PCO2, CHEMICAL-PROPERTIES, AND ESTIMATED NEW PRODUCTION IN THE EQUATORIAL PACIFIC IN JANUARY-MARCH 1991

Measurements of the partial pressure of CO2 (PCO2) at the sea surface, dichlorodifluoromethane (F12), salinity, temperature, oxygen, nutrients, wind, and current velocities were made during a cruise (January-March 1991) in the equatorial Pacific from Panama to Noumea via Tahiti. In the western Pacific (140-degrees-W to 165-degrees-E) the westward South Equatorial Current is well established. Distributions of tracers show extrema near the equator in the eastern Pacific (from 95-degrees-W to 140 W), indicating that the upwelling is especially active in this area. The zonal distribution of chemical tracers is not regular because of intrusions of warmer water from the north associated with equatorial long waves. The temporal changes in PCO2 result from thermodynamic changes, biological activity, and gas exchange with the atmosphere. In order to compare the magnitude of these processes, we assess the variations of PCO2 (dPCO2) between two stations as the sum of thermodynamic changes driven by temperature and salinity changes, air-sea exchange computed from observed wind and difference of PCO2 between the sea and the atmosphere, and the biological activity estimated from the nitrate decrease and C:N ratio (106:16). The resulting assessed change in PCO2 is in agreement with the observed change for 42 pairs of stations. Each of these pairs of stations is thus considered as representing a simple water mass advected by the measured currents between the two stations so that daily fluxes can be estimated. The contribution of CO2 outgassing to dpCO2 is low, between -0.2 to -0.0 mu atm d-1. The thermodynamical dPCO2 averages 0.7 +/- 0.2 mu atm d-1 in the mixed layer. The biological dPCO2 (-1.5 +/- 0.5 mu atm d-1) is the highest in absolute value implying an average value of new production along the equator of 72 +/- 25 mmolC m-2 d-1 (0.9 +/- 0.3 gC m-2 d-1) for the equatorial Pacific (130-degrees-W-165-degrees-E). This value is very high and the overestimation could result from the simplistic description of the advection and mixing of water. An attempt to account for these processes by constraining the net heat flux to 100 W m-2 [Weare et al., 198 1] reduces the estimate of new production to 58 mmolC m-2 d-1 (0.7 gC m-2 d-1). A mean upwelling velocity of 0.5 +/- 0.1 m d-1 east of 140-degrees-W is calculated, based on F12 undersaturations.