5:15 PM - 6:30 PM
[ACG44-P01] Relationship between the variability in dissolved inorganic carbon in surface seawater and the formation volume of subtropical mode water along the 137°E line
Keywords:137°E section, Dissolved inorganic carbon, Subtropical mode water, Isopycnal surface, Decadal scale variability, PDO
1. Introduction
Based on the data of partial pressure of CO2 in seawater (pCO2sea) along the 137°E since the beginning of the 1980s, we previously reported the growth rates of dissolved inorganic carbon (DIC) in surface seawater from subtropic to tropic zone by means of a multiple regression analysis (Ono et al., 2019). In addition, we showed that the rates varied greatly on a decadal time scale in the Kuroshio recirculation. Besides, the paper by Kobashi et al. (2020) indicated the decadal scale variability of the North Pacific subtropical mode water (STMW) and its influence on the pycnocline along the 137°E. In this study, we investigate the relationship between the variability in DIC in surface seawater, STMW thickness and the depth of isopycnal surface at 30°N along the 137°E line.
2. Data & Methods
We mainly used measurement data at 30°N along the 137°E line in each summer acquired by Japan Meteorological Agency (JMA). Based on the measurement data of DIC since 1994, the long-term trend of DIC in surface seawater was calculated by linear regression analysis. And the time-series of DIC was detrended. Using the data acquired by CTD system, we evaluated the temporal variability of the depth of isopycnal surface and STMW thickness which was defined as a layer of potential vorticity (PV) less than 2.0×10-10 m-1s-1. From these data, 3-year running means and correlations of each time-series were calculated.
3. Result & Discussion
Temporal variability of salinity-normalized DIC (nDIC) in surface seawater corresponded with those of STMW thickness and depth of isopycnal surface (Figure a-c). Significant correlations between them were seen and their coefficients were 0.69 (p<0.01) and -0.77 (p<0.01), respectively. These results indicate the thicker STMW and the shallower isopycnal surfaces are, the higher DIC is. Namely, higher values of nDIC around 2015 are considered to be caused by the heaving of isopycnal surfaces and the relatively larger supply of DIC from subsurface to surface by vertical mixing in winter.
It has been reported that the STMW thickness varies corresponding to the decadal variability of the Kuroshio Extension and which are fluctuated in association with the Pacific Decadal Oscillation (PDO) (e.g., Qiu and Chen, 2005). The time lag correlation analysis between nDIC in surface seawater and PDO index showed the maximum coefficient of -0.77 (p<0.01) with 4-year lag applied.
These results suggest that the temporal variability of nDIC in surface seawater in the Kuroshio recirculation along the 137°E line is directly attributed to the fluctuation of STMW formation and isopycnal surface, and originally affected by PDO whose influence is transmitted in around 4 years.
Figure caption
Time-series of (a) STMW thickness, (b) depth of isopycnal surface (σθ=25.0 kg/m3), and (c) detrended nDIC in surface seawater in summer at 30°N, 137°E. The thick yellow lines in (a)-(c) represent 3-year running means. The thin and thick blue lines in (c) indicate monthly means and 3-year running means of PDO index, respectively.
Based on the data of partial pressure of CO2 in seawater (pCO2sea) along the 137°E since the beginning of the 1980s, we previously reported the growth rates of dissolved inorganic carbon (DIC) in surface seawater from subtropic to tropic zone by means of a multiple regression analysis (Ono et al., 2019). In addition, we showed that the rates varied greatly on a decadal time scale in the Kuroshio recirculation. Besides, the paper by Kobashi et al. (2020) indicated the decadal scale variability of the North Pacific subtropical mode water (STMW) and its influence on the pycnocline along the 137°E. In this study, we investigate the relationship between the variability in DIC in surface seawater, STMW thickness and the depth of isopycnal surface at 30°N along the 137°E line.
2. Data & Methods
We mainly used measurement data at 30°N along the 137°E line in each summer acquired by Japan Meteorological Agency (JMA). Based on the measurement data of DIC since 1994, the long-term trend of DIC in surface seawater was calculated by linear regression analysis. And the time-series of DIC was detrended. Using the data acquired by CTD system, we evaluated the temporal variability of the depth of isopycnal surface and STMW thickness which was defined as a layer of potential vorticity (PV) less than 2.0×10-10 m-1s-1. From these data, 3-year running means and correlations of each time-series were calculated.
3. Result & Discussion
Temporal variability of salinity-normalized DIC (nDIC) in surface seawater corresponded with those of STMW thickness and depth of isopycnal surface (Figure a-c). Significant correlations between them were seen and their coefficients were 0.69 (p<0.01) and -0.77 (p<0.01), respectively. These results indicate the thicker STMW and the shallower isopycnal surfaces are, the higher DIC is. Namely, higher values of nDIC around 2015 are considered to be caused by the heaving of isopycnal surfaces and the relatively larger supply of DIC from subsurface to surface by vertical mixing in winter.
It has been reported that the STMW thickness varies corresponding to the decadal variability of the Kuroshio Extension and which are fluctuated in association with the Pacific Decadal Oscillation (PDO) (e.g., Qiu and Chen, 2005). The time lag correlation analysis between nDIC in surface seawater and PDO index showed the maximum coefficient of -0.77 (p<0.01) with 4-year lag applied.
These results suggest that the temporal variability of nDIC in surface seawater in the Kuroshio recirculation along the 137°E line is directly attributed to the fluctuation of STMW formation and isopycnal surface, and originally affected by PDO whose influence is transmitted in around 4 years.
Figure caption
Time-series of (a) STMW thickness, (b) depth of isopycnal surface (σθ=25.0 kg/m3), and (c) detrended nDIC in surface seawater in summer at 30°N, 137°E. The thick yellow lines in (a)-(c) represent 3-year running means. The thin and thick blue lines in (c) indicate monthly means and 3-year running means of PDO index, respectively.