CAFE Project

Science Components: Marine Ecosystems and Fisheries

Scientific Activities

International attention was first drawn to El Niño following the collapse of the Peruvian anchovy fishery in the wake of the 1972-73 El Niño. The hydrographic effects of El Niño caused sea temperatures to elevate, coastal salinity to freshen, and the nutricline to deepen. The biological response to these changes was a marked reduction in the anchovy habitat, a virtual collapse in the number of anchoveta being harvested, and an incursion of sardines replacing the anchovies (Barber and Chavez, 1983). In addition, the centroid of the catch per unit effort for skipjack tuna, residing at the eastern edge of the western Pacific warm pool, is displaced up to 4000 km zonally as a function of ENSO phase (Lehodey et al, 1997). This relationship suggests a predictive capability for this fishery may be possible as a function of SST.

Although the number of hydrographic observations related to El Niño has increased dramatically since the 1982-83 El Niño, in situ biological observations have remained limited. Satellite observations of ocean color have proved valuable for providing spatial coverage of the lowest levels of the marine food chain, i.e., near- surface chlorophyll concentration as an indicator of primary production. The Coastal Zone Color sensor (CZCS) on Nimbus 7 produced periodic views of ecosystem changes during the 1982-83 El Niño. The satellite imagery showed a major phytoplankton bloom downstream, west of the Galapagos Islands, prior to El Niño (Feldman et al., 1984); the phytoplankton concentration dropped dramatically and the remnants shifted east of the islands. Post-El Niño conditions were present by October 1983, and the CZCS imagery indicated a return to westward surface flow and a phytoplankton bloom extending 1000km west of the Galapagos. For the 1992 El Niño, Barber et al. (1996) have estimated primary production to be of order 250 gC/m²/yr and nearly 500 gC/m²/yr during La Niña conditions.

After a hiatus of more than 10 years, space-based ocean color observations resumed in August 1997. The launch of the SeaWiFS sensor coincided with the development of the extreme El Niño in 1997. High quality ocean color data (chlorophyll) provided by the SeaWiFS satellite were analyzed for the first complete year of coverage (October 1997 through September 1998) in the tropical Indo-Pacific basin. The ocean color variability is interpreted using other satellite data such as sea level from TOPEX/Poseidon, and also in terms of the dynamics and thermodynamics of the region from simulations with an ocean general circulation model.

Some of the first images from SeaWiFS were consistent with that seen 15 years earlier during the last major El Niño depicting a highly localized and reduced phytoplankton bloom in the vicinity of the Galapagos, suggestive of minimal nutrient availability. Reductions in equatorial production and the off-equatorial increase in biological activity and their basin-scale evolution were clearly seen for the first time. Persistent northerly wind anomalies resulted in a northward shift of the equatorial divergence and an upwelling Kelvin wave, which signaled the end of the 1997-98 El Niño. The anomalous surface chlorophyll associated with this Kelvin wave was also shifted north of the equator by nearly 300 km and appeared more than a month before the negative sea-level anomalies registered by TOPEX/Poseidon.

In the Indian Ocean, an anomalous phytoplankton bloom was observed by SeaWiFS during October-December 1997 coincident with the anomalous upwelling in the eastern equatorial region and off the coast of Sumatra. A stronger than normal northeast monsoon was seen as higher than climatological values of surface chlorophyll. The open ocean Ekman pumping and shoaling of the thermocline near 6°E and 1°S and the eastward extension of mixed-layer entrainment in the same latitude band was also seen as a region of higher biological activity during the boreal summer.

Although SeaWiFS provides unprecedented quantitative global ocean color data, this view is limited to an optically thin layer of the ocean surface waters. Therefore, to quantitatively explore the interaction of the physical climate system and marine ecosystems, we are developing a fully coupled physical-ecosystem model of the tropical Pacific Ocean. The 1997-98 El Niño and the contemporaneous observations from SeaWiFS provide a valuable opportunity to implement and test such a coupled model.

Initial simulation development has been performed with 1-D ecosystem models containing phytoplankton, zooplankton, and nutrients forced in an offline mode by the output from ocean circulation models. A further development of this work will involve the fuller inclusion of marine ecosystem models consisting of several size classes of phytoplankton and zooplankton within a model of the tropical Pacific Ocean (McClain et al., 1998b). The physical component of the model, which has been extensively tested in regard to El Niño (Gent and Cane, 1990), is based on a reduced gravity, primitive equation, sigma coordinate ocean general circulation model (OGCM), with an embedded variable-depth mixed-layer model (ML) (Chen et al., 1994). The model has also been coupled to an atmospheric mixed-layer model, and subsequently used to simulate sea surface temperature variability in all three tropical oceans (Murtugudde et al., 1996). An active hydrological cycle has been included for the realistic simulation of ocean salinity (Murtugudde and Busalacchi, 1998).

The ecosystem component of the model is based on the one-dimensional ecosystem model of Leonard et al. (1999). It has nine biological components: two phytoplankton functional groups, two zooplankton functional groups, two detrital sizes, nitrate, ammonium, and iron. It is designed to simulate the important biological processes of species succession, grazing pressure, macro-nutrient limitation (nitrate and ammonium), and micro-nutrient limitation (iron), among others. These processes are essential in correctly simulating the processes known or hypothesized to be important during El Niño events.

2000-2001 Progress

Current progress in the Marine Ecosystems components is described in three publications:

• Christian et al. (2001a,b) describes the seasonal and interannual variability of the coupled biogeochemical model including simulations of primary production during the 1997-1998 El Niño as compared to ocean color observations from SeaWiFS. This is also one of the first 3-D coupled ecosystem models to explicitly include the effects of iron geochemistry.
• Murtugudde et al. (2001) illustrates the importance of a realistic treatment of penetrating radiation in climate models as a direct result of the ocean color data sets that now permit estimates of spatially and temporally varying attenuation depths in the ocean.
• Picaut et al. (2001) and Turk et al (2001) describe the use of remotely sensed observations to analyze changes in the physical climate and biological production related to ENSO.

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