10:45 AM - 12:15 PM
[ACG44-P01] Argo float equipped with pH and O2 sensors revealed seasonal variability of DIC in the upper layer of the ocean
Keywords:Biogeochemical Argo, Total dissolved inorganic carbon, Ocean Carbon Cycle
Quantifying the changes and variability in total dissolved inorganic carbon (DIC) in surface layer of the ocean as well as in its interior is essential to quantify and understand the processes controlling the ocean biogeochemical cycles such as net biological production and respiration, net air-sea CO2 exchange, net ocean CO2 transport and storage, and their resultant changes in the acidity of seawater. The development of sustained ocean observing network by Biogeochemical (BGC) Argo coupled with the shipboard observing network like GO-SHIP can be a game changer once a large amount of data of BGC variables from Argo are well quality-controlled, data of pH are converted to DIC and they are made available for a variety of analyses.
Here we present a first result for the variability of DIC in the interior of the subtropical zone near Japan derived from the data taken with Argo floats equipped with pH and oxygen (O2) sensors at high-frequency in time, around every 100 hours, and high resolution in depths, around every 2 dbar from 4 to 2000 dbar. One of these floats (id 2903654) was deployed at 145°E, 31°N on February 22, 2021 to the south of Kuroshio Extension from RV Hakuho-maru and its pH sensor came to its end of life after January 2, 2022 at 144.37°E, 34.45°N at 81st cycle, staying at northern subtropics during its whole life covering nearly a whole seasonal cycle from mid-winter through the next winter.
Data of pH at in-situ temperature, salinity and pressure were corrected for its bias with data of pH in total hydrogen ion concentration scale calculated from the data of DIC and total alkalinity (TA) taken at depths when the float was deployed from RV Hakuho-maru. DIC was calculated from pH, temperature, salinity, pressure, TA, total phosphate (TP), and total silicate (TSi). To do this calculation, we derived empirical equations to estimate the value of latter three biogeochemical variables that are applicable for the western North Pacific at 126.5°E-165°E, 24°N-37.3°N including a portion of East China Sea but not Japan Sea using their high-quality data taken at depths by shipboard measurements by Japan Meteorological Agency between 2010 and 2021. The equations are expressed as a function of potential density for each of four density bands for TA and six density bands for TP and TSi between 21.3 – 27.69 kg m-3 by the method of linear regression. The RMSE of residuals ranges 2.5 – 3.5 μmol kg-1 for TA, 0.01 – 0.08 μmol kg-1 for TP and 0.2 – 7.5 μmol kg-1 for TSi with smaller RMSE in lower density bands. Taking these RMSE and the uncertainty in the systematic error correction for pH-sensor data into considerations, the uncertainty in DIC derived from pH sensor on Argo float (95% c.i.) ranges from 4.9 μmol kg-1 in the upper layer to 6.9 μmol kg-1 in deeper layers.
High-frequency and high-vertical resolution data of DIC and O2 from Argo float equipped with pH and O2 sensors and deployed in the northern subtropics close to Kuroshio Extension (id 2903654) (Fig.1) revealed (1) deepening of mixed layer down to 250m in winter, (2) warming, lowering of salinity and stratification in upper layers in summer, in particular in the upper 50m in June through September, (3) reduction of salinity-normalized DIC due likely to the net biological production in the upper stratified layers in summer, (4) formation of O2 maximum in summer at around 50m in which dissolved O2 is super-saturated but its concentration is lower than in winter mixed layer, implying the decrease of O2 due to its escape to the atmosphere but a partial compensation by the biological O2 supply, (5) increase in nDIC associated with the increase in AOU in spring and autumn at 150-200m, suggesting the occurrence of remineralization in the twilight zone.
Here we present a first result for the variability of DIC in the interior of the subtropical zone near Japan derived from the data taken with Argo floats equipped with pH and oxygen (O2) sensors at high-frequency in time, around every 100 hours, and high resolution in depths, around every 2 dbar from 4 to 2000 dbar. One of these floats (id 2903654) was deployed at 145°E, 31°N on February 22, 2021 to the south of Kuroshio Extension from RV Hakuho-maru and its pH sensor came to its end of life after January 2, 2022 at 144.37°E, 34.45°N at 81st cycle, staying at northern subtropics during its whole life covering nearly a whole seasonal cycle from mid-winter through the next winter.
Data of pH at in-situ temperature, salinity and pressure were corrected for its bias with data of pH in total hydrogen ion concentration scale calculated from the data of DIC and total alkalinity (TA) taken at depths when the float was deployed from RV Hakuho-maru. DIC was calculated from pH, temperature, salinity, pressure, TA, total phosphate (TP), and total silicate (TSi). To do this calculation, we derived empirical equations to estimate the value of latter three biogeochemical variables that are applicable for the western North Pacific at 126.5°E-165°E, 24°N-37.3°N including a portion of East China Sea but not Japan Sea using their high-quality data taken at depths by shipboard measurements by Japan Meteorological Agency between 2010 and 2021. The equations are expressed as a function of potential density for each of four density bands for TA and six density bands for TP and TSi between 21.3 – 27.69 kg m-3 by the method of linear regression. The RMSE of residuals ranges 2.5 – 3.5 μmol kg-1 for TA, 0.01 – 0.08 μmol kg-1 for TP and 0.2 – 7.5 μmol kg-1 for TSi with smaller RMSE in lower density bands. Taking these RMSE and the uncertainty in the systematic error correction for pH-sensor data into considerations, the uncertainty in DIC derived from pH sensor on Argo float (95% c.i.) ranges from 4.9 μmol kg-1 in the upper layer to 6.9 μmol kg-1 in deeper layers.
High-frequency and high-vertical resolution data of DIC and O2 from Argo float equipped with pH and O2 sensors and deployed in the northern subtropics close to Kuroshio Extension (id 2903654) (Fig.1) revealed (1) deepening of mixed layer down to 250m in winter, (2) warming, lowering of salinity and stratification in upper layers in summer, in particular in the upper 50m in June through September, (3) reduction of salinity-normalized DIC due likely to the net biological production in the upper stratified layers in summer, (4) formation of O2 maximum in summer at around 50m in which dissolved O2 is super-saturated but its concentration is lower than in winter mixed layer, implying the decrease of O2 due to its escape to the atmosphere but a partial compensation by the biological O2 supply, (5) increase in nDIC associated with the increase in AOU in spring and autumn at 150-200m, suggesting the occurrence of remineralization in the twilight zone.