Sea-surface partial pressure of carbon dioxide in the eastern equatorial Pacific (August 1991 to October 1992): A multivariate analysis of physical and biological factors

Abstract

Underway PCO 2 measurements were made every three months from a merchant ship in the equatorial Pacific Ocean. The shipping track from Panama to Tahiti crosses the equator near 100°W, collecting PCO 2, sea surface temperature (SST), fluorescence of chlorophyll, nitrate concentration and depth of the mixed layer. The survey started in August 1991, with strong upwelling conditions and PCO 2 peaking at 484 μatm right at the equator. Equatorial warming in November 1991 and decreased upwelling occurred with maximum PCO 2 at 460 μatm, at 2°30′S. With the advent of the 1992 El Niño, and the collapse of equatorial upwelling, PCO 2 was drastically reduced in March 1992. The maximum of PCO 2 was only 397 μatm at 0°40′S, and values greater than 380 μatm spread southward to 6°S. Nitrate concentration, however, was still high between the equator and 10°S. With return to normal conditions in June 1992, PCO 2 rose to 437 μatm at 2°30′S, and in October 1992 strong upwelling brought CO 2-rich water to the surface, with maximum PCO 2 at 0°12′N, at 480 μatm. Multivariate analysis of the data between 7°N and 20°S, after normalization, shows that 60% of the total variance could be explained by the first eigenvector, in which PCO 2, nitrate and fluorescence were opposed to SST. The empirical orthogonal function (EOF) associated to this eigenvector was maximum at or near the equator and decreased abruptly to the north but gradually to the south. This EOF and the corresponding eigenvector could be interpreted as a synthetic representation of the influence of upwelling and of the main processes that modify PCO 2: biological carbon uptake and heating. The variations of the depth of the mixed layer (taken as the depth where temperature was 1°C below SST) were uncorrelated with PCO 2. A strong correlation ( r = 0.97, n = 5) was found between the temperature of upwelled waters (i.e. the minimum SST observed near the equator) and the average PCO 2 between 0° and 5°S.