Geographical distribution of new production in the western/central equatorial Pacific during El Niño and non-El Niño conditions

Abstract

Rates of new production in the warm waters of the tropical Pacific and their seasonal to interannual variability are estimated by measurement of 15N-nitrate uptake along the equator in the western and central (145° E-165° W) equatorial Pacific. Measurements were carried out during non-El Niño conditions and two examples of anomalous conditions: the moderate El Niño in 1994-1995 and the strong El Niño in 1997-1998. Variations in new production are explained in relation to changes in physical, chemical, and biological environments during the El Niño-Southern Oscillation cycle. During non-El Niño conditions in the western region the rates of integrated new production over the euphotic zone were low (0.15 mmol m-2 d-1) and likely limited by availability of nitrate. In the central region, new production was higher (1.37 mmol m-2 d-1) owing to upwelling of nitrate-rich subsurface waters. The east-west asymmetry of observed variables in the western and central equatorial Pacific is altered during El Niño conditions owing to the eastward expansion of the western Pacific Warm Pool. In the western region, during the moderate El Niño in 1994-1995, the integrated new production increased slightly compared to non-El Niño conditions, while during the strong El Niño in 1997-1998, it was higher by a factor of 10 than in non-El Niño conditions. In the central region during the 1994-1995 El Niño, the rates of new production markedly decreased compared to non-El Niño conditions. During the El Niño in 1997-1998, the integrated rates of new production were comparable to non-El Niño conditions, but the vertical distribution showed a downward displacement of maximum rates. The results of our study suggest that interannual variations in new production in the western and central equatorial Pacific correlate well with the change of the nitracline depth during the eastward expansion of the Warm Pool and depend strongly upon the severity of the El Niño event. Copyright 2001 by the American Geophysical Union.