Export flux in the western and central equatorial Pacific: zonal and temporal variability

Particulate organic carbon export fluxes were measured along the equator to resolve the zonal extent of high productivity in the equatorial Pacific du ring two cruises: the French JGOFS FLUPAC study aboard the R/V l'Atalante i n October 1994 and the Zonal Flux study aboard the R/V Thomas G. Thompson i n April 1996. Both cruise tracks went along the equator from 165 degrees E to 150 degrees W. The cruises took place under different seasonal and Fl Ni no-Southern Oscillation (ENSO) conditions: FLUPAC during a strong Fl Nino i n the boreal fall and Zonal Flux during a mild La Nina in the boreal spring . Drifting sediment traps were deployed at the base of the euphotic zone an d calibrated using Th-234. These traps showed over-trapping by 2.7 +/- 1.5 times during FLUPAC and 1.5 +/- 0.7 times during Zonal Flux. During the FLU PAC time-series at 167 degrees E, the upper euphotic zone was devoid of nit rate, and particulate organic carbon export was low (6 +/- 1 mmol m(-2) d(- 1)). The FLUPAC time-series at 150 degrees W had abundant nitrate and much higher particulate organic carbon export (12 +/- 1 mmol m(-2) d(-1)). Simil arly high levels of particulate organic carbon export were observed all alo ng the equator during the Zonal Flux cruise (10 +/- 2 mmol m(-2) d(-1)), wh en cold tongue, high nitrate conditions extended west of 165 degrees E. Synthesis of this data with results from the US Joint Global Ocean Flux Stu dy (JGOFS) equatorial Pacific (EqPac) program allowed a detailed evaluation of equatorial production variability. Data from the TOGA-TAO array illustr ated that both Kelvin Waves and tropical instability waves (TIW) were prese nt during the FLUPAC cruise, while neither wave type was present during Zon al Flux. Comparison with results from the US JGOFS EqPac cruises suggested that the ubiquity of super-mu M nitrate was the major forcing for new produ ction and particle export near the equator, accounting for a doubling of pr oduction over areas with only subsurface nitrate. Within the high nitrate z one, new production and particle export were both found to be enhanced duri ng TIW activity and diminished during Kelvin Wave activity. While the geogr aphical extent of surface nutrients and associated enhanced production is c learly a strong function of season and ENSO. we suggest that equatorially t rapped waves - rather than long-term variability in upwelling velocity - ar e the dominant sources of variability within the equatorial upwelling zone. Comparison of new production and particle export and regressions between n itrate and total organic carbon (TOC) suggest that accumulation and transpo rt of TOC accounts for 17-27% of new production. (C) 2000 Elsevier Science Ltd. All rights reserved.