Distributions and variations in the partial pressure of CO2 in surface waters (pCO2w) of the central and western equatorial Pacific during the 1997/1998 El Niño event

Measurements of partial pressure of CO2 in surface waters (pCO2w) and overlying air (pCO2a) were made in the central and western equatorial Pacific from October 1997 to February 1998 within the period of the 1997/1998 El Niño, which was reported to be the strongest El Niño event on record. The distribution of the pCO2w showed a pattern driven by the eastward movement of western Pacific warm pool and thermodynamic effects (temperature and salinity), which was different from those of the moderate 1986/1987 El Niño and non-El Niño periods. Due to the eastward movement of the warm pool with sea surface temperature (SST) higher than 28.5 °C and sea surface salinity (SSS) lower than 34.5, the pCO2w between 180° and 163°W (347–364 μatm) was almost equal to that of the air (351 μatm). Between 143°E and 180°, the pCO2w tended to increase toward the west (387 μatm at 0°, 144°E in December 1997) along with the SST and SSS. West of 143°E in January 1998, a steep change in pCO2w ranging from 320 to 365 μatm occurred while retaining high SST (>28.5 °C) and SSS (>34.5). This was caused by the advection of surface water from the southern low latitudes that had been affected by biological activity (New Guinea Coastal Current). From December 1997 to January/February 1998, the SSS was usually higher than 34.5 west of 180°, which was significantly high compared to the western equatorial Pacific warm pool. This was probably due to the decrease of the net fresh water input for the western equatorial Pacific and/or the northward migration of surface water from the Southern Hemisphere. The CO2 outflux from the central and western equatorial Pacific (5.5°S–5.5°N, 139.5°E–159.5°W) was estimated to be 0.027 Pg-C/year in December 1997 and 0.038 Pg-C/year in January/February 1998. This presents a significant decrease from the CO2 outflux during the non-El Niño periods (0.34 Pg-C/year in January/February 1989, 0.11 Pg-C/year in September/November 1990) and a slight one from the moderate 1986/1987 El Niño period (0.055 Pg-C/year in January/February 1987). Following the El Niño–Southern Oscillation phenomena, CO2 outflux varied largely in the central equatorial Pacific and little in the western equatorial Pacific.