2:30 PM - 2:45 PM
[ACG50-03] Autumn oxygen emission from ocean around Japan
Keywords:Atmospheric Potential Oxygen (APO), air-sea oxygen flux, BGC floats
1. Introduction
Atmospheric Potential Oxygen (APO) is a tracer calculated as APO = [O2] + 1.1 [CO2], where 1.1 represents the -O2:C fluctuation ratio in the terrestrial biosphere resulting from processes such as photosynthesis and respiration.
Therefore, APO is not influenced by terrestrial biological activity but instead fluctuates due to fossil fuel combustion and the exchange of O2 and CO2 between the atmosphere and the ocean. Since the seasonal variation in fossil fuel consumption is relatively small, the seasonal variation in APO is primarily attributed to the exchange of O2 and CO2 between the atmosphere and the ocean, particularly O2, which has a high gas exchange rate.
In autumn, the solubility of gases increases with the decrease in sea surface temperature, so gases such as O2, which are originally 100% saturated with respect to the atmosphere, are expected to be absorbed from the atmosphere into the ocean. However, Tohjima et al. [2024] compared shipboard APO observations with model calculations based on climatological oxygen fluxes and found that APO observations in the western North Pacific during autumn were higher than the model calculations. This discrepancy suggests O2 release from the ocean to the atmosphere that was not captured by the climatological fluxes. In this study, autumn surface O2 supersaturation is attributed to the entrainment of the subsurface oxygen maximum (SOM) into the mixed layer at 20–40°N and to autumn phytoplankton blooms at 40–60°N.
Kosugi et al. [under review] demonstrated that in the mid-latitudes of the western North Pacific, surface O2 is oversaturated across a wide area of the ocean in autumn based on BGC float observations. Furthermore, they found that surface O2 oversaturation is sustained by the entrainment of SOM, as indicated by vertical O2 profiles, and that oversaturation dissipates when the mixed layer deepens beyond the SOM. This finding is consistent with the formation process of oxygen oversaturation at 20–40°N suggested by Tohjima et al. [2024].
Thus, surface O2 supersaturation in autumn has been elucidated through both atmospheric and oceanic observations; however, many uncertainties remain. In this study, we aim to assess autumn O2 supersaturation, which has previously been examined only in the western North Pacific, by extending the analysis to include the Sea of Japan. Additionally, we evaluate its timing and magnitude across different latitudes, contributing to a more detailed understanding of air-sea O2 flux and APO dynamics.
2. Data
Surface (8–12 m depth) data collected by the Japan Meteorological Agency's oceanographic observation vessels were used in this study. Dissolved O2 concentrations were measured using bottled water samples prior to March 2010 and RINKO sensor values thereafter. The saturation O2 concentration was calculated using the formula of Garcia and Gordon [1992]. Mixed layer depth (MLD) was defined as the layer whose density differs from that at 10 dbar by 0.03 kg m-3 or less.
3. Results
In September and October, surface O2 was supersaturated in nearly all ocean areas in 25–43°N. In particular, strong supersaturation of nearly 110% was observed around 40°N. By November, undersaturated areas began to emerge between 35–40°N, with this trend being most pronounced in the Sea of Japan, where a large amount of data were collected (Figure). By December, supersaturation was only observed south of 30°N, while most areas were undersaturated with respect to the atmosphere.
The presentation will also cover the causes of these seasonal variations and the fluxes between the atmosphere and the ocean.
4. References
Garcia and Gordon, (1992) Oxygen solubility in seawater: Better fitting equations
Kosugi et al., Air-sea oxygen fluxes in mid-latitude western North Pacific quantified by the array of biogeochemical Argo floats, under review
Tohjima et al., (2024) Observed APO Seasonal Cycle in the Pacific: Estimation of Autumn O2 Oceanic Emissions
5. Figure caption
Surface O2 saturation around Japan in (a) October and (b) November. Data was averaged over every 1 degree of latitude and longitude.
Atmospheric Potential Oxygen (APO) is a tracer calculated as APO = [O2] + 1.1 [CO2], where 1.1 represents the -O2:C fluctuation ratio in the terrestrial biosphere resulting from processes such as photosynthesis and respiration.
Therefore, APO is not influenced by terrestrial biological activity but instead fluctuates due to fossil fuel combustion and the exchange of O2 and CO2 between the atmosphere and the ocean. Since the seasonal variation in fossil fuel consumption is relatively small, the seasonal variation in APO is primarily attributed to the exchange of O2 and CO2 between the atmosphere and the ocean, particularly O2, which has a high gas exchange rate.
In autumn, the solubility of gases increases with the decrease in sea surface temperature, so gases such as O2, which are originally 100% saturated with respect to the atmosphere, are expected to be absorbed from the atmosphere into the ocean. However, Tohjima et al. [2024] compared shipboard APO observations with model calculations based on climatological oxygen fluxes and found that APO observations in the western North Pacific during autumn were higher than the model calculations. This discrepancy suggests O2 release from the ocean to the atmosphere that was not captured by the climatological fluxes. In this study, autumn surface O2 supersaturation is attributed to the entrainment of the subsurface oxygen maximum (SOM) into the mixed layer at 20–40°N and to autumn phytoplankton blooms at 40–60°N.
Kosugi et al. [under review] demonstrated that in the mid-latitudes of the western North Pacific, surface O2 is oversaturated across a wide area of the ocean in autumn based on BGC float observations. Furthermore, they found that surface O2 oversaturation is sustained by the entrainment of SOM, as indicated by vertical O2 profiles, and that oversaturation dissipates when the mixed layer deepens beyond the SOM. This finding is consistent with the formation process of oxygen oversaturation at 20–40°N suggested by Tohjima et al. [2024].
Thus, surface O2 supersaturation in autumn has been elucidated through both atmospheric and oceanic observations; however, many uncertainties remain. In this study, we aim to assess autumn O2 supersaturation, which has previously been examined only in the western North Pacific, by extending the analysis to include the Sea of Japan. Additionally, we evaluate its timing and magnitude across different latitudes, contributing to a more detailed understanding of air-sea O2 flux and APO dynamics.
2. Data
Surface (8–12 m depth) data collected by the Japan Meteorological Agency's oceanographic observation vessels were used in this study. Dissolved O2 concentrations were measured using bottled water samples prior to March 2010 and RINKO sensor values thereafter. The saturation O2 concentration was calculated using the formula of Garcia and Gordon [1992]. Mixed layer depth (MLD) was defined as the layer whose density differs from that at 10 dbar by 0.03 kg m-3 or less.
3. Results
In September and October, surface O2 was supersaturated in nearly all ocean areas in 25–43°N. In particular, strong supersaturation of nearly 110% was observed around 40°N. By November, undersaturated areas began to emerge between 35–40°N, with this trend being most pronounced in the Sea of Japan, where a large amount of data were collected (Figure). By December, supersaturation was only observed south of 30°N, while most areas were undersaturated with respect to the atmosphere.
The presentation will also cover the causes of these seasonal variations and the fluxes between the atmosphere and the ocean.
4. References
Garcia and Gordon, (1992) Oxygen solubility in seawater: Better fitting equations
Kosugi et al., Air-sea oxygen fluxes in mid-latitude western North Pacific quantified by the array of biogeochemical Argo floats, under review
Tohjima et al., (2024) Observed APO Seasonal Cycle in the Pacific: Estimation of Autumn O2 Oceanic Emissions
5. Figure caption
Surface O2 saturation around Japan in (a) October and (b) November. Data was averaged over every 1 degree of latitude and longitude.