ; Murata, Akihiko ; Macdonald, Alison M.

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 50° and 20°S. 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 (γn > 28.1 kg m−3) freshening, heating, or isopycnal heave suggests that the deep ocean stratification change is the 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.
Investigating subsurface pathways of Fukushima cesium in the Northwest Pacific

(American Geophysical Union, 2019-06-18) Cedarholm, Ella R. ; Rypina, Irina I. ; Macdonald, Alison M. ; Yoshida, Sachiko

Advective pathways for Fukushima Daiichi Nuclear Power Plant (FDNPP)‐derived cesium observed in 2013 at 166°E south of the Kuroshio Extension (KE) at >500 m on the 26.5σθ isopycnal are investigated. Attention is paid to the KE's role in shaping these pathways. Using a high‐resolution model, particle trajectories were simulated backward and forward in time on 26.5σθ between the 2013 observations and the 2011 source. A large fraction of backtracked trajectories interacted with the mixed layer just offshore of the FDNPP. The likeliest pathway reaching the deepest 2013 observed cesium location runs along the KE out to ~165°E, where it turns sharply southward. Forward trajectory statistics suggest that for 26.5σθ waters originating north of the KE, this current acted as a permeable barrier west of 155–160°E. The deepest 2011 model mixed layers suggest that FDNPP‐derived radionuclides may have been present at 30°N in 2013 at greater depths and densities (700 m; 26.8σθ).
The WOCE–era 3–D Pacific Ocean circulation and heat budget

( 2009-08-17) Macdonald, Alison M. ; Mecking, Sabine ; Toole, John M. ; Robbins, Paul E. ; Johnson, Gregory C. ; Wijffels, Susan E. ; Talley, Lynne D. ; Cook, Margaret F.

To address questions concerning the intensity and spatial structure of the 3–dimensional circulation within the Pacific Ocean and the associated advective and diffusive property flux divergences, data from approximately 3000 high–quality hydrographic stations collected on 40 zonal and meridional cruises have been merged into a physically consistent model. The majority of the stations were occupied as part of the World Ocean Circulation Experiment (WOCE), which took place in the 1990s. These data are supplemented by a few pre–WOCE surveys of similar quality, and time–averaged direct–velocity and historical hydrographic measurements about the equator. An inverse box model formalism is employed to estimate the absolute along–isopycnal velocity field, the magnitude and spatial distribution of the associated diapycnal flow and the corresponding diapycnal advective and diffusive property flux divergences. The resulting large–scale WOCE Pacific circulation can be described as two shallow overturning cells at mid– to low latitudes, one in each hemisphere, and a single deep cell which brings abyssal waters from the Southern Ocean into the Pacific where they upwell across isopycnals and are returned south as deep waters. Upwelling is seen to occur throughout most of the basin with generally larger dianeutral transport and greater mixing occurring at depth. The derived pattern of ocean heat transport divergence is compared to published results based on air–sea flux estimates. The synthesis suggests a strongly east/west oriented pattern of air–sea heat flux with heat loss to the atmosphere throughout most of the western basins, and a gain of heat throughout the tropics extending poleward through the eastern basins. The calculated meridional heat transport agrees well with previous hydrographic estimates. Consistent with many of the climatologies at a variety of latitudes as well, our meridional heat transport estimates tend toward lower values in both hemispheres.
Accelerated freshening of Antarctic Bottom Water over the last decade in the Southern Indian Ocean

(American Association for the Advancement of Science, 2017-01-25) Menezes, Viviane V. ; Macdonald, Alison M. ; Schatzman, Courtney

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.
Spreading pathways of Pilgrim Nuclear Power Station wastewater in and around Cape Cod Bay: Estimates from ocean drifter observations

(Elsevier, 2022-10-19) Rypina, Irina I. ; Macdonald, Alison ; Yoshida, Sachiko ; Manning, James P. ; Gregory, Margaret ; Rozen, Nimrod ; Buesseler, Ken

