Ocean physics and nutrient fields along 180° during an El Niño–Southern Oscillation cold phase

Abstract

This paper presents physical and nutrient results from the Etude du Broutage en Zone Equatoriale cruise, conducted in the equatorial Pacific along the 180° meridian from 8°S to 8°N. Cold conditions of the El Niño–Southern Oscillation cycle were evident during the cruise (October–November 1996), and the equatorial upwelling was in its far western extension. Along the sampled section the nutrient‐enriched area was asymmetric around the equator, and a zone of high remineralization was found from 6° to 3°S. Intensive sampling at two 5‐day time series stations (3°S and 0°) provided some insight on high‐frequency variability. At 3°S a deep density mixed layer showed only small fluctuations in nutrients. On the equator a rapid reduction of surface nutrients during the time series station was attributed to advection of a different water mass from the northeast, in the southward branch of a Tropical Instability Wave (TIW). Measurements from the Tropical Atmosphere‐Ocean (TAO) array in the area confirmed significant contemporaneous TIW activity, which was linked to the cold conditions. Thus, in contrast to previous observations, it is shown that TIWs can contribute to relative decrease of nutrients at the equator. During the two time series sampling efforts, variability at diurnal and semidiurnal periods were found in physical parameters, originating from surface atmospheric forcing and internal wave activity at tidal frequency, respectively. In the 0–150 m layer, where intensive sampling of nutrients was performed, high‐frequency variability did not seem to modify nutrient distribution significantly and physical influences dominated over biology.