Surface pCO2 in the Kuroshio Extension and its recirculation regions in late spring 2018

What is it about?

In the northwestern North Pacific, annual net air-sea CO2 flux is greatest in the Kuroshio Extension (KE) zone, owing to its low annual mean partial pressure of CO2 (pCO2), and it decreases southward across the basin. To quantify the influences of factors controlling the latitudinal gradient in CO2 uptake, sea surface pCO2 and related parameters were investigated in late spring of 2018 in a study spanning the KE, Kuroshio Recirculation (KR), and subtropical zones. We found that the sea-to-air pCO2 difference (ΔpCO2) was negative and at its lowest in the KE zone. ΔpCO2 gradually increased southward across the KR zone, and the sea surface was nearly in air-equilibrium with atmospheric CO2 in the subtropical zone. We found that northward cooling and vertical mixing were the two major processes governing the latitudinal gradient in surface pCO2 and ΔpCO2, while biological influences were relatively minor. In the KE zone affected by upwelling, the vertical-mixing-induced increase in surface pCO2 likely canceled out approximately 61% of the decrease in surface pCO2 caused by cooling and biological activities. Moreover, the prolonged air-sea equilibration for CO2 and relatively short hydraulic retention time jointly led to the low surface pCO2 in the KE zone in spring. Ultimately, the cooling KE current flows out of the region before it can be re-equilibrated with atmospheric CO2.

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Why is it important?

The primary mechanism supporting oceanic CO2 uptake in the KE zone is that the air-sea equilibration time is much longer than the hydrological residence time, indicating that the KE current flows rapidly out of the region before reaching air-sea equilibrium.

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