Taketo Hashioka

On the Southern Ocean CO2 uptake and the role of the biological carbon pump in the 21st century

The Southern Ocean is one of the key regions for global carbon uptake and it is under discussion how physical changes will alter its CO2 balance both directly and indirectly through changes in biological production. nHere we analyse a suite of eight RCP8.5 model simulations until 2100 from the MAREMIP and CMIP5 model intercomparison projects on changes in export production and CO2 uptake. We explore how the counter-acting effects of stronger winds (SAM signal, less stratification) and global warming (more stratification) affect CO2 fluxes in different models and different regions of the Southern Ocean. nThe models simulate a broad range of responses with no agreement on the dominance of the SAM or global warming signal or on nutrient or light as the dominant drivers for changes in export production. There is agreement on an increase in export production south of 58◦S and on a nutrient-driven decrease of export production in the region 30-44◦S (global warming signal). Based on a box-model, we can identify the most important drivers for the future CO2 uptake in the Southern Ocean where the pure increase of atmospheric CO2 has the largest effect, followed by the enhanced biological production and the larger effect of biological production on CO2 uptake at higher Revelle factor. The enhanced upwelling of carbon-rich deep water, and the effects of warming on the CO2 solubility and faster gas-exchange at higher wind-speeds are less important.

Effect of temperature‐dependent organic carbon decay on atmospheric pCO2

[1] Extendingy an almost universal observation that the rate of microbial activity increases with temperature, we propose that marine microbial activity was suppressed during previous glacial periods and allowed proportionally more organic carbon to be exported out of the surface ocean. A stronger organic carbon pump and therefore lower rain ratios of CaCO 3 to organic carbon may have contributed to the low atmospheric CO 2 content during the Last Glacial Maximum. Previous study of temperature-dependent export production (Laws et al., 2000) and our map of data-based, global distribution of the rain ratios lend support to todays rain ratios being controlled at least partly by temperature. A close examination with a high-resolution regional ocean ecosystem model indicates that the correlation between rain ratio and temperature is caused indeed by preferential remineralization of organic matter, but a part of the correlation is also driven by temperature-dependent community composition. An extrapolation of these results to the globe using a global carbon cycle box model with a module for sediments indicates that the drawdown of atmospheric CO 2 by the proposed mechanism is approximately 30 ppm. While this estimate is subject to uncertainty, the fact that it represents nearly one third of the glacial-interglacial variation in atmosphere pCO 2 suggests the potential importance of the new mechanism. Given the historical difficulty in explaining the full CO 2 amplitude with a single cause, we suggest that a set of multiple mechanisms were responsible and that the temperature-dependent POC degradation rate is one of them. We discuss two possible difficulties with our proposal that have to do with the potentially important role that ballasts play in organic carbon export and the possibility that enhanced biological pump is self limiting.

Development of a one‐dimensional ecosystem model including the iron cycle applied to the Oyashio region, western subarctic Pacific

[1]xa0To investigate the iron cycle at Station A4 in the Oyashio region of the western subarctic Pacific, we developed a 1-D ecosystem model consisting of 14 components including the iron cycle. The parameters associated with the iron cycle were optimized by assimilating monthly averaged data from time series observations for depth-integrated net primary production, nitrate, silicate, dissolved and particulate iron within the surface mixed layer (ML) and at two depths (200 and 300 m depth). The model successfully reproduced the observations and demonstrated that (1) on an annual basis, winter mixing of subsurface water supplies more dissolved iron (Fed) to the ML than does dust dissolution, (2) Fed concentration in the ML rapidly declines to near-depletion during the peak period of the diatom bloom in spring, which results in an increasing consumption ratio of silicate to nitrogenous nutrients by diatoms as they become more iron-limited, causing a more rapid decrease of silicate compared to that of nitrogenous nutrients in the ML, followed by the silicate limitation of diatoms, and (3) Fed supplied to the ML by dust dissolution and desorption from particulate iron, by alleviating iron limitation of phytoplankton, supports their continuous utilization of nitrate from spring to fall even though Fed concentration in the ML remains low after the peak spring bloom. The model explained quantitatively the above behavior of Fed and other nutrients associated with Fed over the annual cycle in the Oyashio region.

