Spatial variability and decadal trend of the oceanic CO2 in the western equatorial Pacific warm/fresh water

Data of the partial pressure of carbon dioxide in surface seawater (pCO2sw) collected in the western equatorial Pacific (144°E–160°W, 5°S–5°N) since the 1980s have been used to determine the spatial variability and decadal trends in pCO2sw. A total of 104 cruises since 1983, including 30 cruises since 1990 with total dissolved inorganic carbon (TCO2) measurements, are synthesized for this zone. The western equatorial Pacific warm/fresh surface water where T⩾29.0 °C and S⩽34.8 is nitrate-depleted, and it is moderately supersaturated with CO2 (0<ΔpCO2 (=pCO2sw–pCO2air)/μatm<40). A slight CO2 undersaturation (−20<ΔpCO2/μatm<0) also was observed on many cruises where thin, low density layers (σt<21.4) capped barrier layers. The undersaturation may result from net biological uptake of CO2 due to nitrogen fixation. Excluding the data in such extremely light waters, we determined changes in the linear growth rate of pCO2sw of +0.3±1.3 μatm year−1 for 1985–1990, +2.2±0.7 μatm year−1 for 1990–1999 and −0.2±1.0 μatm year−1 for 1999–2004. The rate of increase was +1.5±0.2 μatm year−1 for the entire period (1985–2004). The variation in the rate of increase of pCO2sw is fairly consistent with the change in the rate of increase of salinity-normalized TCO2 from +2.1±0.4 μmol kg−1 year−1 during 1992–1999 to +0.4±0.9 μmol kg−1 year−1 during 1999–2004. These changes are anti-correlated with the decadal variation in the geostrophic mass transport from subtropics of both hemispheres into the equatorial zone in the Pacific.