Paulette P. Murphy

Assessment of the air‐sea exchange of CO2 in the south Pacific during austral autumn

Measurements of CO2 concentrations in the atmosphere and in the surface waters of the South Pacific Ocean were made by NOAA scientists between 1984 and 1989. These basin-wide measurements were all taken during austral autumn and provide data for evaluation of the seasonal flux of CO2 from this region. The sensitivity of this flux to the uncertainty in the CO2 gas exchange coefficient was evaluated using four different wind data sets and two formulations for the wind dependence of gas transfer velocity. The integrated net flux of CO2 to the atmosphere during austral autumn (February to May) ranges from −0.03 (ocean influx) to +0.09 (ocean efflux) GT of carbon depending on the combination of wind field and wind-dependent exchange coefficient used.

The internal consistency of CO2 measurements in the equatorial Pacific

Abstract During a recent NOAA JGOFS Equatorial Pacific cruise all four analytical parameters of the carbonate system were measured—pH, total alkalinity (TA), total carbon dioxide (TCO2), and the fugacity of carbon dioxide (fCO2). The measurements made during leg 2 on surface waters have been used to examine the internal consistency of the carbon dioxide system in these waters. The internal consistency of the measurements was examined by using various inputs of the measured parameters (pHTA, pHTCO2, pHfCO2, fCO2TA, fCO2TCO2 and TATCO2) to calculate the components of the CO2 system. The results indicate that the measurements have an internal consistency of ±0.003–0.006 in pH, ±5–7 μmol kg−1 in TA, ±5–7 μmol kg−1 in TCO2 and ±6–9 μAtm in fCO2 if reliable constants are used for the dissociation of carbonic acid in seawater. These results indicate that our present understanding of the thermodynamics of the carbonate system in seawater is close to the present accuracy in measuring the various parameters of the system (±0.002 in pH, ±4 μmol kg−1 in TA, ±2 μmol kg−1 in TCO2 and ±2 μAtm in fCO2).

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 Nino 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.

The effect of tropical instability waves on CO2 species distributions along the equator in the eastern equatorial Pacific during the 1992 ENSO event

Tropical instability waves have been shown to have a major impact on the variability of temperature and nutrients along the equatorial wave guide. In order to assess the impact of these features on carbon species distributions during an ENSO event, sea surface temperature, salinity, sigma-t, nitrate, CO[sub 2] fugacity, total inorganic carbon, total alkalinity, and pH along the equator were measured from 130[degrees]W to 100[degrees]W during 8-15 May 1992. Concurrent moored measurements of surface currents and temperature were also made at 0[degrees], 110[degrees]W. Results indicate that tropical instability waves, with periods of 15 to 20 days and zonal wavelengths of 700-800 km, controlled the observed spatial variability of the CO[sub 2] species, nitrate and hydrographic parameters at the equator. 37 refs., 3 figs.

Surface water fCO2 in the eastern equatorial Pacific during the 1992–1993 El Niño

The fugacity of CO 2 in surface water (fCO 2w ) was measured in the eastern equatorial Pacific (EEP) during the boreal spring and fall of 1992 and in the spring of 1993. A prolonged El Nino occurred during this period with anomalously warm sea surface temperatures (SST) during the spring of 1992 and 1993. Correspondingly, the fCO 2w values were lower than historical non-El Nino values at the equator. However, the fCO 2w in the spring of 1993 was up to 50 μatm higher than in the spring of 1992, despite similar SSTs. The trend is attributed to the slower response times of factors causing fCO 2w decrease compared to rapid increase of fCO 2w by upwelling of cold water with high carbon content and subsequent heating. During the fall of 1992, SSTs south of the equator were 5°C cooler than in the spring, which is indicative of vigorous upwelling of water with high CO 2 content from below the thermocline. Decreases in fCO 2w due to net biological productivity and gas exchange take of the order of months, causing the fCO 2w levels during the spring of 1993 to be elevated compared to the spring of the previous year. Our data and data obtained in 1986 and 1989 along 110°W suggest that fCO 2 maxima in the equatorial Pacific can be either associated with temperature minima or temperature maxima. Despite the multitude of factors which influence fCO 2w , most of the variance can be accounted for with changes in nitrate and SST. A multilinear regression of fCO 2w with SST and nitrate for the 1992 data has a standard error in predicted fCO 2w of 10 μatm. Air-sea fluxes of CO 2 in the EEP were estimated to be 30% higher in the spring of 1993 and 10% higher in the fall of 1992 than in the spring of 1992.

North Pacific Ocean CO2 disequilibrium for spring through summer, 1985–1989

Extensive measurements of CO2 fugacity in the North Pacific surface ocean and overlying atmosphere during the years 1985–1989 are synthesized and interpreted to yield a basin-wide estimate of ΔfCO2. The observations, taken from February through early September, suggest that the subtropical and subarctic North Pacific is a small sink for atmospheric CO2 (0.07 to 0.2 Gton C (half year)−1 for the region north of 15°N). Objective analysis techniques are used to estimate uncertainty fields resulting from constructing basin-wide contours of oceanic fCO2 on the basis of individual cruise transects. The uncertainties are significant and imply that future sampling programs need to recognize that estimating oceanic uptake of anthropogenic CO2 from ship-transect observations of oceanic fCO2 alone will require very extensive sampling.