5:15 PM - 6:45 PM
[ACG41-P06] High-frequency time series of dissolved oxygen and atmosphere-ocean oxygen fluxes in the Northwest Pacific Ocean by BGC floats.
Keywords:BGC floatsBGC floats, Air-sea oxygen flux, Hypoxia
(Introduction) Fossil fuel combustion and photosynthesis by plants on land cause simultaneous increases and decreases in atmospheric CO2 and O2 concentrations. In the ocean, on the other hand, both CO2 and O2 exist in dissolved form in seawater, and the effects on atmospheric concentrations of CO2 and O2 through gas exchange between the atmosphere and the ocean are independent of each other. Additionally, there is a long-term decrease in dissolved O2 in seawater, i.e., deoxygenation, due to increasing surface water temperatures and weakening winter mixing in many regions of the world. Against this background, the quantification of O2 exchange (fluxes) between the atmosphere and the ocean plays a major role in estimating the productivity of the ocean, monitoring deoxygenation, and understanding the global carbon cycle.
To date, climatological values of O2 fluxes between the atmosphere and the ocean have been obtained by compiling the results of shipboard observations, but short timescale variability has not been well studied. This study reports on O2 fluxes in the Northwest Pacific calculated on the order of days using observations from Argo floats equipped with O2 sensors.
(Data) Float data used in the study were acquired from 12 BGC floats deployed in the Northwest Pacific between February 2021 and June 2023. All of these floats were equipped with the O2 sensor RINKO, known for its fast response. For more information on float observations, refer to Oka et al.'s study, scheduled to be presented at the A-CG47 session of this conference.
Water temperature, salinity, and dissolved oxygen at 10 dbar obtained from the floats were used as surface values to calculate fluxes. The oxygen solubility of seawater was sourced from Garcia and Gordon [1992], and supersaturation due to wave breaking was considered [Wolf and Thorpe, 1991]. The gas exchange coefficient was extracted from Iida et al. [2021], while the wind speed and pressure used to calculate the fluxes were monthly averaged values from the reanalysis JRA-55 (geographical resolution 1.25°) to calculate oxygen fluxes over a total of 20 years.
(Result) The ocean generally tends to release O2 in summer, while absorbing O2 in winter. During summer, O2 is more likely to be released to the atmosphere due to oxygen supersaturation caused by a decrease in O2 solubility resulting from higher surface water temperatures. However, there were cases where O2 continued to be released to the atmosphere during the autumn season from September onwards, when water temperatures drop, mainly at lower latitudes. This is thought to be because O2 supersaturation was sustained by the uptake of subsurface O2 maxima layer into the mixed layer. In winter, in addition to the increase in O2 solubility due to the decrease in water temperature, the O2 undersaturation of the surface layer became more pronounced due to the entrainment of deep water with undersaturated O2 into the mixing layer through vertical mixing. The degree of O2 undersaturation was particularly high at high latitudes due to the greater depth of vertical mixing.
The relationship between the latitude of each float and the annual flux is depicted in the figure. The annual flux is largely determined by how much O2 is absorbed by the ocean in winter when wind speeds are high, with the ocean absorbing O2 in the north and releasing O2 in the south at around 30°N. This result suggests that O2 is transported from north to south in the ocean interior, with subtropical mode water and central mode water playing a major role in this. Additionally, as the area covered by this study is a CO2 sink, it was found that the oceans generally absorb both O2 and CO2 in the area north of 30°N.
(Figure caption) Relationship between float position and O2 fluxes. The horizontal axis shows latitude, with the most northerly and southerly positions of each float during the year indicated by circle and connected by a line. The vertical axis represents the oxygen flux, with positive values (upper side of the graph) indicating that the ocean absorbs O2 and negative values (lower side of the graph) indicating that the ocean releases O2.
To date, climatological values of O2 fluxes between the atmosphere and the ocean have been obtained by compiling the results of shipboard observations, but short timescale variability has not been well studied. This study reports on O2 fluxes in the Northwest Pacific calculated on the order of days using observations from Argo floats equipped with O2 sensors.
(Data) Float data used in the study were acquired from 12 BGC floats deployed in the Northwest Pacific between February 2021 and June 2023. All of these floats were equipped with the O2 sensor RINKO, known for its fast response. For more information on float observations, refer to Oka et al.'s study, scheduled to be presented at the A-CG47 session of this conference.
Water temperature, salinity, and dissolved oxygen at 10 dbar obtained from the floats were used as surface values to calculate fluxes. The oxygen solubility of seawater was sourced from Garcia and Gordon [1992], and supersaturation due to wave breaking was considered [Wolf and Thorpe, 1991]. The gas exchange coefficient was extracted from Iida et al. [2021], while the wind speed and pressure used to calculate the fluxes were monthly averaged values from the reanalysis JRA-55 (geographical resolution 1.25°) to calculate oxygen fluxes over a total of 20 years.
(Result) The ocean generally tends to release O2 in summer, while absorbing O2 in winter. During summer, O2 is more likely to be released to the atmosphere due to oxygen supersaturation caused by a decrease in O2 solubility resulting from higher surface water temperatures. However, there were cases where O2 continued to be released to the atmosphere during the autumn season from September onwards, when water temperatures drop, mainly at lower latitudes. This is thought to be because O2 supersaturation was sustained by the uptake of subsurface O2 maxima layer into the mixed layer. In winter, in addition to the increase in O2 solubility due to the decrease in water temperature, the O2 undersaturation of the surface layer became more pronounced due to the entrainment of deep water with undersaturated O2 into the mixing layer through vertical mixing. The degree of O2 undersaturation was particularly high at high latitudes due to the greater depth of vertical mixing.
The relationship between the latitude of each float and the annual flux is depicted in the figure. The annual flux is largely determined by how much O2 is absorbed by the ocean in winter when wind speeds are high, with the ocean absorbing O2 in the north and releasing O2 in the south at around 30°N. This result suggests that O2 is transported from north to south in the ocean interior, with subtropical mode water and central mode water playing a major role in this. Additionally, as the area covered by this study is a CO2 sink, it was found that the oceans generally absorb both O2 and CO2 in the area north of 30°N.
(Figure caption) Relationship between float position and O2 fluxes. The horizontal axis shows latitude, with the most northerly and southerly positions of each float during the year indicated by circle and connected by a line. The vertical axis represents the oxygen flux, with positive values (upper side of the graph) indicating that the ocean absorbs O2 and negative values (lower side of the graph) indicating that the ocean releases O2.