The time and space variations of the chlorophyll concentration at the surface of the Tropical Pacific are examined from 1978 to 1989, on the basis of an extensive sampling carried out in cooperation with the crews of merchant ships. The data are distributed into three sets, which correspond to shipping tracks from New Caledonia to Panama (east, 203 transects interpolated at each degree between 7¿N and 23¿S), to North America (center, 182 transects between 15¿N and 15¿S), or to Japan (west, 119 transects between 15¿N and 15¿S). The data are log-transformed, in order to stabilize the variance, and an empirical orthogonal function (EOF) analysis is applied. For each of the three data sets, the first EOF has a uniform eigenvector (spatial pattern) and extracts about 50% of the total variance; it mostly results from biases which affect the transects. In the central and western Paicfic, the second, third and fourth EOFs correspond respectively to the equatorial upwelling, to a north-south balance centered at the equator, and to phenomena which produce maxima at 5¿N or 5¿S, probably related to the equatorial countercurrents. In the eastern Pacific the second EOF opposes the northern to the southern waters and seems to account for the meridional displacements of the thermal front located near the Galapagos Islands.

The multiyear variations caused by the El Ni¿o--Southern Oscillation dominate most EOF time functions (eastern and central Pacific, EOFs 1, 2 and 3; western Pacific, EOFs 2 and 4). Weak seasonal variations are sometimes suggested by these time functions. The results from the EOF analysis are used to reconstruct a tentative, unbiased chlorophyll field from 1979 to 1989. ¿ American Geophysical Union 1992