Part 4 - Oceanic Teleconnections

An ENSO event can have oceanic as well as atmospheric teleconnections with conditions in California. The ocean transmits a signal, possibly by means of a coastally trapped wave, that increases sea-level heights and sea-surface temperatures along the West Coast. During the ENSO event of 1983, for example, sea levels in the San Francisco Bay area were about 10 centimeters higher than predicted and sea-surface temperatures were between 1 and 3 degrees Celsius warmer than normal.

Figure 6. Two ENSO events have been particularly destructive in this century. The February 1926 event produced storm waves that broke second-story windows of the Capitola Hotel (top). The 1983 event was also accompanied by larger sea-level height anomalies, which made it even more destructive. Shown at bottom are waves topping the breakwater at Capitola and battering Venetian Courts. (Top photograph courtesy of the Sandy Lydon Collection; bottom photograph by Sandy Lydon.)

Again the effect of this teleconnection on California is difficult to predict. Sea-level heights near the mouth of San Francisco Bay are forced by a combination of global, regional and local mechanisms. An ENSO event can cause changes in sea level, but so can winds along the central California coast. During some years, the two phenomena reinforce one another, whereas in other years they oppose one another. An additional complication is that the oceanic signal of an ENSO event can be either enhanced or damped by the atmospheric signal.

The ENSO event in 1926 illustrates this modulation. For the most part ENSO events appear as large humps in the filtered record of sea-level heights at the mouth of San Francisco Bay (see Figure 7). The strong ENSO event of 1926 did not produce a sea-level rise like those that occurred during 1941, 1958 and 1983, however.

Figure 7. History of ENSO events demonstrates how the location of the atmospheric pressure anomaly (above) governs delta flow and sea-level height (right). In an ENSO year when the pressure anomaly is close to the West Coast, such as 1941 or 1983, coastal winds amplify the oceanic teleconnection and push sea levels higher than normal. Because the winter storm track passes over California, runoff is high and salinity is low. When the anomaly is far offshore, as it was in 1977, sea levels are lower. Because the winter storm track deviates to the north of California, runoff is low and salinity is high.

The large-scale sea-level pressure patterns were similar during the four ENSO events, which suggests that the wind patterns were also similar. A small deviation from the pattern appeared in January 1926, however. A regional high developed north of California in January, faded in February and reappeared in March.

This comparatively small perturbation in the atmospheric patterns had a major effect on California weather. For most of the winter, central Pacific storms tracked to the north rather than to the east, avoiding the regional high-pressure cell and California. Thus the regional high effectively blocked the large-scale storminess and the sea-level-height anomalies associated with it. When the blocking ridge temporarily disappeared in February, there was a spate of storms over the West Coast and central California and a minor rise in sea-level heights off the coast. By contrast, in 1941, 1958 and 1983 there were storms throughout the winter and the coastal sea-level-height anomalies were much larger and more persistent.

Even though conditions during February 1926 set no records, they were fairly impressive. At Seacliff, about 40 miles south of San Francisco, a developer named W. I. Morgan had been building a seawall designed to withstand "the greatest waves imaginable." In February 1926, however, the ocean destroyed this and other seawalls along the coast. Huge combers even broke second-story windows of the ocean-front Capitola Hotel.

During the ENSO event of 1983, unlike that in 1926, everything conspired to increase sea levels. A huge ENSO event in the Pacific propagated up the coast in the form of a wave, increasing sea levels. The sea-level increase happened to coincide with high astronomical tides and heavy river flows. For all three reasons the coast was unusually vulnerable to high storm waves. As a result of the 1983 storms, damage to public beach and pier structures alone was estimated at 50 million dollars. Damage to private property was estimated in the hundreds of millions of dollars.

Because higher sea levels at the mouth of the bay push salt water farther into the estuary, the sea-level signal of an ENSO event is an example of atmospheric coupling through oceanic processes to estuarine variables. This coupling, however, probably has minimal influence on estuarine salinities. The regional atmospheric response to an ENSO event (that is, the atmospheric signal) is probably more important. A high winter CPA is associated with stronger northerly (equatorward) winds that tend to increase the coastal upwelling of saline water. A low winter CPA is associated with southerly winds that do not favor upwelling and may even encourage downwelling. Since high-CPA winters tend to be dry and low-CPA winters to be wet, the CPA-governed changes in precipitation and coastal salinities act in concert on estuarine salinities.

Part 5