The 1997-98 El Nino - Increased potential for winter storms,
flooding and coastal impacts in California


Dan Cayan1,2, Tim Barnett1, Larry Riddle2
1Climate Research Division, Scripps Institution of Oceanography
and 2Water Resources Division, US Geological Survey


A large El Nino event has developed in the tropical Pacific Ocean, increasing the potential for a stormy winter in Southern California. El Nino is a natural climate fluctuation that involves elements of both the tropical ocean and global atmosphere. El Ninos happen every 2 to 7 years but not with perfectly regularity. When an El Nino does develop, it often brings stormy weather.

The latest map of departures of sea surface temperatures (SSTs) from normal shows that an El Nino pattern has developed in the eastern tropical Pacific, with SST departures from normal exceeding 4 °C (7 °F) near the South American coast. The current El Nino warming is the strongest we have seen since 1982-83, which was the largest El Nino that had been seen during the last 100 years (that is, during the period when we have instrumental observations). Very crucial to weather patterns over the North Pacific/western North America this winter is the extent to which these anomalously warm tropical temperatures maintain their strength into the winter: a strong El Nino enhancement of winter storms in the Pacific would be more likely if the current SST patterns persist through January-March, as was the case during 1983. We will be watching tropical ocean and atmospheric conditions attentively as fall and winter progress this year.

Note also the warm SST anomalies along the coast of California, part of a broad warm pool that lies along virtually the entire West Coast. If this tropical episode runs true to form of past El Ninos, the warmth along the California coast will persist and possibly strengthen over the next several months.

The evolution of the current El Nino can be seen from TOPEX satellite altimetric sea level anomaly estimates. Weakened trade winds across the basin produce heightened sea levels (and warmer upper ocean temperatures) in the eastern tropical Pacific and lower sea levels (and cooler water) in the western tropical Pacific. This pattern began to develop early this spring (first signs in February to March, 1997), and has progressed to the point where sea level is raised about 25 cm (10 inches) above normal along the Peru/Ecuador coast. Note that higher than normal sea levels are also propagating northward along the west coast of North America. This is typical of El Nino. Currently, sea levels are running about 15 cm (6 inches) above normal at Scripps Pier in La Jolla. Anomalously high sea levels are relatively minor perturbations unless they are accompanied by high tides and storms; however they are very important as visible (to satellites) signs of broad oceanic changes.

Usually the North Pacific has a very active storm season during the winter period of an El Nino year. The attached map is a snapshot of the North Pacific during March 1983 which saw an impressive number of storms tracking across the North Pacific. Storm tracks are given by successive location of Low Pressure centers as they cross the North Pacific. The storm patterns associated with El Nino result in anomalously wet weather with increased flood frequencies across the Southwest U.S. and anomalously dry weather across the Northwest. In California, the southern part of the state (e.g. south of Pt. Concepcion) exhibits the strongest link to the El Nino phenomenon. Historically, El Ninos have resulted in a variety of Southern California wet-season (October-March) precipitation totals (see the red dots on the graph attached), ranging from very dry to very wet. Of the 20 El Nino cases presented on this graph, 10 were significantly wetter than average, 6 were moderate, and 4 were significantly drier than average. On the other hand, for the 12 cases when the tropical Pacific was in a cool (La Nina) phase, that is opposite of the temperature patterns during El Ninos, 9 were quite dry, 3 were moderate, and none were wet.

Southern California has had a significant increase in the number of large precipitation days during El Nino years as compared to the number of those wet days during La Nina years. Note that not all El Nino years result in an excessive number of very wet days, however--some are actually quite dry. On the other hand, La Nina years very seldom generate many wet days.

Streamflow "hydrographs" collected by the US Geological Survey are much more likely to exhibit high flows (floods) during El Nino years (red curves) than during La Nina years. While El Nino years do not always exhibit high flows, many of the southwest's largest floods have occurred during El Nino years. Approximately half of the El Nino years in the record since World War II have produced a significantly greater number of winter days with heavy precipitation and high streamflow than normal.

A crucial aspect of the storms this winter, if they do occur, is whether they occur during the few day periods of the winter months when tides are high (new moon and full moon) or not. With the likelihood of anomalously high sea levels associated with El Nino, storm surge and big waves would increase the likelihood of coastal hazards and damage. Because North Pacific winter storms are strengthened, El Nino years have produced some of the largest waves observed along the Southern California coast during the last two decades. Close attention should be given to short range weather forecasts during these periods with high tides. In the 1982-83 event, there was massive damage along the California Coast. High sea levels increase the threat of flooding various reclaimed farmlands of the San Francisco Bay/Delta.



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