Variability of CO2 distributions and sea-air fluxes in the central and eastern equatorial Pacific during the 1991-1994 El Nino

As part of the U.S. JGOFS Program and the NOAA Ocean-Atmosphere Carbon Exchange Study (OACES), measurements of CO2 partial pressure were made in the atmosphere and in the surface waters of the central and eastern equatorial Pacific during the boreal spring and autumn of 1992, the spring of 1993, and the spring and autumn of 1994. Surface-water pCO(2) data indicate significant diurnal, seasonal, and interannual variations. The largest variations were associated with the 1991-1994 ENSO event, which reached maximum intensity in the spring of 1992. The lower values of surface-water Delta pCO(2) observed during the 1991-1994 ENSO period were the result of the combined effects of both remotely and locally forced physical processes. The warm pool, which reached a maximum eastward extent in January-February of 1992, began in September of 1991 as a series of westerly wind events lasting about 30 days. Each wind event initialed an eastward-propagating Kelvin wave which caused a deepening of the thermocline. By the end of January 1992 the thermocline was at its maximum depth, so that the upwelled water was warm and CO2-depleted. In April of the same year, the local winds were weaker than normal, and the upwelling was from shallow depths. These changes resulted in a lower-than-normal CO2 flux to the atmosphere. The results show that for the one-year period from the fall of 1991 until the fall of 1992, approximately 0.3 GtC were released to the atmosphere; 0.6 GtC were released in 1993, and 0.7 GtC in 1994, in good agreement with the model results of Ciais et al. [Science, 269, 1098-1102; J. Geophys. Res., 100, 5051-5070]. The net reduction of the ocean-atmosphere CO2 flux during the 1991-1994 El Nino was on the order of 0.8-1.2 GtC. Thus, the total amount of CO2 sequestered in the equatorial oceans during the prolonged 1991-1994 El Nino period was about 25% higher than the severe El Nino of 1982-1983. (C) 1998 Elsevier Science Ltd. All rights reserved.