The impact of equatorial Pacific tropical instability waves on hydrography and nutrients: 2004-2005

This paper documents the variability in physics and nutrients during two cruises spanning 110°W to 140°W in December 2004 and September 2005. The goal of this work is to set the hydrographic framework for companion papers which quantify the role of iron, silicon and grazing in maintaining the high nitrate, low chlorophyll (HNLC) conditions in the equatorial Pacific. The two cruises were conducted almost a year apart, at different phases of the El Niño cycle, but during similarly intense tropical instability wave (TIW) seasons. The higher phytoplankton biomass observed on the 2005 cruise was due to a combination of time of year and a weakening El Niño. A general relationship between TIWs and Si cycling is described. TIWs advect the equatorial upwelling plume alternately to the north and south, and also generate localized enhanced upwelling. The distorted upwelling plume is a region of enhanced biogenic silica production and export. Away from regions of active upwelling, Si remineralization is enhanced and export is significantly reduced or absent.