Seasonal and Inter-Annual Variations in pCO2sea 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 (pCO2air-pCO2sea) 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 pCO2sea, we have evaluated the effect of the factors controlling pCO2sea: sea surface temperature (SST), salinity (SSS), dissolved inorganic carbon (TCO2), and total alkalinity (AT). In the WNP, not only SST but also TCO2 plays an important role in the seasonal pCO2sea variation, while the SST could only explain most of the pCO2sea variation in the ENP. From 1999 to 2006, pCO2sea increased at a significantly lower rate (0.53 ± 0.11 µatm yr-1) than pCO2air (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.