Near-surface drifter observations were used to study the spreading pathways in and around the Cape Cod Bay from a source region located just offshore of the Pilgrim Nuclear Power Station. The study was motivated by the recent closing of the power plant and a possible release of accumulated wastewater. The investigation applies several different techniques to the drifter data set to estimate and quantify various aspects of the circulation and spreading. Our goal was two-fold: first, to better understand and predict the fate of the Pilgrim wastewater should it be released; and second, to review, compare, and contrast several useful techniques that can be applied to drifter datasets in other parts of the global ocean. Our analysis suggests weaker spreading of the wastewater plume inside the Bay than outside, and sensitivity of the advection pathways to the location of the release. Statistical techniques predicted that part of the plume would likely be advected cyclonically around the inner coastline of the Bay towards the more quiescent eastern regions, while another part of the plume would likely pass close to the tip of Cape Cod and the beaches of the Outer Cape. For the soluble radionuclides, the levels observed in our statistical model offshore of Provincetown and Dennis/Brewster will be at least 100 times smaller than the initial concentrations.
Fukushima-derived radionuclides in the ocean and biota off Japan

(National Academy of Sciences, 2012-04-02) Buesseler, Ken O. ; Jayne, Steven R. ; Fisher, Nicholas S. ; Rypina, Irina I. ; Baumann, Hannes ; Baumann, Zofia ; Breier, Crystaline F. ; Douglass, Elizabeth M. ; George, Jennifer ; Macdonald, Alison M. ; Miyamoto, Hiroomi ; Nishikawa, Jun ; Pike, Steven M. ; Yoshida, Sachiko
Short-term dispersal of Fukushima-derived radionuclides off Japan : modeling efforts and model-data intercomparison

(Copernicus Publications on behalf of the European Geosciences Union, 2013-07-24) Rypina, Irina I. ; Jayne, Steven R. ; Yoshida, Sachiko ; Macdonald, Alison M. ; Douglass, Elizabeth M. ; Buesseler, Ken O.

The Great East Japan Earthquake and tsunami that caused a loss of power at the Fukushima nuclear power plants (FNPP) resulted in emission of radioactive isotopes into the atmosphere and the ocean. In June of 2011, an international survey measuring a variety of radionuclide isotopes, including 137Cs, was conducted in surface and subsurface waters off Japan. This paper presents the results of numerical simulations specifically aimed at interpreting these observations and investigating the spread of Fukushima-derived radionuclides off the coast of Japan and into the greater Pacific Ocean. Together, the simulations and observations allow us to study the dominant mechanisms governing this process, and to estimate the total amount of radionuclides in discharged coolant waters and atmospheric airborne radionuclide fallout. The numerical simulations are based on two different ocean circulation models, one inferred from AVISO altimetry and NCEP/NCAR reanalysis wind stress, and the second generated numerically by the NCOM model. Our simulations determine that > 95% of 137Cs remaining in the water within ~600 km of Fukushima, Japan in mid-June 2011 was due to the direct oceanic discharge. The estimated strength of the oceanic source is 16.2 ± 1.6 PBq, based on minimizing the model-data mismatch. We cannot make an accurate estimate for the atmospheric source strength since most of the fallout cesium had left the survey area by mid-June. The model explained several key features of the observed 137Cs distribution. First, the absence of 137Cs at the southernmost stations is attributed to the Kuroshio Current acting as a transport barrier against the southward progression of 137Cs. Second, the largest 137Cs concentrations were associated with a semi-permanent eddy that entrained 137Cs-rich waters, collecting and stirring them around the eddy perimeter. Finally, the intermediate 137Cs concentrations at the westernmost stations are attributed to younger, and therefore less Cs-rich, coolant waters that continued to leak from the reactor in June of that year.
International Quality-Controlled Ocean Database (IQuOD) v0.1: the temperature uncertainty specification

(Frontiers Media, 2021-06-11) Cowley, Rebecca ; Killick, Rachel E. ; Boyer, Tim ; Gouretski, Viktor ; Reseghetti, Franco ; Kizu, Shoichi ; Palmer, Matthew D. ; Cheng, Lijing ; Storto, Andrea ; Le Menn, Marc ; Simoncelli, Simona ; Macdonald, Alison M. ; Domingues, Catia M.

