Part 5 - Long-Term Trends in Salinity
ENSO events are relatively short-term fluctuations in atmospheric circulation patterns, although they provide useful tests of our understanding of atmospheric forcing of bay variables. Are there long-term trends in North Pacific atmospheric circulation patterns? If so, how are they affecting the bay?
The 1920s and 1930s were dry in California, and river-flow anomalies were persistently negative. Estimates of total annual delta flow, which were first recorded in the early 1920s, indicate that the total flow today is almost the same as the flow during this dry era. Since then, the average flow has not increased because increased precipitation has been offset by a nearly equivalent increase in human consumption. Although the total annual flow has remained relatively constant, the distribution of flow over the annual cycle has changed. Over time, delta flow has increased in early winter and decreased in spring.
Figure 8. Spring salinities in San Francisco Bay have increased gradually over the past several decades. Shown here are the trends in the May salinities at Fort Point, which is near the mouth of bay, and at Alameda, which lies across the bay from San Francisco.
This long-term decline in spring flow into the bay explains most of the long-term rise in spring salinities and, in turn, is mostly the result of increasing agricultural consumption. Indeed, we estimate very roughly that water exports from the rivers and delta account for at least 80 percent of the salinity trend.
Figure 9. Spring salinities in Suisun Bay are correlated with water diversions both from year to year and over the long term. A greater percentage of the total flow is exported in dry years, and a smaller percentage of the total flow is exported in wet years. At the same time, the diversion of water in the spring has been increasing, just as have spring salinity values. (The annual rather than the spring export is shown here, but more water is exported in spring than in other seasons of the year.) As these correlations suggest, water exports account for most of the salinity trend, although there is also a small climate-related contribution.
To complicate matters, however, the decline also includes climate-driven contributions. The main climate effect is a trend toward warmer winters, which has led to less snowpack accumulation and therefore to less snowmelt discharge in spring. The decline in spring runoff, which was discovered by Maurice Roos of the California Department of Water Resources, was, in fact, what tipped meteorologists off to the winter warming. It has been suggested by some that the winter warming might be a local manifestation of greenhouse warming. The local warming can as easily be explained, however, by the fact that over the past several decades the winter wind field over the North Pacific has been displaced progressively southward. On balance, we think the winter warming is more likely to prove to be a natural fluctuation of the atmospheric circulation pattern than a unidirectional trend in the global climate.
A second climate effect is a small decrease in spring precipitation. This effect may be traceable to a recent tendency for high-pressure zones south and west of San Francisco to strengthen and migrate northward in the spring. The high-pressure zones tend to divert storms approaching California from the west, so that the spring remains dry.
The same atmospheric pattern has also tended to increase spring sea-surface salinities at the mouth of the bay. The stronger off shore high-pressure zones have strengthened equatorward wind components in spring. These winds lower the coastal sea level and encourage coastal upwelling, which increases coastal salinities. Andrew Bakun of the National Oceanic and Atmospheric Administration, who first discovered the spring increase in coastal upwelling, identified a similar trend at coastal locations around the world and therefore argued that stronger upwelling was an indirect effect of greenhouse warming. Coastal upwelling is known for its short-period intermittence, however, and it has also been shown to vary over periods of thousands years. For this reason we again find no reason to assume that the recent change in the upwelling was caused by human activity rather than by the natural wandering of the climate system.
Figure 10. Intensified spring upwelling in the coastal ocean is a trend in estuarine dynamics that may reflect the influence of climate. Andrew Bakun of the National Oceanic and Atmospheric Administration, who discovered this intensification, argued that it was an indirect effect of greenhouse warming. The authors think that the stronger upwelling along the West Coast might instead reflect a low-frequency variation in the regional atmospheric circulation. Using the California Pressure Anomaly as an index of the strength of winds that favor upwelling, they were able to simulate the fluctuation in coastal salinities at the Farallon Islands, about 45 kilometers offshore from the bay, from 1926 to 1942 and from 1957 to 1986.
Both of the springtime trends--less precipitation and more coastal upwelling--are understandable in terms of an observed long-term rise in the spring CPA. Mathematical analysis indicates that the CPA rise has been sufficient to account for the spring decline in the fraction of delta flow not explained by water diversions, for the slight increase in spring coastal salinities, and for the combined effect of drier springs and a saltier ocean on salinities within the bay.
