S. Nakaoka

A Global Surface Ocean fCO2 Climatology Based on a Feed-Forward Neural Network

AbstractA feed-forward neural network is used to create a monthly climatology of the sea surface fugacity of CO2 (fCO2) on a 1° × 1° spatial resolution. Using 127 880 data points from 1990 to 2011 in the track-gridded database of the Surface Ocean CO2 Atlas version 2.0 (Bakker et al.), the model yields a global mean fCO2 increase rate of 1.50 μatm yr−1. The rate was used to normalize multiple years’ fCO2 observations to the reference year of 2000. A total of 73 265 data points from the normalized data were used to model the global fCO2 climatology. The model simulates monthly fCO2 distributions that agree well with observations and yields an anthropogenic CO2 update of −1.9 to −2.3 PgC yr−1. The range reflects the uncertainty related to using different wind products for the flux calculation. This estimate is in good agreement with the recently derived best estimate by Wanninkhof et al. The model product benefits from a finer spatial resolution compared to the product of Lamont–Doherty Earth Observatory (T...

North Pacific dissolved inorganic carbon variations related to the Pacific Decadal Oscillation

We elucidated multiyear variations of sea surface dissolved inorganic carbon (DIC) concentrations in the North Pacific from 2002 to 2008 by using monthly DIC maps derived from partial pressure CO2 observations. The Pacific Decadal Oscillation (PDO) was related to an east-west seesaw pattern in the North Pacific DIC anomaly field. In the western North Pacific, DIC concentrations were relatively high from mid-2002 to mid-2005 and low after late 2007 compared with climatological values, and in the eastern North Pacific the opposite change was observed. Changes of the forcing factors associated with the PDO could explain the DIC east-west seesaw pattern: horizontal advection, freshwater fluxes and vertical mixing in most regions, CO2 fluxes south of 40°N, and biological production in the subarctic.

Observation-based trends of the southern ocean carbon sink

The Southern Ocean (SO) carbon sink has strengthened substantially since the year 2000, following a decade of a weakening trend. However, the surface ocean pCO2 data underlying this trend reversal are sparse, requiring a substantial amount of extrapolation to map the data. Here we use nine different pCO2 mapping products to investigate the SO trends and their sensitivity to the mapping procedure. We find a robust temporal coherence for the entire SO, with eight of the nine products agreeing on the sign of the decadal trends, that is, a weakening CO2 sink trend in the 1990s (on average 0.22 ± 0.24 pg C yr−1 decade−1), and a strengthening sink trend during the 2000s (−0.35 ± 0.23 pg C yr−1 decade−1). Spatially, the multiproduct mean reveals rather uniform trends, but the confidence is limited, given the small number of statistically significant trends from the individual products, particularly during the data-sparse 1990–1999 period. Plain Language Summary The Southern Ocean plays an important role in regulating Earth’s climate as it takes up a substantial amount of carbon dioxide from the atmosphere, thereby limiting the effect of global warming. However, this part of the global ocean is also the least well observed and observational data are sparse. Therefore, to study Southern Ocean carbon uptake, data interpolation methods are used to estimate the variability of the carbon uptake from the few existing observations. This poses the question on how reliable these estimates are. The Surface Ocean CO2 Mapping intercomparison project aims to do exactly that, that is, test how reliable current estimates are by comparing results from different methods. Here we compare the results from nine data interpolation methods in the Southern Ocean from 1990 to 2010 and find a broad and encouraging agreement regarding decadal carbon uptake signals, whereas a spatially more refined analysis reveals much less agreement locally, illustrating the need to continue the measurement effort in the Southern Ocean.

Long-term variability of surface nutrient concentrations in the North Pacific

We present the spatial distributions and temporal changes of the long-term variability of surface nutrient concentrations in the North Pacific by using nutrient samples collected by volunteer ships and research vessels from 1961 to 2012. Nutrient samples are optimally interpolated onto 1° × 1° monthly grid boxes. When the Pacific Decadal Oscillation is in its positive phase, nutrient concentrations in the western North Pacific are significantly higher than the climatological means, and those in the eastern North Pacific are significantly lower. When the North Pacific Gyre Oscillation is in its positive phase, nutrient concentrations in the subarctic are significantly higher than the climatological means. The trends of phosphate and silicate averaged over the North Pacific are −0.012 ± 0.005 µmol l−1 decade−1 and −0.38 ± 0.13 µmol l−1 decade−1, whereas the nitrate trend is not significant (0.01 ± 0.13 µmol l−1 decade−1).

Reconstructing the Carbon Dioxide Absorption Patterns of World Oceans Using a Feed-Forward Neural Network: Software Implementation and Employment Techniques

Oceans play a major role in the global carbon budget, absorbing approximately 27% of anthropogenic carbon dioxide (CO2). As the degree to which an ocean can serve as a carbon sink is determined by the partial pressure of CO2 in the surface water, it is critical to obtain an accurate estimate of the spatial distributions of CO2 and its temporal variation on a global scale. However, this is extremely challenging due to insufficient measurements, large seasonal variability, and short spatial de-correlation scales. This paper presents an open source software package that implements a feed-forward neural network and a back-propagation training algorithm to solve a problem with one output variable and a large number of training patterns. We discuss the employment of the neural network for global ocean CO2 mapping.

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.

In situ observation of atmospheric oxygen and carbon dioxide in the North Pacific using a cargo ship

Atmospheric oxygen (O2) and carbon dioxide (CO2) variations in the North Pacific were measured aboard a cargo ship, the New Century 2 (NC2), while it cruised between Japan and the United States between December 2015 and November 2016. A fuel cell analyzer and a nondispersive infrared analyzer were used for the measurement of O2 and CO2, respectively. To achieve parts-per-million precision for the O2 measurements, we precisely controlled the flow rates of the sample and reference air introduced into the analyzers and the outlet pressure. A relatively low airflow rate (10 cm3 min−1) was adopted to reduce the consumption rate of the reference gases. In the laboratory, the system achieved measurement precisions of 3.8 per meg for δ(O2 /N2), which is commonly used to express atmospheric O2 variation, and 0.1 ppm for the CO2 mole fraction. After the in situ observation started aboard NC2, we found that the ship’s motion caused false wavy variations in the O2 signal with an amplitude of more than several tens of ppm and a period of about 20 s. Although we have not resolved the problem at this stage, hourly averaging considerably suppressed the variation associated with ship motion. Comparison between the in situ observation and flask sampling of air samples aboard NC2 showed that the averaged differences (in situ–flask) and the standard deviations (±1σ ) are −2.8± 9.4 per meg for δ(O2 /N2) and−0.02± 0.33 ppm for the CO2 mole fraction. We compared 1 year of in situ data for atmospheric potential oxygen (APO; O2 +1.1×CO2) obtained from the broad middle-latitude region (140 E–130W, 29 N–45 N) with previous flask sampling data from the North Pacific. This comparison showed that longitudinal differences in the seasonal amplitude of APO, ranging from 51 to 73 per meg, were smaller than the latitudinal differences.