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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/S0717-71782002030100002 

The Role of El Niño, La Niña and
Climate Change in the Pacific Eastern
Boundary Currents:

An Integrated Introductory View*

Wolf Arntz

Alfred Wegener Institute for Polar and Marine
Research, Section for Comparative Ecosystem
Research, Postfach 12 01 61, 27515 Bremerhaven,
Germany, E-mail: warntz@awi-bremerhaven.de

Eastern Boundary Currents (EBC) play a decisive part in the budgets of both the Pacific and the Atlantic Ocean, including latitudinal heat exchange, ocean-atmosphere interactions, organism transport and genetic interchange. They are the great motors of the circular oceanic current systems, transporting cold water into subtropical and tropical climates, and highly efficient ecosystems in which nutrients are returned to the surface layers of the ocean where they can be used by photosynthetic primary producers. There are obvious and well-known similarities in all EBCs, however there are also regional differences which make each of them unique.

The symposium „Impacts of EN and Basin-Scale Climate Change on Ecosystems and Living Marine Resources - a comparison between the California and the Humboldt current systems" is an attempt at a direct dialogue between researchers working in the two principal Pacific EBCs. Despite the dependance of these currents, at least to a large extent, on the same basin-wide mechanisms and causal chains, and despite mirror-like, often identical impacts on both sides of the equator, this dialogue has never taken place, with a few noteworthy exceptions. Each upwelling system is investigated by specific researchers who meet during their regional conferences and at best know their colleagues, who are working on the same subject in a parallel system, from the literature. This situation is worsened by the English-Spanish language barrier. Hopefully, the Viña meeting can overcome these problems, including also other EBCs such as the Benguela Current, which shares important characteristics with the systems in the East Pacific.

Which are going to be the major subjects of the planned conference? One will certainly be the scales of climate change, from the ENSO subcycles (El Niño, La Niña and the „normal" situation in between) via inter- or multidecadal towards centennial variability, and from local disturbances to basin-wide climate change. We hope to learn about the interrelations between the short ENSO cycle and the mid- to longterm variability underlying it and about direct forcing and teleconnections that link local to regional and global events.

Taking into account the enormous efforts undertaken in measuring variables that might be involved in the dynamics of the ENSO cycle, the identification of the relations behind this phenomenon has so far yielded disappointing results, and the same is obviously true for EN prediction. „Postdiction is easier than prediction", we wrote a few years ago, and the situation has hardly changed. EN can be predicted once it is underway, either physically by tracking Kelvin waves and regional warming or by biological indicator species, but what would be required to minimize danger is a secure long-term prediction 1 - 2 years before the event, which - despite all affirmations by the various modelling clans - is not in sight.

A third subject, judging by the presentations that have been offered, will be the different manifestations of climate change. We are now all aware that ENSO means much more than changes in sea surface temperature, precipitation or dissolved oxygen, and many such variables are continuously registered via buoys, satellites, from shipboard, land-based stations or gear employed by individual scientists. But are these manifestations comparable in the two Pacific (and other) EBCs? Is increased oxygen at the seafloor during EN a masterfactor only for the Humboldt current, which is characterized by large amounts of sulfur bacteria under „normal" conditions? To what extent does temperature variability coincide in the two systems? Why did the very strong EN 1997-98 show up in the entire East Pacific but was almost indistinguishable in the Benguela region? Which characteristics, after all, are responsible for the severity of an ENSO event in the different regions or subsystems?

Manifestations of climate change are available not only for recent times but also for the past, and palaeo-climatology is presently one of the most fascinating facets of climate research. High-resolution records of annual, interdecadal and centennial variability of ocean climate have been derived from a variety of sources to identify extended warm and cold periods or major El Niño-events. However, sources of, for example, varve deposition, fish scales in sediments, shell accumulations, growth rings in coral structures or reports of unusual precipitations require validation by comparison to be generally acceptable. Comparisons in different EBCs present a great challenge for the study of climate change patterns in the past.

Finally, one major subject during this conference will be the impact of climate change, from its ecological consequences in marine ecosystems via economical effects on fisheries (and agriculture, traffic etc.) to social implications. There is, again, the question of „severity" and of the „positive" or „negative" effects, and the answer is likely to be different depending on which of these facets is considered. Whether EN 1982-83 or EN 1997-98 was indeed „more severe" may well be a question of regionality and of the specific impact that is considered. Talking about EN's effect on life in the sea, the critical point is clearly not only the amount of deviation from the long-term mean as revealed by certain parameters, the distance from the equator or life at greater depths, which both may mitigate many effects considerably, but also the time of onset of the anomaly, the situation immediately before and after the change, and the duration of the event. Impacts on fishery resources may depend to a large extent on the management situation of the exploited stocks. And there still is a lively discussion to what extent catastrophes and disasters occurring during an EN or LN event can be causally attributed to these events.

Changes that marine ecosystems undergo during major El Niños or regime shifts can be used as a proxy of how these systems may have responded to past climate variability and will respond to future change. The range of such responses is very large, from changes in species composition, dominance and diversity through antecological consequences for reproductive biology, population dynamics, food intake and behaviour to the collapse of entire food webs or establishment of new ecosystem structures and flows. Latitudinal range expansions and contractions of the habitats of many species, together with enhanced exchange in longitudinal direction, represents a very efficient instrument of genetic exchange and biodiversity maintenance.

An introduction can only give hints to what might be themes of interest in a scientific meeting. From this short summary of subjects, which is by no means exhaustive, it is evident, however, that we are looking forward to an extremely interesting conference on climate change in EBCs.


* Keynote presentation.

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