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Abrupt Climate Change: Inevitable Surprises
ENSO and ENSO-Related Variability
A weakening of the trade winds in the equatorial Pacific and attendant warming of the sea surface (or lack of cooling by upwelled cold water) is known as an El Niño event. Such events alternate with an opposing state, popularly referred to as “La Niña,” with strong trade winds and upwelling of cold waters off Peru and along the Equator. The few-year oscillation between those different states is the El Niño/Southern Oscillation. The coupled oscillation of the tropical ocean and atmosphere is important in global climate, with impacts that extend far beyond the tropical Pacific to the tropical Atlantic and Indian Oceans, to the Southern Ocean, and to middle to high latitudes in the Northern Hemisphere. There are speculations that greenhouse warming is sufficient to put the world into a warmer, near-perpetual El Niño state (e.g., Timmerman et al., 1999; Federov and Philander, 2000), but there is no strong consensus.
ENSO might be linked to another of the leading patterns of variability, the so-called Pacific North American (PNA) pattern, which exerts a strong influence on distribution of rainfall and surface temperature over western North America. Like the AO, the PNA pattern fluctuates randomly from one month to the next, but also exhibits what appear to be systematic variations on a much longer time scale. Since 1976-1977, the positive polarity of the PNA pattern—marked by a tendency toward relatively mild winters over Alaska and western Canada, below-normal rainfall and stream flows over the Pacific Northwest, and above-normal rainfall in the southwestern United States—has been prevalent, whereas during the preceding 30-year period the opposite conditions prevailed.
The abrupt shift toward the positive polarity of the PNA pattern in 1976-1977 was coincident with and believed to be caused by a widespread pattern of changes throughout the Pacific Ocean. Sea-surface temperatures along the equatorial belt and along the coast of the Americas became warmer, while farther to the west at temperate latitudes the sea surface became cooler (Nitta and Yamada, 1989; Trenberth, 1990; Graham, 1994). An array of changes in the marine ecosystem occurred around the same time (Ebbesmeier et al., 1991). For example, salmon recruitment underwent a major readjustment toward more abundant harvests along the Alaskan coast accompanied by deteriorating conditions in southern British Columbia and the US Pacific Northwest (Francis and Hare, 1994). Another basin-wide “regime shift” that was analogous in many respects to the one that occurred in 1976-1977, but in the opposite sense, was observed during the 1940s (Zhang et al., 1997; Minobe and Mantua, 1999), and there are