Ocean temperature observations are crucial for a host of climate research and forecasting activities, such as climate monitoring, ocean reanalysis and state estimation, seasonal-to-decadal forecasts, and ocean forecasting. For all of these applications, it is crucial to understand the uncertainty attached to each of the observations, accounting for changes in instrument technology and observing practices over time. Here, we describe the rationale behind the uncertainty specification provided for all in situ ocean temperature observations in the International Quality-controlled Ocean Database (IQuOD) v0.1, a value-added data product served alongside the World Ocean Database (WOD). We collected information from manufacturer specifications and other publications, providing the end user with uncertainty estimates based mainly on instrument type, along with extant auxiliary information such as calibration and collection method. The provision of a consistent set of observation uncertainties will provide a more complete understanding of historical ocean observations used to examine the changing environment. Moving forward, IQuOD will continue to work with the ocean observation, data assimilation and ocean climate communities to further refine uncertainty quantification. We encourage submissions of metadata and information about historical practices to the IQuOD project and WOD.
Reviews and syntheses : Ocean iron fertilization experiments – past, present, and future looking to a future Korean Iron Fertilization Experiment in the Southern Ocean (KIFES) project

(Copernicus Publications on behalf of the European Geosciences Union, 2018-10-05) Yoon, Joo-Eun ; Yoo, Kyu-Cheul ; Macdonald, Alison M. ; Yoon, Ho-Il ; Park, Ki-Tae ; Yang, Eun Jin ; Kim, Hyun-Cheol ; Lee, Jae Il ; Lee, Min Kyung ; Jung, Jinyoung ; Park, Jisoo ; Lee, Jiyoung ; Kim, Soyeon ; Kim, Seong-Su ; Kim, Kitae ; Kim, Il-Nam

Since the start of the industrial revolution, human activities have caused a rapid increase in atmospheric carbon dioxide (CO2) concentrations, which have, in turn, had an impact on climate leading to global warming and ocean acidification. Various approaches have been proposed to reduce atmospheric CO2. The Martin (or iron) hypothesis suggests that ocean iron fertilization (OIF) could be an effective method for stimulating oceanic carbon sequestration through the biological pump in iron-limited, high-nutrient, low-chlorophyll (HNLC) regions. To test the Martin hypothesis, 13 artificial OIF (aOIF) experiments have been performed since 1990 in HNLC regions. These aOIF field experiments have demonstrated that primary production (PP) can be significantly enhanced by the artificial addition of iron. However, except in the Southern Ocean (SO) European Iron Fertilization Experiment (EIFEX), no significant change in the effectiveness of aOIF (i.e., the amount of iron-induced carbon export flux below the winter mixed layer depth, MLD) has been detected. These results, including possible side effects, have been debated amongst those who support and oppose aOIF experimentation, and many questions concerning the effectiveness of scientific aOIF, environmental side effects, and international aOIF law frameworks remain. In the context of increasing global and political concerns associated with climate change, it is valuable to examine the validity and usefulness of the aOIF experiments. Furthermore, it is logical to carry out such experiments because they allow one to study how plankton-based ecosystems work by providing insight into mechanisms operating in real time and under in situ conditions. To maximize the effectiveness of aOIF experiments under international aOIF regulations in the future, we therefore suggest a design that incorporates several components. (1) Experiments conducted in the center of an eddy structure when grazing pressure is low and silicate levels are high (e.g., in the SO south of the polar front during early summer). (2) Shipboard observations extending over a minimum of ∼ 40 days, with multiple iron injections (at least two or three iron infusions of ∼ 2000kg with an interval of ∼ 10–15 days to fertilize a patch of 300km2 and obtain a ∼ 2nM concentration). (3) Tracing of the iron-fertilized patch using both physical (e.g., a drifting buoy) and biogeochemical (e.g., sulfur hexafluoride, photosynthetic quantum efficiency, and partial pressure of CO2) tracers. (4) Employment of neutrally buoyant sediment traps (NBST) and application of the water-column-derived thorium-234 (234Th) method at two depths (i.e., just below the in situ MLD and at the winter MLD), with autonomous profilers equipped with an underwater video profiler (UVP) and a transmissometer. (5) Monitoring of side effects on marine/ocean ecosystems, including production of climate-relevant gases (e.g., nitrous oxide, N2O; dimethyl sulfide, DMS; and halogenated volatile organic compounds, HVOCs), decline in oxygen inventory, and development of toxic algae blooms, with optical-sensor-equipped autonomous moored profilers and/or autonomous benthic vehicles. Lastly, we introduce the scientific aOIF experimental design guidelines for a future Korean Iron Fertilization Experiment in the Southern Ocean (KIFES).
Drifter-based estimate of the 5 year dispersal of Fukushima-derived radionuclides

(John Wiley & Sons, 2014-11-28) Rypina, Irina I. ; Jayne, Steven R. ; Yoshida, Sachiko ; Macdonald, Alison M. ; Buesseler, Ken O.

