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: During the last decade, the analysis of the ocean color satellite imagery has allowed determining the dominant phytoplankton groups in surface waters through the development of bio-optical models aimed at identifying the main phytoplankton functional types (PFTs) or size classes from space. One of these bio-optical model is PHYSAT, which is a global method applied for oceanic Case I water and used to identify in satellite pixels specific dominant phytoplankton groups, such as nanoeukaryotes, Prochlorococcus, Synechococcus, diatoms, Phaeocystis-like and coccolithophores. Here, we present a regionalized version of the PHYSAT method that has been specifically developed for the Mediterranean Sea due to the peculiarities of phytoplankton assemblages and succession than can be found in the basin and its particular optical properties. The updated version of the method, the so called PHYSAT-Med, has been validated successfully with large in situ datasets available for this oceanic region, mainly for nanoeukaryotes, Prochlorococcus, Synechococcus and diatoms. PHYSAT-Med allows to include a much higher number of pixels for the Mediterranean than PHYSAT does, through the use of a new Look-Up-Table created specifically for this oceanic region. Results provided by PHYSAT-Med showed the dominance of Synechococcus versus prochlorophytes throughout the year at the basin level, although nanoeukaryotes were more abundant during winter months. In addition, PHYSAT-Med data identified a rise in the eukaryote biomass (mainly diatoms) during the spring period (March to April), especially in the Ligurian and Adriatic seas. PHYSAT-Med represents a useful tool for the spatio-temporal monitoring of different dominant phytoplankton functional types in Mediterranean surface waters at a high resolution.Remote Sensing of Environment 09/2014; 152:557–575. · 5.10 Impact Factor
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ABSTRACT: Recent analyses of physical measurements show the existence of a central Pacific type of El Niño (CPEN) with a sea surface temperature warming pattern distinct from that of the "classical" eastern Pacific El Niño (EPEN). In this study, we analyze the surface chlorophyll signature of El Niño-Southern Oscillation (ENSO), using monthly maps of satellite-derived chlorophyll anomalies between September 1997 and December 2010. We identify five typical ENSO structures. The first structure describes the lonely 1997-1998 EPEN of the period, the second and third represent La Niña, the fourth illustrates intermediate conditions, and the fifth characterizes CPEN. During the 1997-1998 EPEN, a large eastward shift of the oligotrophic warm pool and a reduction of equatorial upwelling result in negative chlorophyll anomalies east of 170°E between 10°S and 10°N. During the four CPEN events, a reduced eastward shift yields negative chlorophyll anomalies in the equatorial band, within about 160°E and 160°W only. Westward surface current in the central basin limits the expansion of the anomaly core. Negative chlorophyll anomalies that extend eastward from the equatorial anomaly core probably result from reduced upward iron fluxes linked to the deepening of the Equatorial Undercurrent. During La Niña, the westward expansion of the equatorial upwelling results in positive chlorophyll anomalies west of the date line. Away from the equatorial band, advection of oligotrophic warm pool waters by enhanced eastward countercurrents drives negative anomalies within 8-10°N and toward the Marquesas Islands during CPEN, while reduced countercurrents lead to positive chlorophyll anomaly during La Niña.Journal of Geophysical Research (Oceans). 04/2012; 117(C4):4007-.
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ABSTRACT: With the emergence of decadal predictability simulations, research toward forecasting variations of the climate system now covers a large range of timescales. However, assessment of the capacity to predict natural variations of relevant biogeochemical variables like carbon fluxes, pH, or marine primary productivity remains unexplored. Among these, the net primary productivity (NPP) is of particular relevance in a forecasting perspective. Indeed, in regions like the tropical Pacific (30$,^circ$N--30$,^circ$S), NPP exhibits natural fluctuations at interannual to decadal timescales that have large impacts on marine ecosystems and fisheries. Here, we investigate predictions of NPP variations over the last decades (i.e., from 1997 to 2011) with an Earth system model within the tropical Pacific. Results suggest a predictive skill for NPP of 3 y, which is higher than that of sea surface temperature (1 y). We attribute the higher predictability of NPP to the poleward advection of nutrient anomalies (nitrate and iron), which sustain fluctuations in phytoplankton productivity over several years. These results open previously unidentified perspectives to the development of science-based management approaches to marine resources relying on integrated physical-biogeochemical forecasting systems.Proceedings of the National Academy of Sciences. 07/2014;