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Investigaciones marinas

versión On-line ISSN 0717-7178

Investig. mar. v.30 n.1 supl.Symp Valparaíso ago. 2002

http://dx.doi.org/10.4067/S0717-71782002030100018 

SESSION 3: PELAGIC

Biogeochemical and Ecological
Consequences of El Niño in Eastern
Pacific Upwelling Ecosystems*

Francisco Chavez

Monterey Bay Aquarium Research Institute, 7700
Sandholdt Rd., Moss Landing, CA 95039-9644, USA,
E-mail: chfr@mbari.org

The fisherman of northern Peru coined the name El Niño in the late nineteenth century for a warm southward current that appeared every year around Christmas (the Christ child= El Niño). It was not until the 1960's that the warming of the coastal ocean off Peru (and the equatorial Pacific) was linked with larger scale climatic phenomena. It was then that a relatively small and inoffensive coastal current was associated with dramatic global weather disturbances. Over the last 30 years the influence of El Niño on oceanic and atmospheric conditions throughout the globe has been well documented. More recently, in the 1980's, Peruvian scientists coined the name La Niña for anomalously cool temperatures along Peru. This phenomenon also has significant climatic impacts.

Even though El Niño has been extensively studied many aspects remain an enigma. It has been difficult to predict the timing and intensity of the phenomenon. Once initiated, the developmental sequence of events in the equatorial Pacific and along the coast of Peru is relatively well understood. Westerly wind bursts initiate Kelvin waves that propagate along the equator, raising sea level and deepening the thermocline. The deep thermocline then influences a variety of physical, chemical and biological processes. Along the South American coast the importance of thermocline displacements associated with equatorial Kelvin waves on biological productivity has been well documented. At higher latitudes of the eastern Pacific, and off the western coast of North America in particular, the developmental sequence has not been as clear. In some cases, oceanic warming and anomalies associated with El Niño have been ascribed by some authors primarily to local meteorological anomalies caused by atmospheric teleconnections while other authors ascribed them primarily to propagation through the oceanic wave-guide.

Two very strong El Niño episodes affected the eastern Pacific during 1982-83 and 1997-98. During both cases the anomalous warming was accompanied by dramatic ecological changes, including reductions in primary and secondary production, and redistribution and disappearance of species from their typical habitat. In this contribution we analyze satellite, in situ and fish catch data and make two comparisons: 1) the effects of El Niño on upwelling ecosystems off Peru and California, and 2) the different biological responses of the 82-83 and 97-98 El Niños.

Comparison between Peru and California

The Peruvian upwelling ecosystem and the Humboldt Current upwelling system in general are very tightly linked to the equatorial wave-guide. The signature of equatorially-generated Kelvin waves can be traced clearly, using sea level, from the western Pacific across the basin on the equator and then poleward as coastally-trapped waves along the South American coast. These waves deepen the thermocline and push the nutrient rich water deeper into the water column. Even though upwelling-favorable winds continue during El Niño, the water upwelled is warm and nutrient-poor. The size of the productive habitat and primary productivity are significantly reduced in many cases by over 75%. Associated with the thermocline displacements are changes in the typical current systems and in the distribution of species. Oceanic waters impinge on to the coast and tropical waters move poleward both at the surface and below. These anomalous currents displace the typical species and larger organisms, primarily fish, migrate large distances to more favorable habitats. These large scale movements have made it difficult to quantify the impact of reduced primary production on lower trophic levels. Lower fat and body weight seem to indicate that this impact is substantial.

At first glance the biological effects off California are very similar. However, the Gulfs of Panama and California and possibly the coastline orientation all seem to act to diminish the intensity of the northward propagating coastally-trapped waves. The propagating sea level signature is harder to identify in the northern hemisphere. In addition there are significant atmospheric teleconnections (not clear that these are not important off Chile), in particular a shift in the typical position of the Aleutian Low. The onshore movement of oceanic waters, and the reductions in the size of the productive habitat and primary production have been attributed to local meteorological effects, i.e. decreases in upwelling. Observations during 1997-98, however, confirm the importance of the oceanic wave-guide in driving the physical and biological anomalies. The response of California was then remarkably similar to that in the Humboldt Current system.

Comparison between 82-83 and 97-98

Based on available sampling coverage, which is not ideal, we can make comparisons between 1982-83 and 1997-98 El Niños. There are various opinions as to which of these events was stronger, suffice to say that both of these exhibited large SST anomalies. The general pattern of hydrographic, nutrient and productivity fields were extremely similar at both sites. The response of lower trophic levels, however, was extremely different in both North and South America. Fish catch records from the coast of Peru show a remarkable recovery of anchoveta after 1998 while no similar rebound occurred during 1983. Similar patterns were reported for salmon off Oregon. Why was this? The 1982-83 El Niño was bounded by weak warm anomalies prior to and after the event. On the other hand early 1997 and late 1998 were abnormally cool. The different ecosystem responses to El Niño are then perhaps related to the longer-term background climate state of the Pacific Ocean. The 1982-83 and 1992-93 El Niños occurred during the positive/warm eastern Pacific phase of the Pacific Decadal Oscillation (PDO), a 20-40 year cycle of Pacific temperatures. The PDO appears to have changed sign in the late nineties apparently ameliorating the biogeochemical and ecological impact of the 1997-98 El Niño. We suggest that rather than using the term Pacific Decadal Oscillation, since it is really a multidecadal oscillation, we use the term El Viejo for the positive phase and La Vieja for the negative phase of the PDO.


* Keynote presentation.

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