Climatological mean and decadal change in surface ocean pCO_2, and net sea-air CO_2 flux over the global oceans

摘要

A climatological mean distribution for the surface water pCO_2 over the global oceans in non-El Nino conditions has been constructed with spatial resolution of 4° (latitude) × 5° (longitude) for a reference year 2000 based upon about 3 million measurements of surface water pCO_2 obtained from 1970 to 2007. The database used for this study is about 3 times larger than the 0.94 million used for our earlier paper [Takahashi et al., 2002. Global sea-air CO_2 flux based on climatological surface ocean pCO_2, and seasonal biological and temperature effects. Deep-Sea Res. II, 49, 1601-1622]. A time-trend analysis using deseasonalized surface water pCO_2 data in portions of the North Atlantic, North and South Pacific and Southern Oceans (which cover about 27% of the global ocean areas) indicates that the surface water pCO_2 over these oceanic areas has increased on average at a mean rate of 1.5μatmy~(-1) with basin-specific rates varying between 1.2±0.5 and 2.1 ± 0.4 μatmy~(-1). A global ocean database for a single reference year 2000 is assembled using this mean rate for correcting observations made in different years to the reference year. The observations made during El Nino periods in the equatorial Pacific and those made in coastal zones are excluded from the database.rnSeasonal changes in the surface water pCO_2 and the sea-air pCO_2 difference over four climatic zones in the Atlantic, Pacific, Indian and Southern Oceans are presented. Over the Southern Ocean seasonal ice zone, the seasonality is complex. Although it cannot be thoroughly documented due to the limited extent of observations, seasonal changes in pCO_2 are approximated by using the data for under-ice waters during austral winter and those for the marginal ice and ice-free zones.rnThe net air-sea CO_2 flux is estimated using the sea-air pCO_2 difference and the air-sea gas transfer rate that is parameterized as a function of (wind speed)~2 with a scaling factor of 0.26. This is estimated by inverting the bomb ~(14)C data using Ocean General Circulation models and the 1979-2005 NCEP-DOE AMIP-II Reanalysis (R-2) wind speed data. The equatorial Pacific (14°N-14°S) is the major source for atmospheric CO_2, emitting about +0.48Pg-Cy~(-1) and the temperate oceans between 14° and 50° in the both hemispheres are the major sink zones with an uptake flux of -0.70Pg-Cy~(-1) for the northern and -1.05Pg-Cy~(-1) for the southern zone. The high-latitude North Atlantic, including the Nordic Seas and portion of the Arctic Sea, is the most intense CO_2 sink area on the basis of per unit area, with a mean of -2.5 tons-C month~(-1) knrr~2. This is due to the combination of the low pCO_2 in seawater and high gas exchange rates. In the ice-free zone of the Southern Ocean (50-62°S), the mean annual flux is small (-0.06Pg-Cy~(-1)) because of a cancellation of the summer uptake CO_2 flux with the winter release of CO_2 caused by deepwater upwelling. The annual mean for the contemporary net CO_2 uptake flux over the global oceans is estimated to be -1.6 + 0.9Pg-Cy~(-1) which includes an undersampling correction to the direct estimate of -1.4 + O^Pg-Cy~(-1). Taking the pre-industrial steady-state ocean source of 0.4 ±0 .2Pg-Cy~(-1) into account, the total ocean uptake flux including the anthropogenic CO_2 is estimated to be -2.0 + l.0Pg-Cy~(-1) in 2000.