What are the implications of these findings for the future of the bay? Assuming that we are witnessing climate fluctuations rather than climate trends, the climate-induced portion of the spring salinity trend might reverse direction at any time, acting to oppose rather than to exacerbate the impacts of water exports. This does not mean that we can withdraw more water from the delta with impunity, because the natural fluctuations could also change so as to increase salinity trends even more. Although these findings serve to remind us that not all estuarine variability is anthropogenic, they show further that, just as human beings may not be able to claim all the blame for salinity increases, neither can we expect to claim complete control over future variations.
The upstream watersheds, the offshore ocean and, as a unifying force, the atmosphere overhead all affect estuarine variables. On the time scales considered in this article, estuarine variability is linked to variations in atmospheric circulation through precipitation and runoff from the upland river basin and through wind-driven salinity variations in the coastal ocean. In the past several decades the climate has tended to increase salinities in the bay, but in our judgment this is just a passing effect of an endlessly varying North Pacific climate system. To paraphrase Heraclitus, not all is flux, but some certainly is.
We would like to thank Curtis Ebbesmeyer, Joe Hlebica, Robert Hirsch, and Andrew Spieker for reviewing the manuscript of this article and to Richard Smith, Lucenia Thomas, and Larry Riddle for technical assistance. The studies described here have recently been expanded as part of the U.S. Department of Interior's San Francisco Bay Ecosystem Initiative.
Aguado, E., D. Cayan, L. Riddle and M. Roos. 1993. Climatic fluctuations and the timing of West Coast streamflow. Journal of Climate 5:1468-1483.
Cayan, D. R., and D. H. Peterson. 1989. The influence of North Pacific atmospheric circulation on streamflow in the west. In Geophysical Monograph 55. Washington D.C.: American Geophysical Union.
Cloern, J. E., and F. H. Nichols (eds.). 1985. Temporal Dynamics of an Estuary. Dordrect, The Netherlands: Junk.
Conomos, T. J. (ed.) 1979. San Francisco Bay: The Urbanized Estuary. San Francisco: American Association for the Advancement of Science.
Dean, R. G., G. A. Armstrong and N. Sitar. 1984. California Coastal Erosion and Storm Damage During the Winter of 1982-83. Washington, D.C.: National Academy Press.
Dettinger, M. D., and D. R. Cayan. In press. Large-scale atmospheric forcing of recent trends toward early snowmelt runoff in California. Journal of Climate.
Fox, J. P., T. R. Morgan and W. J. Miller. 1991. Reply to discussion by D. R. Helsel and E. D. Andrews of "Trends in freshwater inflow to San Francisco Bay from the Sacramento-San Joaquin Delta." Water Resources Bulletin 27:327-330.
Helsel, D. R., and E. D. Andrews. 1991. Discussion of "Trends in freshwater inflow to San Francisco Bay from the Sacramento-San Joaquin Delta," by J. P. Fox, T. R. Morgan and William J. Miller. Water Resources Bulletin 27:317-319.
Jassby, A. D. 1993. Isohaline position as a habitat indicator for estuarine resources: San Francisco Bay estuary. Appendix B. In Managing Freshwater Discharge to the San Francisco Bay/Sacramento-San Joaquin Delta Estuary: The Scientific Basis for an Estuarine Standard. Report to the San Francisco Estuary Project, San Francisco, Calif.
Lydon, S. 1990. Time and tide show us no mercy. Santa Cruz Sentinel. October 21.
Nichols, F. H., et al. 1986. The modification of an estuary. Science 231:567-573.
Peterson, D. H, et al. 1989. Climatic variability in an estuary: Effects of river flow on San Francisco Bay. In Geophysical Monograph 55. Washington, D.C.: American Geophysical Union.
Philander, S. G. 1989. El Niño and La Niña. American Scientist 77:451-459.
Smith, D. E., M. Letter and G. MacKiernan (eds.). 1992. Oxygen Dynamics in the Chesapeake Bay. Sea Grant Colleges of Maryland and Virginia.