Seasonal and lateral variations in carbon chemistry of surface water in the eastern equatorial Pacific during 1992

Abstract

During the (boreal) spring and fall of 1992 the NOAA Ocean-Atmosphere Carbon Exchange Study did an intensive survey of upper water column (⪡1000 m) chemistry in the eastern equatorial Pacific from 110°W to 170°W. The spring-time conditions were influenced by an El Niño that had disappeared before the fall cruises. This contributed to a large seasonal contrast in surface temperature, carbon and nutrient concentrations, and thermocline depth. Nitrate, total inorganic carbon, and fugacity of CO 2 values were significantly lower in the spring, while sea surface temperatures south of the equator were higher. The seasonal change in surface water chemistry at the equator is due to changes in upwelling of nutrient and carbon enriched water. Oxygen and CO 2 anomalies at the surface point to approximately a three-fold increase in upwelling of thermocline water in the fall compared to the spring. The large-scale spatial variations in the surface chemistry patterns remained unchanged between spring and fall. There was a westward decrease in surfacewater carbon and nitrate concentrations and a strong north to south asymmetry with higher carbon and nitrate values south of the equator. This pattern is attributed to input of carbon and nutrients with the South Equatorial Current from the east. Using velocities obtained from surface drifters tracks, along with reasonable gas exchange estimates, and a “Redfield analysis” to account for export biological production, this westward decrease in carbon and nutrients can be quantitatively accounted for in the region from 0 to 3°S and 110°W to 140°W in the spring. In the fall the calculated concentration decrease is greater than observed, which is attributed to input from local equatorial upwelling along the pathway of water transit.