Potential impact of global warming on North Pacific spring blooms projected by an eddy-permitting 3-D ocean ecosystem model.

[1] Using an eddy-permitting ecosystem model with a projected physical environment from a high-resolution climate model, we explored the potential impact of global warming on spring blooms in the western North Pacific. We focused on statistically significant signals compared with natural variability. Considering 2xCO 2 conditions, maximum biomass during the spring bloom is found to occur 10 to 20 days earlier due to strengthened stratification, and in the subarctic region, the bloom to decrease in magnitude relative to pre-industrial simulation. However, in the northern part of the Kuroshio extension region where photosynthesis is not strongly limited by nutrients, the maximum biomass increases by 20 to 40% associated with rising temperatures, even though the annually averaged biomass slightly decreases. Our results reveal that even if global warming weakly affects annually averaged quantities, it could strongly affect certain species and biogeochemical processes which depend on seasonal events such as blooms.

Impacts of climate change on growth, migration and recruitment success of Japanese sardine (Sardinops melanostictus) in the western North Pacific

We developed a multi-trophic level ecosystem model by coupling physical, biogeochemical-plankton and fish models. An oceanic general circulation model was coupled with a lower trophic level ecosystem model and a Japanese sardine migration model, and applied to the western North Pacific. To investigate the impact of global warming on the pelagic fish ecosystem, such as Japanese sardine, we conducted numerical experiments of growth and migration of Japanese sardine using physical fields for the present day and future with a global warming scenario simulated by a high-resolution climate model. The model results demonstrated possible impacts of global warming on the growth and migration pattern of Japanese sardine. The growths of fish in the current main spawning region under the global warming scenario were significantly slower than those under the present climate scenario. Fish in this region will be at disadvantage for their recruitment under the global warming condition. Prey conditions in the spawning region were projected not to markedly change under global warming condition while water temperature increased. As a result sardine spawning ground was projected to shift towards more north areas. During the feeding migration period in summer, geographical distribution of juveniles fish was projected to shift northwards by one to two degrees latitude under the global warming condition following the change in the distribution of optimal temperature region for feeding. However, this northwards shift of the optimal temperature for feeding was minimized adjacent to the western North Pacific by the cooler water supply by the intensification of the Oyashio.

Coupled 1-D physical–biological model study of phytoplankton production at two contrasting time-series stations in the western North Pacific

A vertical one-dimensional physical–biological model is applied to clarify the mechanisms controlling the seasonality and interannual variability of primary production in the surface layer at two contrasting time-series stations, K2 (in the subarctic gyre) and S1 (in the subtropical gyre), in the western North Pacific. Using forcing based on realistic atmospheric and oceanic data, the model reproduces seasonal differences in the degree to which different controlling factors affect primary production between these two stations, primarily as a result of differences in the physical environment. At station K2, light intensity is an important factor controlling primary production in summer. After April, the mixed layer depth (MLD) becomes shallow, resulting in higher average light intensity, and the water column remains stratified until September; these sustain high primary production during this period. In contrast, at station S1, the supply of nutrients via entrainment is vital to sustaining production, because light intensity remains sufficient throughout the year. In summer, the relationship between nutricline depth and euphotic layer is a controlling factor. The simulations forced by the different atmospheric conditions for each year, respectively, show different MLD. In the 2012 simulation, the deep winter MLD (200xa0m) enhances primary production in the surface layer as compared to the other twoxa0years (2010 and 2011) simulations.

