Part 3 - Atmospheric Teleconnections
Understanding the net effect of the many interacting physical processes that govern the state of an ecosystem is never easy, but it is particularly difficult in the case of an estuary. Lying on the boundary between the land and the sea, an estuary is subject to both terrestrial and oceanic physical processes and to varied and interesting climatic effects. If each contributing process and its interactions with other processes had to be individually considered, the problem would be overwhelmingly complex. This is why it is useful to recognize that the large-scale patterns in atmospheric circulation couple with and organize geophysical processes. These overarching patterns allow us to understand estuarine dynamics without oversimplifying them.
The San Francisco area has a Mediterranean climate characterized by warm dry summers and cool, wet winters. The climate is governed by a big high-pressure cell that blossoms over the North Pacific in the summer. This cell deflects storms to the north, preventing measurable precipitation over California. During winter the cell migrates south and becomes less intense. As the Pacific High weakens, the Aleutian-Alaskan Low strengthens. Temperature and pressure gradients between the tropics and the pole become steeper, and many more weather disturbances stream across the Pacific. Since California is no longer blocked by the Pacific High, the winter storm track passes over it.
At least, this is the pattern in a typical year. We are concerned here with variations in delta flow, which are driven by variations from this typical sequence of atmospheric events. One prominent large-scale pattern that causes unusual weather over California is an El Niño/Southern Oscillation, or ENSO, event. An ENSO event is a large irregularity in the coupled atmospheric and oceanic systems along the equatorial Pacific. It was named for the independently discovered but dynamically linked reversal of the pressure distribution over the tropical and subtropical Pacific, called the Southern Oscillation, and the Christmastime warming of the ocean off the coast of South America, called an El Niño event.
Although ENSO events are concentrated in the tropical Pacific Basin, they generate very slowly varying waves that propagate away from the region through both the atmosphere and the ocean. These waves produce very large-scale climatic correlations, or teleconnections, and through these ENSOs have far-flung global effects.
In the tropical and subtropical Pacific, seasonal climate variability is remarkably tuned to the Southern Oscillation Index, the standard measure of ENSO intensity. But in extratropical regions, ENSO influences are more tenuous, and regional climatic influences and variations modulate the ENSO signal. In general the northwestern United States and southwestern Canada tend to be dry during the winter of a mature phase of an ENSO event. The southwestern United States tends to be wet during the same phase, as does the Gulf Coast and south Florida.
California, on the other hand, is located at the geographic boundary between these two responses and can experience either dry or wet weather. It turns out that precipitation in northern and central California during an ENSO event is better predicted by the location of the Aleutian-Alaskan Low than by the value of the Southern Oscillation Index.
The low is usually more intense than normal during an ENSO event, but its location varies. When the Aleutian-Alaskan Low forms farther east than usual, that is, nearer to the West Coast, as it did during the winter of 1983, storms penetrate into central California and the winter is wet. (By meteorological convention, the winter of 1983 includes December 1982 and January and February of 1983.) When the low forms farther west, that is, nearer the International Date Line, as it did in the winter of 1977, high-pressure anomalies tend to be found off the California coast. These anomalies deflect storms northward, keeping California dry.
ENSO events may appear on either side of the precipitation balance sheet, but they are usually associated with extreme weather conditions. For example, the ENSO winters of 1941, 1958, 1983 and 1993 were very wet because central North Pacific storms took a southern path into California. On the other hand, the ENSO winters of 1977, 1987 and 1992 were extremely dry over much of the state because high pressure developed over the West Coast and North Pacific storms were diverted to the north.
Although ENSO events alone do not predict California weather, two of the authors (Cayan and Peterson) have defined a regional index that captures the effect of an ENSO event on Californian weather. This index, called the California Pressure Anomaly, or CPA, is calculated from sea-level pressure anomalies in a small region off the coast of California where the pressure anomalies have historically exhibited the strongest correlation with river-flow variability. The CPA region, which measures about 15 degrees longitude by 10 degrees latitude, is centered at 40 degrees north latitude and 135 degrees west longitude.
Years with high CPA winters are characterized by anomalously high pressure that deflects moisture-bearing storms to the north, resulting in reduced precipitation, lower delta flow and higher bay salinities. Years with low CPA winters are stormier, resulting in increased snowpack, greater delta flow and relatively low salinities. Unfortunately, meteorologists are not yet able to predict the CPA.
Figure 5. El Niño/Southern Oscillation (ENSO) events do not have a unique Californian signature. Precipitation patterns are correlated instead with the east-west location of the Aleutian-Alaskan Low. If the low is farther west than normal, as it was in the winter off 1977, a high-pressure cell forms over California, protecting it from winter storms (top). If the low is farther east than normal, as it was in 1983, storms penetrate into central California and the winter is wet (middle). Brown corresponds to monthly river flow in the lowest quartile and green to a monthly river flow in the highest quartile. Isobar units are millibars. Actual flows for the two years are shown in the graph at bottom.