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Investigaciones marinas

versión On-line ISSN 0717-7178

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

El Niño and La Niña Effects on the NE
Pacific Circulation, with Some
Comparison to the SE Pacific*

P. Ted Strub, Corinne James

College of Oceanic and Atmospheric Sciences, 104
Ocean Admin. Bldg., Oregon State University
Corvallis, OR U.S.A. 97331-5503,

Altimeter measurements of sea surface height (SSH) along the eastern Pacific margin and over the eastern half of the basin are used to characterize the surface currents and transports during the 1997-1998 El Niño and the strong La Niña conditions that followed it. Large-scale atmopheric pressure fields serve as proxies for the basin-scale winds, while coastal upwelling indices derived from sea level pressure gradients for local alongshore wind stress.

The initial development and eastward propagation of the SSH signal along the Equator is clear and appears to follow several periods of westerly wind anomalies in early 1997. SSH rose along the coast of equatorial South America in May-July 1997, relaxed in August-September, and rose again in October-December 1997. At the same time, the mid-latitude atmosphere and SST structure resembled patterns that have been suggested as precursers to the relaxation of the trade winds and onset of El Niño conditions (see prior presentations by F. Schwing and G. Shaffer here). Thus, separating the effects of mid-latitude and western equatorial winds remains somewhat ambiguous.

Movement of the high SSH signal into both hemispheres along the coastal wave guide was fairly symmetric within 15 deg of the equator in May-June, 1997. Poleward of that region, the symmetry breaks down. The SSH signal quickly propagated poleward to the tip of South America in May 1997, but stopped at the mouth of the Gulf of California (~22N) in the Northern Hemisphere. Both the geometry of the Gulf of California and the normal spring-summer equatorward winds and currents along Baja and Central California may have played a role in slowing the SSH signal's progress in this early phase of the El Niño off North America. In the Southern Hemisphere, the normal fall-winter poleward winds were in the right direction to aid poleward propagation of the high coastal SSH signal. During the second period of high equatorial SSH (October-December 1997) conditions were reversed, with poleward winds off North America and equatorward winds at mid-latitudes off South America. The high SSH signal spread quickly around the Gulf of Alaska but did not affect as strongly the mid- and high-latitude coastal regions off South America.

Details of the poleward movement of the signal off North America are presented in a number of recent papers and also reviewed here by F. Chavez. CalCOFI cruises found warm and salty subsurface anomalies off southern California in July, as did mooring measurements near Monterey and San Francisco (central California). Off Oregon the July fields gave some suggestions of "spicier" water (advected from the south) and by September the El Niño signal was clearly present. Changes in species distributions off Oregon also indicated that species favoring warmer water were more abundant during the El Niño. These patterns of increased SSH, temperatures, salinities and warm water species were also observed off Vancouver Island, British Columbia (~49N). Much farther along the coast near Seward Alaska (~60N), it was more difficult to pinpoint a time for the arrival of an El Niño signal in the water column, due to the lack of measurements prior to fall 1997. However there is a sharp contrast between conditions in late 1997-1998 (fresher due to high precipitation) and those in 1999-2000.

Coastal ocean conditions along North America after March 1998 changed either abruptly (SSH dropped) or more gradually (temperature and salinity decreased, cold water species appeared). More importantly, those conditions did not return to those preceding the El Niño (1993-1996), but evolved into a cooler coastal ocean, more like the climatology from the 1960's. SSH values remained lower and satellite-derived chlorophyll pigment values were higher in a broad band next to North America. Zooplankton species were more typical of Boreal conditions in summer, similar to those found in the 1960's and early 1970's. In addition, salmon survival increased along the Pacific Northwest, again suggesting a return to conditions found prior to the mid-1970's "regime shift."

A new analysis of altimeter and atmospheric pressure data quantifies the differences between conditions prior to the El Niño (October 1992 to October 1996) and those found following it (October 1998 to October 2001). These differences may represent simply a change from pre-El Niño to La Niña conditions; or they may represent a transition of one regimem to another, with a strong El Niño in between them.

Motivated by the observations that the zooplankton species off Oregon and British Columbia indicate a change from more southern to more northern species, we asked the question, "Do the altimeter transport residuals show greater transport from the north in the region within 200-300 km of the coast following the El Niño?" In general the answer is "no" from Oregon to British Columbia, except for a several month period right after the El Niño. But the resolution of the data are very coarse and cannot distinguish between transport in the truly coastal zone within 20-40 km of the coast and the region between 40-250 km offshore.

What do the 2-D transport patterns (considering both east-west and north-south transports) show about the NE Pacific basin? During the El Niño, the Alaska Gyre intensified and the California Current weakend. After the El Niño, the Aleutian Low pressure system and the Alaska Gyre remained intensified, while the North Pacific High pressure system and California Current have also increased in strength. In addition, transport in the West Wind Drift strengthened and appeared to deliver more water to the eastern boundary around 50N, where the boundary currents would incorporate these offshore water properties and species. These changes in large-scale transport patterns may help to explain observations that the offshore species in summer off Oregon are similar to those found everywhere off Oregon in winter, while the nearshore species in summer are more typical of boreal communities, with an affinity for colder water. Upwelling may simply keep the offshore, transition zone species some distance from the coast in summer, favoring the cold-loving boreal species, whereas the transition zone species are advected into the boundary current in winter (and the boreal species carried north in the poleward Davidson Current.

* Keynote presentation.

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