Abstract. Air–sea CO2 fluxes over the Pacific Ocean are known to be characterized by coherent large-scale structures that reflect not only ocean subduction and upwelling patterns, but also the combined effects of wind-driven gas exchange and biology. On the largest scales, a large net CO2 influx into the extratropics is associated with a robust seasonal cycle, and a large net CO2 efflux from the tropics is associated with substantial interannual variability. In this work, we have synthesized estimates of the net air–sea CO2 flux from a variety of products, drawing upon a variety of approaches in three sub-basins of the Pacific Ocean, i.e., the North Pacific extratropics (18–66° N), the tropical Pacific (18° S–18° N), and the South Pacific extratropics (44.5–18° S). These approaches include those based on the measurements of CO2 partial pressure in surface seawater (pCO2sw), inversions of ocean-interior CO2 data, forward ocean biogeochemistry models embedded in the ocean general circulation models (OBGCMs), a model with assimilation of pCO2sw data, and inversions of atmospheric CO2 measurements. Long-term means, interannual variations and mean seasonal variations of the regionally integrated fluxes were compared in each of the sub-basins over the last two decades, spanning the period from 1990 through 2009. A simple average of the long-term mean fluxes obtained with surface water pCO2 diagnostics and those obtained with ocean-interior CO2 inversions are −0.47 ± 0.13 Pg C yr−1 in the North Pacific extratropics, +0.44 ± 0.14 Pg C yr−1 in the tropical Pacific, and −0.37 ± 0.08 Pg C yr−1 in the South Pacific extratropics, where positive fluxes are into the atmosphere. This suggests that approximately half of the CO2 taken up over the North and South Pacific extratropics is released back to the atmosphere from the tropical Pacific. These estimates of the regional fluxes are also supported by the estimates from OBGCMs after adding the riverine CO2 flux, i.e., −0.49 ± 0.02 Pg C yr−1 in the North Pacific extratropics, +0.41 ± 0.05 Pg C yr−1 in the tropical Pacific, and −0.39 ± 0.11 Pg C yr−1 in the South Pacific extratropics. The estimates from the atmospheric CO2 inversions show large variations amongst different inversion systems, but their median fluxes are consistent with the estimates from climatological pCO2sw data and pCO2sw diagnostics. In the South Pacific extratropics, where CO2 variations in the surface and ocean interior are severely undersampled, the difference in the air–sea CO2 flux estimates between the diagnostic models and ocean-interior CO2 inversions is larger (0.18 Pg C yr−1). The range of estimates from forward OBGCMs is also large (−0.19 to −0.72 Pg C yr−1). Regarding interannual variability of air–sea CO2 fluxes, positive and negative anomalies are evident in the tropical Pacific during the cold and warm events of the El Niño–Southern Oscillation in the estimates from pCO2sw diagnostic models and from OBGCMs. They are consistent in phase with the Southern Oscillation Index, but the peak-to-peak amplitudes tend to be higher in OBGCMs (0.40 ± 0.09 Pg C yr−1) than in the diagnostic models (0.27 ± 0.07 Pg C yr−1).
AbstractWe estimated long-term trends of ocean acidification in surface waters in latitudinal zones from 3°N to 33°N along the repeat hydrographic line at 137°E in the western North Pacific Ocean. Estimates were based on the observational records of oceanic CO2 partial pressure and related surface properties over the last two decades. The computed pH time series both for 25 yr in winter (late January.early February) and for 21 yr in summer (June.July) exhibited significant decreasing trends in the extensive subtropical to equatorial zones, with interannual variations that were larger in summer. The calculated rates of pH decrease ranged from 0.0015 to 0.0021 yr-1 (average, 0.0018 ± 0.0002 yr-1) in winter and from 0.0008 to 0.0019 yr-1 (average, 0.0013 ) 0.0005 yr-1) in summer. The thermodynamic effects of rising sea surface temperature (SST) accounted for up to 44% (average, 15%) of the trend of pH decrease in the subtropical region in winter, whereas a trend of decreasing SST slowed the pH decrease in the northern subtropical region (around 25°N) in summer. We used the results from recent trends to evaluate future possible thermodynamic changes in the upper ocean carbonate system.
