Variation of CO2 partial pressure in surface seawater in the equatorial Pacific Ocean

The partial pressure of CO2 gas in surface seawater (pCO2sea) and in the atmosphere (pCO2air) was continually measured in the equatorial Pacific Ocean during two US JGOFS cruises in 1992. Time-series measurements at the equator and 140°W show that the pCO2 of surface seawater varied by 30 μatm during the 2 week period of the experiment with a diurnal variation of close to 8 μatm. The diurnal variations of pCO2sea were associated with the diurnal variations in sea surface water temperature, but the larger variation of pCO2sea on a weekly time-scale was associated with water movements (equatorial upwelling and passage of a tropical instability wave) rather than heating or cooling of surface seawater. The water movements are characterized by change in temperature as well as salinity, total inorganic carbon and total alkalinity. During the upwelling period, pCO2sea increased whereas sea surface temperature decreased. From the equator to 13°S, pCO2sea decreased from 430 μatm to 340 μatm whereas the sea surface temperature increased from 25.4°C to 28.3°C. However, there was no overall correlation between pCO2sea and sea surface temperature. Indeed, the pCO2sea varied irregularly with temperature across a complex system of fronts and eddies. The observed diurnal variations of pCO2sea while the ship was in transit between Tahiti and the equator were up to 8 μatm.