A lagrangian method combined with high-resolution ocean general circulation model to evaluate CO2 ocean sequestration

Publisher Summary A high-resolution model with a Lagrangian method is used to simulate the distribution and concentration of CO 2 injected into the mid-depth ocean over timescales ranging from a week to a few years. A high-resolution model with a Lagrangian tracer enables effects of eddy activities on dispersion of particles to be well represented. Comparison of Lagrangian with Eulerian tracers shows that the Lagrangian tracer avoids artificial diffusion and enables CO 2 maximum concentration at specific sites to be predicted, which helps in assessing CO 2 impact on the marine ecosystem. Ensemble experiments using high-resolution models explicitly deal with dispersion by advection due to mesoscale eddies which cannot be achieved by coarse-resolution models with implicit eddy diffusion, and enable the distribution of CO 2 maximum concentrations to be predicted. A high resolution model is demonstrated with this method is a powerful tool for assessing the effects of CO 2 injection on the marine environment.

Comparison of carbon cycle between the western Pacific subarctic and subtropical time-series stations: highlights of the K2S1 project

A comparative study of ecosystems and biogeochemistry at time-series stations in the subarctic gyre (K2) and subtropical region (S1) of the western North Pacific Ocean (K2S1 project) was conducted between 2010 and 2013 to collect essential data about the ecosystem and biological pump in each area and to provide a baseline of information for predicting changes in biologically mediated material cycles in the future. From seasonal chemical and biological observations, general oceanographic settings were verified and annual carbon budgets at both stations were determined. Annual mean of phytoplankton biomass and primary productivity at the oligotrophic station S1 were comparable to that at the eutrophic station K2. Based on chemical/physical observations and numerical simulations, the likely “missing nutrient source” was suggested to include regeneration, meso-scale eddy driven upwelling, meteorological events, and eolian inputs in addition to winter vertical mixing. Time-series observation of carbonate chemistry revealed that ocean acidification (OA) was ongoing at both stations, and that the rate of OA was faster at S1 than at K2 although OA at K2 is more critical for calcifying organisms.

Seasonal Response of North Western Pacific Marine Ecosystems to Deposition of Atmospheric Inorganic Nitrogen Compounds from East Asia

The contribution of the atmospheric deposition of inorganic nitrogen compounds produced in East Asia to the marine ecosystems of the North Western Pacific Ocean (NWPO) was investigated in this study using a 3-D lower trophic-marine ecosystem model (NEMURO) combined with an atmospheric regional chemical transport model (WRF-CMAQ). The monthly mean values for the wet and dry deposition of nitrogen compounds, including gases (HNO3 and NH3) and aerosol particles (NO3− and NH4+), were determined using the WRF-CMAQ for the NWPO from 2009–2016. These values were input into the NEMURO as an additional nitrogen source. The NEMURO indicated that the annual average chlorophyll mass concentration at the surface in the subtropical region (20°N–30°N; 125°E–150°E) of the NWPO increased from 0.04 to 0.10u2009mg/m3. Similarly, the gross primary productivity, integrated over sea depths of 0–200u2009m, increased from 85 to 147u2009mgu2009C/m2/day because of this deposition. This study indicates that the supply of atmospheric inorganic nitrogen compounds from East Asia to the NWPO could have a high nutrient impact on the marine ecosystem in the subtropical region.

Basin-scale distribution of NH4+ and NO2− in the Pacific Ocean

We used more than 25,000 nutrient samples to elucidate for the first time basin-scale distributions and seasonal changes of surface ammonium (NH4+) and nitrite (NO2−) concentrations in the Pacific Ocean. The highest NH4+, NO2−, and nitrate (NO3−) concentrations were observed north of 40°N, in the coastal upwelling region off the coast of Mexico, and in the Tasman Sea. NH4+ concentrations were elevated during May–October in the western subarctic North Pacific, May–December in the eastern subarctic North Pacific, and June–September in the subtropical South Pacific. NO2− concentrations were highest in winter in both hemispheres. The seasonal cycle of NH4+ was synchronous with NO2−, NO3−, and satellite chlorophyll a concentrations in the western subtropical South Pacific, whereas it was synchronous with chlorophyll-a but out of phase with NO2− and NO3− in the subarctic regions.