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.
N2O dynamics in the western Arctic Ocean during the summer of 2017

(Nature Research, 2021-06-15) Heo, Jang-Mu ; Kim, Seong-Su ; Kang, Sung-Ho ; Yang, Eun Jin ; Park, Ki-Tae ; Jung, Jinyoung ; Cho, Kyoung-Ho ; Kim, Ju-Hyoung ; Macdonald, Alison M. ; Yoon, Joo-Eun ; Kim, Hyo-Ryeon ; Eom, Sang-Min ; Lim, Jae-Hyun ; Kim, Il-Nam

The western Arctic Ocean (WAO) has experienced increased heat transport into the region, sea-ice reduction, and changes to the WAO nitrous oxide (N2O) cycles from greenhouse gases. We investigated WAO N2O dynamics through an intensive and precise N2O survey during the open-water season of summer 2017. The effects of physical processes (i.e., solubility and advection) were dominant in both the surface (0–50 m) and deep layers (200–2200 m) of the northern Chukchi Sea with an under-saturation of N2O. By contrast, both the surface layer (0–50 m) of the southern Chukchi Sea and the intermediate (50–200 m) layer of the northern Chukchi Sea were significantly influenced by biogeochemically derived N2O production (i.e., through nitrification), with N2O over-saturation. During summer 2017, the southern region acted as a source of atmospheric N2O (mean: + 2.3 ± 2.7 μmol N2O m−2 day−1), whereas the northern region acted as a sink (mean − 1.3 ± 1.5 μmol N2O m−2 day−1). If Arctic environmental changes continue to accelerate and consequently drive the productivity of the Arctic Ocean, the WAO may become a N2O “hot spot”, and therefore, a key region requiring continued observations to both understand N2O dynamics and possibly predict their future changes.
Multi-iteration approach to studying tracer spreading using drifter data

(American Meteorological Society, 2017-01-31) Rypina, Irina I. ; Fertitta, David ; Macdonald, Alison M. ; Yoshida, Sachiko ; Jayne, Steven R.

A novel multi-iteration statistical method for studying tracer spreading using drifter data is introduced. The approach allows for the best use of the available drifter data by making use of a simple iterative procedure, which results in the statistically probable map showing the likelihood that a tracer released at some source location would visit different geographical regions, along with the associated arrival travel times. The technique is tested using real drifter data in the North Atlantic. Two examples are considered corresponding to sources in the western and eastern North Atlantic Ocean, that is, Massachusetts Bay–like and Irish Sea–like sources, respectively. In both examples, the method worked well in estimating the statistics of the tracer transport pathways and travel times throughout the entire North Atlantic. The role of eddies versus mean flow is quantified using the same technique, and eddies are shown to significantly broaden the spread of a tracer. The sensitivity of the results to the size of the source domain is investigated and causes for this sensitivity are discussed.
Estimating remineralized phosphate and its remineralization rate in the northern East China Sea during Summer 1997 : a snapshot study before Three-Gorges Dam construction

(Terrestrial, Atmospheric and Oceanic Sciences, 2016-12) Kim, Hyun-Cheol ; Kim, Il-Nam ; Macdonald, Alison M. ; Park, Ki-Tae ; Kim, Ju-Hyoung ; Yoon, Joo-Eun ; Lee, Tongsup

The northern East China Sea (a.k.a., “The South Sea”) is a dynamic zone that exerts a variety of effects on the marine ecosystem due to Three-Gorges Dam construction. As the northern East China Sea region is vulnerable to climate forcing and anthropogenic impacts, it is important to investigate how the remineralization rate in the northern East China Sea has changed in response to such external forcing. We used an historical hydrographic dataset from August 1997 to obtain a baseline for future comparison. We estimate the amount of remineralized phosphate by decomposing the physical mixing and biogeochemical process effect using water column measurements (temperature, salinity, and phosphate). The estimated remineralized phosphate column inventory ranged from 0.8 to 42.4 mmol P m-2 (mean value of 15.2 ± 12.0 mmol P m-2). Our results suggest that the Tsushima Warm Current was a strong contributor to primary production during the summer of 1997 in the study area. The estimated summer (June - August) remineralization rate in the region before Three-Gorges Dam construction was 18 ± 14 mmol C m-2 d-1.
Spatial and temporal variabilities of spring Asian dust events and their impacts on chlorophyll-a concentrations in the western North Pacific Ocean

