versión On-line ISSN 0717-7178
Investig. mar. v.30 n.1 supl.Symp Valparaíso ago. 2002
El Niño and Multidecadal Climate
Change: A Global Perspective*
Franklin B. Schwing
Pacific Fisheries Environmental Laboratory, NOAA/
NMFS, 1352 Lighthouse Ave., Pacific Grove, CA
93950-2097, E-mail: email@example.com
The global atmosphere and ocean interact and vary on interannual scales due to the climate phenomenon commonly called El Niño-Southern Oscillation (ENSO). Aside from the seasonal cycle, interannual ENSO fluctuations - associated most notably with its extremes, El Niño (EN) and La Niña (LN) events - are the strongest and most familiar signals of ocean variability. The influence of EN/LN extends far beyond the tropical Pacific, and is truly a global phenomenon. EN and LN have pronounced regional effects on the physical, chemical, and biological nature of the Peru-Chile (Humboldt) and California Current Systems, the Pacific Ocean's eastern boundary current (EBC) ecosystems. A key to understanding the impact of these climate events on these ecosystems is knowing how typical EN/LN events evolve and transfer their signals between the tropics and higher latitudes, and how individual events vary.
The atmosphere and oceans also fluctuate abruptly on multidecadal scales, termed climate regime shifts. Interannual events vary because of, or at least in the context of, regime-scale variability. The changes that oceans and their ecosystems undergo during regime shifts may be a rough proxy of how these systems have responded to past climate variability, and will respond to future change.
This presentation will (1) characterize the global atmospheric and oceanic patterns of typical EN/LN events that influence Pacific EBCs; (2) characterize global multidecadal regime anomaly patterns; (3) compare and contrast EN/LN and regime patterns, the sources and mechanisms of their forcing, and the ocean's response; and (4) compare and contrast southern and northern hemispheric anomaly patterns on EN/LN and regime scales. Finally, coupling between large-scale processes and regional and meso-scale phenomena in EBCs on these time scales will be discussed.
While each EN/LN event is unique to some degree, there are enough similarities in their timing and pattern to characterize a typical event, using composites of previously observed events. Sea temperature anomalies (STAs) in the upper 100-200 m during EN (LN) are generally: cool (warm) in the western tropical Pacific and central north and south Pacific; and warm (cool) in the central and eastern tropical Pacific and in the EBCs. Pacific STAs are approximately symmetric about the equator. The mechanisms by which extratropical oceanic anomalies are generated by regional wind anomalies - including Ekman processes, geostrophic advection, surface heat fluxes, and mixing - are associated with a characteristic spatial relationship between upper ocean and atmospheric forcing anomalies.
During EN, tropical Pacific near-surface wind anomalies are eastward, indicating weak trade winds and strong wind convergence in the central and eastern tropical Pacific. This leads to anomalous equatorial downwelling and eastward currents, which contribute to positive STA anomalies in the central and eastern tropical Pacific. The opposite anomalous state occurs during LN events.
The trade wind anomalies also are part of coherent extratropical wind anomalies that emanate out of high atmospheric pressure systems in the eastern regions of the north and south Pacific. These atmospheric anomalies are roughly symmetric about the equator. For example, large cyclonic surface wind anomalies during EN in the northeast and southeast Pacific are associated with negative sea level pressure anomaly (SLPA) centers and weaker trade winds. Over the northeast (southeast) Pacific, negative SLPAs correspond to a stronger Aleutian Low and weaker North Pacific High (South Pacific High) during EN. The result is anomalous forcing of the upper ocean by, for example, cyclonic (clockwise in the southern hemisphere) wind stress anomalies during EN, including poleward (downwelling-favorable) winds in the EBCs, and the opposite forcing anomalies in LN.
Two main mechanisms are responsible for the remote forcing of oceanic anomalies in Pacific EBCs during tropical EN/LN: (1) atmospheric teleconnections involving wave trains from the western and central tropical Pacific; and (2) oceanic poleward propagating coastal-trapped waves, principally baroclinic Kelvin waves.
Eastward-propagating Kelvin waves, which are produced by tropical wind anomalies, continue propagating along the eastern boundaries where they are modified by local wind forcing and topographic scattering. These oceanic signals are generally more important in the Peru-Chile Current region due to its geometry and proximity to the equator.
Evidence for atmospheric teleconnections into the north and south Pacific is clear in upper tropospheric geopotential heights. Their patterns are qualitatively similar from the upper troposphere to the Earth's surface. In the northern hemisphere, this pattern (positive over the western subtropical north Pacific, negative over most of the northeast Pacific, and positive over North America during EN), agrees with the Pacific/North American (PNA) pattern, a familiar example of an anomalous wave train associated with greater (reduced) tropospheric heating in the central and eastern tropical Pacific during EN (LN). An analogous teleconnections pattern is established in the southern hemisphere. Anomalous wave trains also emanate from east Asia and arch east over the north Pacific and North America. These are associated with tropospheric heating anomalies over southeast Asia and the western tropical Pacific.
In analyzing EN and LN events and their impacts on EBCs, several fundamental points must be considered. (1) EN/LN are largely tropical phenomena but have important extratropical impacts. (2) Anomalies that occur during EN/LN can be produced by other phenomena and processes. (3) There is a characteristic spatial relationship between atmospheric and upper ocean anomalies. (4) Because of the slow response of the ocean to atmospheric change (principally due to its thermal and mechanical inertia), the oceanic impacts of EN/LN events can linger long after the atmospheric processes that caused them have dissipated. (5) The relative importance of local and remote forcing of oceanic anomalies is temporally variable, due to the timing of individual EN/LN events and the complex interaction of preferred spatial patterns with the previously established ocean state and synoptic weather events.
Atmospheric and oceanic anomaly fields associated with multidecadal climate regimes are spatially very similar to those computed for EN/LN events, and similar in magnitude. However, they appear to be amplified in the higher latitudes, particularly in the northeast Pacific (EN anomalies are relatively greater in the tropics). Because of their longer duration, they are associated with significant changes in fishery resources and their socioeconomic consequences. For example, the collapse of the California sardine fishery out of Monterey Bay in the early 1940's, documented in popular literature by John Steinbeck, occurred following a shift toward cooler ocean conditions. Hemispherical differences in EN/LN and decadal anomalies are probably due to differences in the total area and distribution of land masses.