La Niña and El Niño impacts in the eastern North Pacific
Warren Wooster
University of Washington
Seattle, WA. USA
wooster@u.washington.edu
Time series plots of equatorial SST anomalies show occurrences of La Niña as clearly as they do El Niño. The two phenomena have similar start months (usually spring or early summer), end months (usually in the early in the year), and duration (average 11 - 12 months). There seem to be fewer significant La Niña events (five since 1950) than El Niño events (seven in the same period) and some of the latter are much more intense1 (e.g., 2.2°C, 3-0°C, and 3.4°C for the major events of 1972-73, 1982-83, and 1997-98 compared with 1.9°C and 1.7°C for the cold events of 1954-56 and 1973-74).
Changes in ocean conditions in the eastern North Pacific are commonly attributed to unusual equatorial warming or cooling, but particularly in the case of warm events, the relationship is not always strong. Some major El Niño events (e.g., 1957-58, 1982-83, 1991-92, and 1997-98) are accompanied by significant extra-tropical warming, others are not (e.g., 1972-73), and some minor events (e-g., 1976-77) strongly impact higher latitudes.
Cold events seem to be more strongly coupled to higher latitudes. The five major La Niña events, from 1954-56 to 1988-89, were all associated with pronounced extra-tropical cooling as were some of the minor events. In most such cases, cooling at higher latitudes appeared to be simultaneous with that at the equator.
Warm and cold equatorial events are not distributed evenly through the record. For example, in the fifteen years since the El Niño of 1982-83, there have been five significant El Niños and only two La Niñas. In contrast, three of the five major La Niña events occurred in the six years between May 1970 and March 1976. The cold event of 1970 initiated a period of intense higher latitude cooling, only slightly interrupted at low latitudes by the 1972-73 El Niño, which continued until the minor El Niño of 1976-77. One might wonder if this long cool period set the stage for the subsequent regime shift.
It has become commonplace to attribute various consequences, for example changes in distribution or abundance of fish stocks, to "El Niño" (the same logic would apply to La Niña), whereas at higher latitudes the causative factors are probably more local -- for example, unusual warming in the Gulf of Alaska or eastern Bering Sea2 is more likely to impact recruitment directly than is the distant tropical event that may (or may not) have been a cause of that warming.
1. Intensity is defined as the average of the extreme anomalies during three consecutive months.
2. Called El Niño North by Wooster (1998).
Wooster, W., 1985: El Niño North: Niño Effects in the Eastern Subarctic Pacific Ocean. Seattle, Washington. Sea Grant Program, Univ. of Washington.