Unveiling controls of the latitudinal gradient of surface pCO2 in the Kuroshio Extension and its recirculation regions（northwestern North Pacific） in late spring

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...

Unveiling controls of the latitudinal gradient of surface pCO2 in the Kuroshio Extension and its recirculation regions (northwestern North Pacific) in late spring

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.