(John Wiley & Sons, 2017-02-15) Yoon, Joo-Eun ; Kim, Kitae ; Macdonald, Alison M. ; Park, Ki-Tae ; Kim, Hyun-Cheol ; Yoo, Kyu-Cheul ; Yoon, Ho-Il ; Yang, Eun Jin ; Jung, Jinyoung ; Lim, Jae-Hyun ; Kim, Ju-Hyoung ; Lee, Jiyoung ; Choi, Tae-Jun ; Song, Jae-Min ; Kim, Il-Nam

As the western North Pacific Ocean is located downwind of the source regions for spring Asian dust, it is an ideal location for determining the response of open waters to these events. Spatial analysis of spring Asian dust events from source regions to the western North Pacific, using long-term daily aerosol index data, revealed three different transport pathways supported by the westerly wind system: one passing across the northern East/Japan Sea (40°N–50°N), a second moving over the entire East/Japan Sea (35°N–55°N), and a third flowing predominantly over the Siberian continent (>50°N). Our results indicate that strong spring Asian dust events can increase ocean primary productivity by more than 70% (>2-fold increase in chlorophyll-a concentrations) compared to weak/nondust conditions. Therefore, attention should be paid to the recent downturn in the number of spring Asian dust events and to the response of primary production in the western North Pacific to this change.
Changes in ocean heat, carbon content, and ventilation : a review of the first decade of GO-SHIP Global Repeat Hydrography

( 2015-05-30) Talley, Lynne D. ; Feely, Richard A. ; Sloyan, Bernadette M. ; Wanninkhof, Rik ; Baringer, Molly O. ; Bullister, John L. ; Carlson, Craig A. ; Doney, Scott C. ; Fine, Rana A. ; Firing, Eric ; Gruber, Nicolas ; Hansell, Dennis A. ; Ishii, Masayoshi ; Johnson, Gregory ; Katsumata, K. ; Key, Robert M. ; Kramp, Martin ; Langdon, Chris ; Macdonald, Alison M. ; Mathis, Jeremy T. ; McDonagh, Elaine L. ; Mecking, Sabine ; Millero, Frank J. ; Mordy, Calvin W. ; Nakano, T. ; Sabine, Chris L. ; Smethie, William M. ; Swift, James H. ; Tanhua, Toste ; Thurnherr, Andreas M. ; Warner, Mark J. ; Zhang, Jia-Zhong

The ocean, a central component of Earth’s climate system, is changing. Given the global scope of these changes, highly accurate measurements of physical and biogeochemical properties need to be conducted over the full water column, spanning the ocean basins from coast to coast, and repeated every decade at a minimum, with a ship-based observing system. Since the late 1970s, when the Geochemical Ocean Sections Study (GEOSECS) conducted the first global survey of this kind, the World Ocean Circulation Experiment (WOCE) and Joint Global Ocean Flux Study (JGOFS), and now the Global Ocean Ship-based Hydrographic Investigations Program (GO-SHIP) have collected these “reference standard” data that allow quantification of ocean heat and carbon uptake, and variations in salinity, oxygen, nutrients, and acidity on basin scales. The evolving GO-SHIP measurement suite also provides new global information about dissolved organic carbon, a large bioactive reservoir of carbon.
Confirmation of ENSO-Southern Ocean teleconnections using satellite-derived SST

(MDPI AG, 2018-02-23) Ferster, Brady S. ; Subrahmanyam, Bulusu ; Macdonald, Alison M.

The Southern Ocean is the focus of many physical, chemical, and biological analyses due to its global importance and highly variable climate. This analysis of sea surface temperatures (SST) and global teleconnections shows that SSTs are significantly spatially correlated with both the Antarctic Oscillation and the Southern Oscillation, with spatial correlations between the indices and standardized SST anomalies approaching 1.0. Here, we report that the recent positive patterns in the Antarctic and Southern Oscillations are driving negative (cooling) trends in SST in the high latitude Southern Ocean and positive (warming) trends within the Southern Hemisphere sub-tropics and mid-latitudes. The coefficient of regression over the 35-year period analyzed implies that standardized temperatures have warmed at a rate of 0.0142 per year between 1982 and 2016 with a monthly standard error in the regression of 0.0008. Further regression calculations between the indices and SST indicate strong seasonality in response to changes in atmospheric circulation, with the strongest feedback occurring throughout the austral summer and autumn.
Latitudinal distributions and controls of bacterial community composition during the summer of 2017 in western Arctic surface waters (from the Bering Strait to the Chukchi Borderland)

