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 during two cruises: the French JGOFS FLUPAC study aboard the R/V lʹAtalante in October 1994 and the Zonal Flux study aboard the R/V Thomas G. Thompson in April 1996. Both cruise tracks went along the equator from 165°E to 150°W. The cruises took place under different seasonal and El Niño-Southern Oscillation (ENSO) conditions: FLUPAC during a strong El Niño in the boreal fall and Zonal Flux during a mild La Niña in the boreal spring. Drifting sediment traps were deployed at the base of the euphotic zone and calibrated using 234Th. These traps showed over-trapping by 2.7±1.5 times during FLUPAC and 1.5±0.7 times during Zonal Flux. During the FLUPAC time-series at 167°E, the upper euphotic zone was devoid of nitrate, and particulate organic carbon export was low (6±1 mmol m−2 d−1). The FLUPAC time-series at 150°W had abundant nitrate and much higher particulate organic carbon export (12±1 mmol m−2 d−1). Similarly high levels of particulate organic carbon export were observed all along the equator during the Zonal Flux cruise (10±2 mmol m−2 d−1), when cold tongue, high nitrate conditions extended west of 165°E. sis of this data with results from the US Joint Global Ocean Flux Study (JGOFS) equatorial Pacific (EqPac) program allowed a detailed evaluation of equatorial production variability. Data from the TOGA-TAO array illustrated that both Kelvin Waves and tropical instability waves (TIW) were present during the FLUPAC cruise, while neither wave type was present during Zonal Flux. Comparison with results from the US JGOFS EqPac cruises suggested that the ubiquity of super-μM nitrate was the major forcing for new production and particle export near the equator, accounting for a doubling of production over areas with only subsurface nitrate. Within the high nitrate zone, new production and particle export were both found to be enhanced during TIW activity and diminished during Kelvin Wave activity. While the geographical extent of surface nutrients and associated enhanced production is clearly a strong function of season and ENSO, we suggest that equatorially trapped waves — rather than long-term variability in upwelling velocity — are the dominant sources of variability within the equatorial upwelling zone. Comparison of new production and particle export and regressions between nitrate and total organic carbon (TOC) suggest that accumulation and transport of TOC accounts for 17–27% of new production.