Revisiting the La Niña 1998 phytoplankton blooms in the equatorial Pacific
ABSTRACT A biogeochemical model of the tropical Pacific has been used to assess the impact of interannual variability in a western Pacific iron source on the iron-limited ecosystem of the central and eastern Pacific during the 1997–1998 El Niño. A reference simulation and two simulations with an iron source in the western Pacific have been performed. The two “source” simulations differed only in the temporal variability of the iron source. In the variable source simulation, the iron concentration in the source region was proportional to the velocity of the New Guinea Coastal Undercurrent (NGCUC). In the constant source simulation, the same time-averaged concentration of iron was imposed with no temporal variability. The variable source was designed to mimic variations of iron flux from the northeast slope of New Guinea to the NGCUC due to modulation of sedimentary iron resuspension as previously hypothesized. Through the comparison of these simulations, it appeared that: (i) an iron source in the NGCUC, regardless of its source variability, increases biomass in the eastern equatorial Pacific because of the greater eastward iron flux by the Equatorial Undercurrent and (ii) a variable NGCUC iron source does not change the temporal variability of eastern Pacific chlorophyll, and in particular the timing and intensity of the June 1998 bloom. To explain eastern Pacific biological variability, local rather than remote processes are needed, such as wind-driven upwelling, the local depth of the thermocline, tropical instability waves and biological processes such as high grazing pressure. Therefore, while the western Pacific sources of dissolved iron are important in our model to sustain annually integrated equatorial Pacific production, they are unlikely to strongly constrain the timing of blooms in the central and eastern Pacific such as during the 1998 La Niña.
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ABSTRACT: Three-dimensional monthly velocity fields from an ocean general circulation model are used to study the annual mean mass balance of the Pacific Equatorial Undercurrent (EUC). Eulerian diagnostics are used to evaluate the various meridional, vertical, and zonal mass fluxes related to the EUC. There are several distinct regimes along the equator, showing clear asymmetries between the western and eastern parts of the basin, and between the northern and southern edges of the EUC. Meridional fluxes are decomposed into pure Ekman divergence and geostrophic convergence, and it is shown that the asymmetries are mainly related to the spatial structure of the Ekman divergence, and thus to that of the trade winds. Lagrangian calculations are used to evaluate accurately the mass transfers between various sections of the EUC and between the EUC domain and the Tropics. The authors show that geostrophic convergence only ventilates the upper layers of the EUC and that the EUC really is a tongue of water flowing from the western Pacific to the Galapagos Islands and beyond. Finally, Lagrangian integrations extended to extratropical regions show that the EUC contributes to an exchange of water between the southern and northern Pacific (and the Indian Ocean through the Indonesian Throughflow): The equatorial zonal pressure gradient draws water from the western boundary currents that originate mostly in the south subtropical gyre. The poleward Ekman divergence associated with the equatorial upwelling distributes EUC water over the surface, with significant recirculation within the EUC (more than 15% of the total transport at 150°W).Journal of Physical Oceanography 01/1997; 27. · 3.18 Impact Factor
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ABSTRACT: The Nd isotopic composition (IC) and Rare Earth patterns of hydrodynamic structures of the Equatorial Pacific Ocean were characterized along 140°W. The Nd IC of Antarctic Intermediate Water (AAIW) and of the lower layer of the Equatorial Undercurrent (EUC) at 140°W (13°C Water) are much more radiogenic at the equator than at their origin in the South Equatorial Current (12°S), revealing that these water masses have been in contact with the highly radiogenic Papua New Guinea (PNG) slope. In both cases, only a small fraction (less than 9%) of the sediment deposited on the PNG slope is required to be exchanged or dissolved to explain these Nd IC variations, whereas the hydrographic properties of the same water masses remain unchanged. This confirms the usefulness of this tracer to identify pathways of water masses. These results emphasize the importance of jets in transporting lithogenic material into the subsurface layers of remote areas, where aeolian inputs are particularly weak and corroborate the previous results on Fe and Al maximum in this area [M.L. Wells, G.K. Vallis, E.A. Silver, Nature 398 (1999) 601–604]. The Nd IC of the upper layer of the EUC contrasts strongly to that of the subpycnocline layer, indicating that the equatorial upwelling only affects the surface waters and is not effective between 120 and 150 m. We calculate that the Nd imprint of the PNG input is likely to vanish from this surface layer as it traverses the basin, due to the replacement of upwelled waters by non-radiogenic ones.Earth and Planetary Science Letters. 01/2001;
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ABSTRACT: A recent set of oceanographic measurements in the western equatorial Pacific has revealed the existence of previously undescribed major ocean currents and upper-ocean mixed-layer structure. The measurements also confirm a 20-year-old hypothesis on the water-mass origins of the Equatorial Undercurrent in the Pacific Ocean.12/1987; 330(6148):533-537.