Pacific warm pool and divergence: temporal and zonal variations on the equator and their effects on the biological pump

Abstract

The equatorial Pacific consists of two regions that have distinct hydrological features and ecosystem dynamics: the warm pool to the west and the Pacific Equatorial Divergence (PEQD). The two regions are separated by well-defined fronts in salinity, pCO2, and macronutrients. Both the input of macronutrients in the two regions and their zonal extension are influenced primarily by the El Niño Southern Oscillation (ENSO) variability, which may be depicted by the Southern Oscillation Index (SOI). Thus, a quantitative relationship can be established between the longitude of the front and the SOI (Geophys. Res. Lett. 23 (1996) 1781–1784), allowing an assessment of the zonal extension of the two regions. In the PEQD, equatorial transects have shown that there is much temporal and spatial stability of biomass and fluxes of the biological pump, regardless of variations in macronutrient concentrations. This is a consequence of the High Nutrient Low Chlorophyll (HNLC) status of the waters, where primary production is not limited by the input of macronutrients. Moreover, the lack of an E–W gradient in the biological parameters supports the view that micronutrient inputs also should be more or less constant with longitude along the equator. A review of temporal variability on scales ranging from ENSO cycles to diel cycles leads to the conclusion that much of the biomass and flux variations along equatorial transects may be ascribed to temporary meridional transport by Tropical Instability Waves (TIW). In the warm pool, primary production is regulated by the input of macronutrients to the photic zone. These inputs from the deep nutricline depend on the ENSO cycle and temporary events. However, primary production is less than in the PEQD. A monthly estimate of the biological pump was made for the period 1980–2000 and the 1°N–1°S band of the divergence, west of the Galapagos Islands, on the assumption that: the longitude of the front was directly related to the SOI; there were no zonal variations in biological parameters; and mean rate values could therefore be used for primary production, new production, particle sinking and fluxes due to diel migration. The average longitude of the front was 178°W, and mean new production was 106–134 Mt C yr−1; it was 76–96 Mt C yr−1 during strong El Niño years and 114–144 Mt C yr−1 during other years. These variations between the two ENSO situations are less than those of published CO2 fluxes to the atmosphere, a result that can be ascribed to the particularly stable HNLC system.