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, nutrien ts, wind, and current velocities were made during a cruise (January-Ma rch 1991) in the equatorial Pacific from Panama to Noumea via Tahiti. In the western Pacific (140-degrees-W to 165-degrees-E) the westward S outh 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 thi s area. The zonal distribution of chemical tracers is not regular beca use of intrusions of warmer water from the north associated with equat orial long waves. The temporal changes in PCO2 result from thermodynam ic changes, biological activity, and gas exchange with the atmosphere. In order to compare the magnitude of these processes, we assess the v ariations of PCO2 (dPCO2) between two stations as the sum of thermodyn amic changes driven by temperature and salinity changes, air-sea excha nge computed from observed wind and difference of PCO2 between the sea and the atmosphere, and the biological activity estimated from the ni trate decrease and C:N ratio (106:16). The resulting assessed change i n PCO2 is in agreement with the observed change for 42 pairs of statio ns. 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 C O2 outgassing to dpCO2 is low, between -0.2 to -0.0 mu atm d-1. The th ermodynamical 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 abs olute value implying an average value of new production along the equa tor of 72 +/- 25 mmolC m-2 d-1 (0.9 +/- 0.3 gC m-2 d-1) for the equato rial Pacific (130-degrees-W-165-degrees-E). This value is very high an d the overestimation could result from the simplistic description of t he advection and mixing of water. An attempt to account for these proc esses by constraining the net heat flux to 100 W m-2 [Weare et al., 19 8 1] reduces the estimate of new production to 58 mmolC m-2 d-1 (0.7 g C 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.