Nitrate and chlorophyll distributions in relation to thermohaline and current structures in the western tropical Pacific during 1985–1989

Abstract

Fourteen transects carried out from 1985 to 1989 permit us to describe the nitrate and chlorophyll a distributions and their temporal variability in the tropical western Pacific (165°E, 20°S-10°N). Sections representative of the moderate 1986–1987 El Niño, the strong 1988–1989 La Niña and an equatorial westerly wind burst were compared to the January 1986 transect, which we define as a “reference” section. Along 165°E, there appears to be four characteristic distributions of nitrate and chlorophyll relative to the thermohaline structure. (1) In the south (20-17°S), a seasonal cycle was observed in both nitrate and chlorophyll distributions. Variability of surface chlorophyll and depth of the deep chlorophyll maximum (DCM) was associated with variations in the vertical mixing and available amount of light. (2) In the north (6–10°N), seasonal and interannual changes remained in the subsurface layer because of the strong stratification. Ekman pumping is one cause of the changes of the DCM depth. (3) In the 2°S–2°N band, the seasonal variability of the DCM depth was associated with variations in precipitation and eastward advection of low salinity water. During the 1986–1987 El Niño, elevation and intensification of the pycnocline and shoaling of the nutrient reservoir were the result of basin-wide changes. Consequences included abnormally low surface chlorophyll concentrations and an increase of the primary production. Intra-El Niño variations of the DCM depth were associated with changes of the thickness of the low salinity surface layer. During the subsequent La Niña, upwelling developed and vertical nitrate and chlorophyll distributions were strongly modified. Surface nitrate and chlorophyll concentrations for this period were the highest of the dataset, and primary production values were about twice the El Niño values. The main differences between upwelling in the western and central Pacific are attributed to the existence of two atmospheric convergence zones in the western basin, and especially to the tendency of the Intertropical Convergence Zone to migrate south of the equator. Intraseasonal Kelvin waves influence the variations of nutrient concentration and phytoplankton biomass. During the December 1989 westerly wind burst, there were strong modification of the thermohaline and nitrate distributions, associated with a geostrophic adjustment, which were not echoed on the chlorophyll distribution. (4) At 10°S, a doming of the different properties was frequently observed. Transient surface enrichments in nitrate or chlorophyll result from combined effects of the divergence between the South Equatorial Counter Current and the southern branch of the South Equatorial Current, Ekman pumping favourable to upwelling and local wind events. In all cases, nitrate and chlorophyll distributions were closely linked to the density structure for which salinity can be the controling factor. Low-frequency variability (seasonal, intraseasonal and interannual scales) of the nitrate and chlorophyll distributions were determined primarily by local or remote physical processes which controlled stability of the water column that governs the vertical displacements of phytoplankton.