9:15 AM - 9:30 AM
[AOS23-02] Underway measurements of surface pCO2 and total alkalnity in Kuroshio-Oyashio transition region
Keywords:Kuroshio-Oyashio transition region, Total alkalinity, pCO2
Surface TA (open circle in Figure 1) showed fine spatial variation which can not be captured by the interpolation of the bottle sampling and measurement of TA (open square in Figure 1) at CTD station which located every 2 degrees in longitude. The estimation of TA by Lee et al.  (gray dot in Figure 1) overestimated the measurement by up to 30 μmol/kg.
We calculated surface DIC from TA and pCO2 obtained by the underway mesurements. Calulated DIC was in good agreement with the measured DIC taken from nearby CTD station. Difference and standard deviation between calculated and measured value were 0.8 and 5.4 μmol/kg respectively (N = 38). Underway measurement of pCO2 and TA can reproduce other carbonate parmeters such as DIC and pH accurately. This method is beneficial to understand carbon cycling in coastal region and Kuroshio-Oyashio transition region where spatial variation of TA is large.
The variation of TA by precipitation and evapolation can be excluded by salinity normalization. Normalized alklainity to Salinity = 35 (NTA35) has a large meridional gradient in the western North Pacific. NTA35 in the subtropical region and subarctic gyre were about 2300 and 2370 μmol/kg respectively [Takatani et al. 2014]. In our observation, NTA35 also showed large zonal varitaion and ranged 2310-2355 μmol/kg along 41°N and 2300-2325 μmol/kg along 37.5°N. These dynamic spatial variation of NTA35 was attributable to the complicated distribution of Kuroshio and Oyashio water. pCO2 was concave against NTA35 and the smallest around NTA35 = 2320 μmol/kg. Below this NTA35, pCO increased thermodynamically due to temperature rise. On the other hand, nutrients was significanly high above this NTA35. This indicated that pCO2 was high because the DIC supplied to the surface by the winter mixing had not been substantially reduced by biological production.