near-decadal oscillations, and thus close in period to the 6-year repeat period of another prominent lunisolar tidal cycle.
Is it possible that the oceanic tides influence global temperature? Perceptions of cyclic behavior in the climatic record have in the past involved so many exceptions and inconsistencies that the subject does not have a good reputation among scientists. Compelling evidence that a recognized periodicity is actually caused by an identifiable extraterrestrial forcing agent is difficult to find. Our task is not made any easier by the nonstationarity of the lunisolar tidal cycles resulting from the incommensurability of the various astronomical periodicities. Although patterns in the strength of tidal forcing often recur, they don’t repeat identically even after hundreds of years. Evidence of a tidal connection therefore cannot rely solely on the usual past practice of looking for a correlation with temperature at a single tidal periodicity.
Although we have delved into properties of the tides in some detail to test whether a correlation of tidal strength with temperature exists, much more might be accomplished by a closer attention to the possible physical basis for the correlations found. Until now, to mount such an effort has not seemed worthwhile, given the small perceived likelihood that any lunisolar tidal connection to climate exists. We have only touched upon a possible cause by proposing that strong tides increase vertical mixing in the oceans and thereby episodically cool the sea surface. Also, we have explored in detail only 6to 10-year periodicities seen in records of both temperature and tidal forcing. We propose, nevertheless, that the near synchronicities seen at these periodicities argue sufficiently in favor of a tidal-forcing hypothesis, to justify further investigation of a possible tidal mechanism of temperature and climate variability.
We are grateful to many who gave generously of their time to discuss the subject of tides, solar phenomena, and climatic variation with us in the course of preparation of this article. We specifically thank Phillip Jones, David Cartwright, Amy Ffield, James Hansen, Reid Bryson, Harry van Loon, Thomas Wigley, Christopher Garrett, Thomas Royer, David Parker, Henry Diaz, Thomas Karl, and Fergus Wood. We are also grateful for discussions with Timothy Barnett, Robert Bacastow, Daniel Cayan, Myrl Henderschott, Ralph Keeling, and Walter Munk, at the University of California at San Diego. We further thank Dr. Jones and his coworkers for supplying us with their temperature data sets. Computer time was provided by the San Diego Supercomputer Center. Financial support was from the National Science Foundation (Grant ATM-91-21986) and from the U.S. Department of Energy (Grants FG03-90ER-60940 and FG03-95ER-62075).
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