Distribution of chemical tracers in the eastern equatorial Pacific during and after the 1982–1983 El Niño/Southern Oscillation event

Abstract

two cruises were conducted in the central equatorial Pacific to determine the effects of the El Nifio/Southern Oscillation (ENSO) event on the sea-air exchange of CO2. Measurements of total carbon dioxide, pCO2, total alkalinity, freon-l l, salinity, temperature, oxygen, nutrients, and wind and current velocities were made along three meridional transects (158øW, April 1983; 150øW, March 1984; 170øW, April 1984). The cessation of upwelling during the ENSO event caused pCO2 concentrations in surface waters to decrease to near-saturation levels along the equator. The calculated net flux of CO2 across the sea-air interface was essentially negligible in the eastern equatorial Pacific during this period. However, when normal trade winds and consequent upwelling returned in late 1983 and early 1984, the equatoriai region returned to a pCO2 condition of highly supersaturated •urface waters in the region between 8øN and 8øS along the 150 ø and 170øW transects. The calculated post-ENSO sea-air flux of CO2 exceeded 6.0 mmol CO2 m-2d -•. The mean annual CO2 flux in the equatorial Pacific was estimated to be 0.02 Gt of carbon during the 1982-1983 ENSO event as compared with 0.6 Gt during 1984. the year-to-year changes in the atmospheric growth rate of CO2 tended to reach a minimum of near-zero growth during the summer-fall period of 1982 followed by an anomalously high growth rate of•approximately 2-3 ppm yr -• during the summer-fall period of 1983. They suggested that the striking variations during the strong 1982-1983 ENSO event were due in part to variations in upwelling-induced CO2 exchange along th e equator. All of these earlier studies have been limited by a lack of pCO2 data for equato,rial surface waters which could be utilized to determine the variability of the rate of exchange of carbon dioxide between the ocean and the atmosphere. In this paper we describe the distributions of physical and chemical properties along the equator during and after the 1982-1983 ENSO event with a view toward assessing the physical factor• controlling the variability of CO2 upwelling and exchange in the eastern equatorial Pacific. Detailed measurements of the distribution of pCO2 in surface waters of the equatorial Pacific have been limited to a single time series study and a number of individual cruises over the past 25 years. The earlies• data were obtained on various ScriPps Institution of Oceanography (SIO) cruises, and the results were summarized in several papers by Keeling and coworkers [Keeling et al., 1965; Keeling, 1968]. Supersaturations of pC02 of UP to approximately 30% were observed in an equatorial zone between 10øN and 20øS. The GEOSECS data for 1973 showed similar pCO2 supersaturations along 150øW [Takahashi et al., 1980]. The only time series measurements were made by Weiss et al. [1982] during the 1979-1980 First Global Geophysical. Experiment expedition, during which pCO2 measurements were taken along 150øW on a bimonthly basis for a period of one year. The preliminary results, reported by Keeling [1983], indicated...

Carbon Dioxide in Surface Seawaters of the Western Tropical Pacific Ocean

“…Scatters of plotted points partly come from insufficient temperature control of sample seawater and partly come from pCO2 values of different water masses due to horizontal advection, because there is no appreciable equatorial upwelling during the present experiment. Vertical profiles of pCO2 measured in the eastern equatorial Pacific (Feely et al, 1987) are similar to our results. We can image the situation that the increase in pCO2 with increasing depth is associated with the decomposition of dead organisms which have sunk from the overlying surface water.…”

“…Although their pCO2 values in surface seawater are not smaller than those in the atmosphere, it is noted that low pCO2 values like our results have been obtained during periods of high surface temperature. Feely et al (1987) also reported that low pCO2 (nearly equilibrated with the atmosphere) was obtained during cessation of upwelling near central and eastern equatorial Pacific areas. These results denote the possibility that the equatorial oceans act as a sink of the atmospheric carbon dioxide under conditions of high seawater temperature during cessation of upwelling.…”

Temporal and Spatial Variations in Atmospheric and Oceanic CO₂ in the Western North Pacific from 1990 to 1993

“…The values of the CO2 flux have been estimated over the tropical Pacific during both ENSO and non-ENSO periods (Feely et al, 1987;Inoue and Sugimura,1992;Wong et al, 1993). The results can be roughly summarized as follows.…”

“…A number of previous studies have successfully related the variations in atmospheric and oceanic CO2 to ENSO events (along et al, 1984;Gammon et al, 1985;Keeling and Revelle, 1985;Feely et al, 1987;Fushimi, 1987;Conway et al, 1988;Gaudry et al, 1991, Inoue andSugimura, 1992;Wong et al, 1993). Therefore, in this section, the relationships between variations in the atmospheric and oceanic CO2 and the 1991/92 ENSO event are discussed.…”

“…One of the main results from these studies is that atmospheric CO2 concentrations show a weak or negative growth rate during (c) 1996, Meteorological Society of Japan the early stage of ENSO events, while they exhibit a strong growth rate in the later stage. In addition to the surveys of temporal variations in atmospheric CO2 concentrations associated with ENSO events, results of direct measurements of mole fractions or partial pressures of oceanic CO2 in both the western and central Pacific during ENSO events have been reported (Feely et al, 1987;Fushimi, 1987;Inoue et al, 1987;Inoue and Sugimura, 1992;Wong et al, 1993). These studies can be roughly summarized as follows.…”