[A-CG45] Biogeochemical linkages between the surface ocean and atmosphere

[ACG45-07] Seasonal variation in pCO₂ within a cold eddy formed to the north of Kuroshio Large Meander

*Naohiro Kosugi¹, Masao Ishii¹ (1.Meteorological Research Institute)

Keywords:Kuroshio Large Meander, CO2, Ocean Acidification

1. Introduction
The Kuroshio Large Meander (KLM) that occurred in August 2017 continues as of February 2023. This is the longest duration on record. In this period, many researches about the impact of KLM on climate and sea level variation have been reported. Also, researches about the biochemical impact of KLM are required since possible impact of KLM on fisheries is a matter of concern. In this study, we discuss about the seasonal variation in the partial pressure of CO₂ (pCO₂) within and outside the cold eddies that occur north of the KLM.
2. Data
Surface pCO₂ data were obtained by Japan Meteorological Agency’s research vessels “Ryofu-Maru” and “Keifu-Maru” within 30-33°N and 135-140°E between December 2017 and March 2022. Water temperature and salinity were observed concurrently. The reach of KLM was judged from subsurface temperature. The areas with water temperature below 11°C and over 18°C at 200 m depth in MOVE/MRI.COM 10-days reanalysis were defined as “cold eddy areas” and “Kuroshio areas”, respectively. In this study, the long-term increasing trend in pCO₂ was not considered and the monthly mean pCO₂ and surface temperature were calculated for the cold eddy areas and the Kuroshio areas for all years.
3. Results
Surface temperatures were lower in the cold eddy areas than in the Kuroshio areas in all months (Figure 1a). The difference between two areas was large and up to about 3°C in winter. The pCO₂ in both areas showed similar seasonal variations, with maximum in summer and minimum in winter (Figure 1b). Such seasonal variation suggested that the dominant factor of the variation was temperature. The annual mean pCO₂ was 364.0 μatm in the cold eddy areas and 366.4 μatm in the Kuroshio areas. There were no significant differences between the two areas.
4. Disscussion
The cold eddy region areas showed similar seasonal variations in pCO₂ to the Kuroshio areas, despite larger seasonal variations in surface temperature. To explain this quantitatively, we separated pCO₂ variations into thermal and non-thermal variations in following equations after Takahashi et al. [2002].
Thermal pCO₂ = pCO_2am × exp{0.0423 × (T - T_am)}
Non-thermal pCO₂ = pCO₂ × exp{0.0423 × (T_am - T)}
where, T and pCO₂ are the monthly mean surface temperature and pCO₂, respectively. T_am and pCO_2am are the annual mean of surface temperature and pCO₂, respectively. Seasonal variations of thermal pCO₂ and non-thermal pCO₂ in the cold-water eddy region were about 1.4 times larger than those in the Kuroshio region (Figures 1c and d). In winter, despite the lower temperature in the cold eddy areas than in the Kuroshio areas, there were no clear differences in pCO₂ between two areas. This suggested that dissolved inorganic carbon (DIC) was higher in cold eddy areas than in the Kuroshio areas. Subsequently, from spring to summer, the decrease in non-thermal pCO₂ was larger in the cold eddy areas than in the Kuroshio region, suggesting a greater consumption of DIC by primary production. From late-autumn to winter, larger increase in non-thermal pCO₂ in cold eddy areas suggested a greater entrainment of DIC with the deepening of the mixed layer. Simultaneously, greater nutrients were supplied to the surface layer. These nutrients should fuel primary production in subsequent spring.

Figure caption
Monthly mean of (a) surface temperature, (b) pCO₂, (c) Thermal pCO₂ and (d) Non-thermal pCO₂. Orange and blue lines indicate the values of Kuroshio areas and cold eddy areas respectively. In (a) and (b), absolute values are plotted. In (c) and (d), anomalies from annual mean are shown.