(Nature Research, 2019-11-14) Lee, Jiyoung ; Kang, Sung-Ho ; Yang, Eun Jin ; Macdonald, Alison M. ; Joo, Hyoung Min ; Park, Junhyung ; Kim, Kwangmin ; Lee, Gi Seop ; Kim, Ju-Hyoung ; Yoon, Joo-Eun ; Kim, Seong-Su ; Lim, Jae-Hyun ; Kim, Il-Nam

The western Arctic Ocean is experiencing some of the most rapid environmental changes in the Arctic. However, little is known about the microbial community response to these changes. Employing observations from the summer of 2017, this study investigated latitudinal variations in bacterial community composition in surface waters between the Bering Strait and Chukchi Borderland and the factors driving the changes. Results indicate three distinctive communities. Southern Chukchi bacterial communities are associated with nutrient rich conditions, including genera such as Sulfitobacter, whereas the northern Chukchi bacterial community is dominated by SAR clades, Flavobacterium, Paraglaciecola, and Polaribacter genera associated with low nutrients and sea ice conditions. The frontal region, located on the boundary between the southern and northern Chukchi, is a transition zone with intermediate physical and biogeochemical properties; however, bacterial communities differed markedly from those found to the north and south. In the transition zone, Sphingomonas, with as yet undetermined ecological characteristics, are relatively abundant. Latitudinal distributions in bacterial community composition are mainly attributed to physical and biogeochemical characteristics, suggesting that these communities are susceptible to Arctic environmental changes. These findings provide a foundation to improve understanding of bacterial community variations in response to a rapidly changing Arctic Ocean.
Observed eastward progression of the Fukushima 134Cs signal across the North Pacific

(John Wiley & Sons, 2015-09-09) Yoshida, Sachiko ; Macdonald, Alison M. ; Jayne, Steven R. ; Rypina, Irina I. ; Buesseler, Ken O.

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.
Pacific anthropogenic carbon between 1991 and 2017

(American Geophysical Union, 2019-04-29) Carter, Brendan ; Feely, Richard A. ; Wanninkhof, Rik ; Kouketsu, Shinya ; Sonnerup, Rolf E. ; Pardo, Paula Conde ; Sabine, Christopher L. ; Johnson, Gregory C. ; Sloyan, Bernadette M. ; Murata, Akihiko ; Mecking, Sabine ; Tilbrook, Bronte ; Speer, Kevin G. ; Talley, Lynne D. ; Millero, Frank J. ; Wijffels, Susan E. ; Macdonald, Alison M. ; Gruber, Nicolas ; Bullister, John L.

We estimate anthropogenic carbon (Canth) accumulation rates in the Pacific Ocean between 1991 and 2017 from 14 hydrographic sections that have been occupied two to four times over the past few decades, with most sections having been recently measured as part of the Global Ocean Ship‐based Hydrographic Investigations Program. The rate of change of Canth is estimated using a new method that combines the extended multiple linear regression method with improvements to address the challenges of analyzing multiple occupations of sections spaced irregularly in time. The Canth accumulation rate over the top 1,500 m of the Pacific increased from 8.8 (±1.1, 1σ) Pg of carbon per decade between 1995 and 2005 to 11.7 (±1.1) PgC per decade between 2005 and 2015. For the entire Pacific, about half of this decadal increase in the accumulation rate is attributable to the increase in atmospheric CO2, while in the South Pacific subtropical gyre this fraction is closer to one fifth. This suggests a substantial enhancement of the accumulation of Canth in the South Pacific by circulation variability and implies that a meaningful portion of the reinvigoration of the global CO2 sink that occurred between ~2000 and ~2010 could be driven by enhanced ocean Canth uptake and advection into this gyre. Our assessment suggests that the accuracy of Canth accumulation rate reconstructions along survey lines is limited by the accuracy of the full suite of hydrographic data and that a continuation of repeated surveys is a critical component of future carbon cycle monitoring.

Macdonald Alison M.

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