Simultaneous observations of atmospheric potential oxygen (APO=O2+1.1×CO2) and air–sea O2 flux, derived from dissolved oxygen in surface seawater, were carried out onboard the research vessel MIRAI in the northern North Pacific and the Arctic Ocean in the autumns of 2012–2014. A simulation of the APO was also carried out using a three-dimensional atmospheric transport model that incorporated a monthly air–sea O2 flux climatology. By comparing the observed and simulated APO, as well as the observed and climatological air–sea O2 fluxes, it was found that the large day-to-day variation in the observed APO can be attributed to the day-to-day variation in the local air–sea O2 fluxes around the observation sites. It was also found that the average value of the observed air–sea O2 fluxes was systematically higher than that of the climatological O2 flux. This could explain the discrepancy between the observed and simulated seasonal APO cycles widely seen at various northern hemispheric observational sites in the fall season.
We used a new underway measurement system to investigate the partial pressure of methane (CH4) in surface seawater and overlying air in the Southern Ocean from late November 2012 to mid-February 2013. The underway system consisted of a cavity ring-down spectroscopy analyser and a shower-head type equilibrator. The monthly mean atmospheric CH4 mixing ratios obtained agreed well (within 5 ppb) with those recorded at onshore baseline stations. CH4 saturation ratios (SR, %), defined as CH4 concentration in seawater divided by CH4 concentration equilibrated with atmospheric CH4, varied between 85 and 185%; most of the ratios we calculated indicated supersaturation, except for those from south of the Southern limit of Upper Circumpolar Deep Water. SR was higher at the lower latitudes, including coastal areas north of the Sub-Antarctic Front, but decreased gradually and monotonously between the Sub-Antarctic Front and the Upper Circumpolar Deep Water. At high latitudes south of the Polar Front, SR decreased to below 100% due to the effects of upwelling and vertical mixing. We found a strong linear correlation between SR and apparent oxygen utilisation (AOU) south of the Polar Front. Observed SR decreased with increasing AOU and reached 85% at high AOU (41 µmol kg−1) and low temperature (–1.8 °C). On the basis of the linear relationship between SR and AOU, we evaluated the climatological sea–air flux of CH4 from December to February for the entire Southern Ocean south of 50°S: Sea–air CH4 emission was estimated to be 0.027 Tg yr−1 in December, 0.04 Tg yr−1 in January, and 0.019 Tg yr−1 in February.
We estimated long-term trends of ocean acidification in surface waters in latitudinal zones from 3°N to 33°N along the repeat hydrographic line at 137°E in the western North Pacific Ocean.Estimates were based on the observational records of oceanic CO 2 partial pressure and related surface properties over the last two decades.The computed pH time series both for 25 years in winter (late January to early February) and for 21 years in summer (June to July) exhibited significant decreasing trends in the extensive subtropical to equatorial zones, with interannual variations that were larger in summer.The calculated rates of pH decrease ranged from 0.0015 to 0.0021 yr -1 (average, 0.0018 ± 0.0002 yr -1 ) in winter and from 0.0008 to 0.0019 yr -1 (average, 0.0013 ± 0.0005 yr -1 ) in summer.The thermodynamic effects of rising sea surface temperature (SST) accounted for up to 44% (average, 15%) of the trend of pH decrease in the subtropical region in winter, whereas a trend of decreasing SST slowed the pH decrease in the northern subtropical region (around 25°N) in summer.We used the results from recent trends to evaluate future possible thermodynamic changes in the upper ocean carbonate system.
Although much attention has been paid to describing the distribution of oceanic dimethyl sulfide (DMS) concentrations, establishing robust relationships between DMS concentrations and biological, physical, and chemical variables is still challenging. Previous studies have proposed semiempirical parameterizations by combining multiple physical and biogeochemical parameters to better understand and reproduce the global distribution of sea surface DMS. However, none of these parameterization schemes could reconcile regionally elevated DMS peaks found in high‐resolution DMS measurements made in the western subarctic Pacific. Here we found that DMS concentrations are highly correlated with the net community production, a parameter that integrates biological activity over time. We anticipate that this relationship may be exportable to other regions with high primary productivity, such as the Southern Ocean or upwelling regions, and can be used as an important parameterization scheme, combined with solar radiation dose relationship.
