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614 VARIABILITY, MULTIDECADAL FRANCISCO CHAVEZ Monterey Bay Aquarium Research Institute Changes in climate, ocean circulation, and ocean ecosystems with periods of about years have been recently recognized and referred to as Pacific multidecadal variability. These climate or regime shifts have particularly large impacts on small pelagic fish such as anchovies and sardines. These populations shift synchronously in Japan, California, and Peru, with sardines dominating for about years and anchovies for the following years. Impact on other ocean ecosystems, such as those on rocky shores, are unknown but likely. NATURAL CLIMATE CHANGE As farmers and fishermen have known for centuries, climate fluctuations are both important and normal. Climate varies on daily, weather-system, and seasonal time scales, and we are now learning about longer term fluctuations that occur within the bounds of natural variability. These cycles strongly affect humans, their economies, and the ecosystems on which they depend. But humans are also affecting climate by increasing atmospheric CO, and the biological consequences of the resulting global warming are not at present predictable and are potentially catastrophic . Understanding human-induced climate change will require characterization of natural climate variability and the use of natural cycles as models of climate change. Prior to this growing awareness of natural climate variations, ecologists viewed the physical environment as a stable background against which biotic interactions drive population change and structure communities. Over the past two decades, strong El Niños, the ozone hole, and the looming specter of global warming forced the uncomfortable realization that the physical environment is changing, even on the relatively short time scales of ecological study, and that human activities may affect climate in unforeseen ways. Climate and the physical environment have thus reemerged as major themes in ecological science. In the oceans, it is clear that natural climate variability can have large impacts on ecosystem structure and biological productivity. The correlation between climate variability and the productivity and structure of ocean ecosystems has been well established. Climate-driven changes in ocean circulation, ocean mixing, dust deposition, or both, can regulate the overall productivity of an ocean ecosystem by changing the supply of a limiting nutrient. Changes in primary productivity then cascade through every trophic level. Climate can also influence animal populations directly through effects on recruitment, competitive advantages, or predation. Of particular interest are relationships between abiotic (bottom-up climate impacts on overall ecosystem productivity) and biotic (top-down climate impacts on competition and predation) effects. CLIMATE IS NEVER AVERAGE In general dictionaries, climate is defined as the average course or condition of the weather at a place, usually over a period of years as exhibited by temperature, wind velocity , and precipitation. However, the average itself changes, depending on the period used. For example, sea surface temperature (SST) for the coasts of Peru and California for the years – was warmer than during –, and during the – El Niño, SST was extremely warm. These changes in temperature were accompanied V V by fluctuations in ocean productivity, with warmer years being less productive than cooler years. What causes climate to change from year to year and decade to decade? What are the consequences of this climate variability? Complete answers to these questions are still forthcoming, although significant progress has been made over the past several decades, particularly in understanding the consequences of climate variability. For example, the warm SST observed during – was a result of a strong El Niño that was well documented in the equatorial Pacific and along the west coast of the Americas. El Niño is a prime example of the insight gained on the consequences of climatic variability . Although El Niño had been recognized off Peru since the ancient civilizations of the Incas, it was the large – El Niño that brought international attention to the phenomenon . Following the – El Niño it became clear that the oceanic perturbations in the tropical Pacific had global effects on climate. Oceanic effects were originally thought to be restricted to the tropical and eastern Pacific, but careful studies in the center of the Pacific Ocean close to Hawaii uncovered El Niño effects there as well. The past several decades have seen growing awareness of La Niña, the counterpart or opposite condition of El Niño. THE STORY OF EL VIEJO Recently, focus has shifted to longer decade-scale changes that show remarkable basin-wide coherence and, again, strong impacts on oceans ecosystems. These multidecadal changes help explain the differences...
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