Seasonal and Inter-Annual Variations in pCO₂^sea and Air-Sea CO₂ Fluxes in Mid-Latitudes of the Western and Eastern North Pacific during 1999-2006: Recent Results Utilizing Voluntary Observation Ships

Abstract

We have investigated the seasonal and inter-annual variations of the difference in partial pressure of CO₂ between surface seawater (pCO₂sea) and overlying air (pCO₂air) and the air-sea CO₂ flux in the mid-latitudes of the western North Pacific (WNP; 25-40°N, 140-170°E) and eastern North Pacific (ENP; 25-40°N, 120-150°W) from 1999 to 2006 using the latest voluntary observation ship data. In the WNP and ENP, the area-averaged ΔpCO₂ (pCO₂^air-pCO₂^sea) was at its minimum in late summer (-4.6 to 6.7 µatm in the WNP and -32.5 to -20.5 µatm in the ENP) and at its maximum in late winter (51.0 to 59.8 µatm in the WNP and 35.1 to 46.2 µatm in the ENP). The WNP acts as a moderate sink for atmospheric CO₂ (4.1 to 5.5 mmol m^-2d^-1), while the ENP acts as a weak sink (1.1 to 1.9 mmol m^-2d^-1). Because ΔpCO₂ is mainly controlled by pCO₂^sea, we have evaluated the effect of the factors controlling pCO₂^sea: sea surface temperature (SST), salinity (SSS), dissolved inorganic carbon (TCO₂), and total alkalinity (A_T). In the WNP, not only SST but also TCO₂ plays an important role in the seasonal pCO₂^sea variation, while the SST could only explain most of the pCO₂^sea variation in the ENP. From 1999 to 2006, pCO₂^sea increased at a significantly lower rate (0.53 ± 0.11 µatm yr^-1) than pCO₂^air (1.81 ± 0.01 µatm yr^-1) in the WNP, and at a slightly lower rate in the ENP (1.32 ± 0.16 µatm yr^-1). The air-sea CO₂ flux increased at a rate of 0.19 ± 0.05 mmol m^-2d^-1 yr^-1 in the WNP and 0.09 ± 0.03 mmol m^-2d^-1 yr^-1 in the ENP, suggesting that the WNP is a stronger sink for atmospheric CO₂.