Sea-Level Change (1990) / Chapter Skim
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3 Large-Scale Coherence of Sea Level at Very Low Frequencies
3 Large-Scale Coherence of Sea Level at Very Low Frequencies
Pages 63-72
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From page 63... ...
The sea-level signals in the eastern Pacific appear to be dominated by propagating Rossby waves, so that the variability, having periods of 5 to 8 yr (e.g., between San Francisco and Honolulu) is coherent but out of phase by several years.
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From page 64... ...
However, if the observed rise is primarily the result of continuing adjustment since the last deglaciation, we would expect the sea-level rise to be a simple linear trend when viewed on appropriate time scales. Local tectonic effects, such as the differences between the east and west coasts of the United States, are not addressed here.
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From page 65... ...
The intermediate periods, from 1 month to 10 months, are based on S-hourly filtered data, smoothed with three Hanning passes. Monthly data at San Francisco are used for periods longer than 10 months and are smoothed with five Hanning passes.
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From page 66... ...
Second, there is a substantial phase difference; San Francisco leads. While this is clearly a straightforward result in terms of Rossby wave propagation, it has important implications for the "sea-level rise problem," as discussed in a later section.
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From page 67... ...
Frequencyresponse curve, lower right, shows the ratio of the coherent power at Honolulu to that at San Francisco. On the phase plot, upper right, the 90 percent confidence intervals are shown.
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From page 68... ...
The coherence is highest, however, when a 3- to 4-yr lead is introduced into the Mayport data-which is in the "wrong direction." The most plausible explanation is that direct atmospheric forcing is more important in these records than is ocean wave propagation. This lag direction for maximum coherence is also supported by the comparisons between San Francisco and Cascais, as discussed in the next section.
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From page 69... ...
That is, in the example of San Francisco and Honolulu, the observed coherence results from oceanic wave propagation. Any coherence between San Francisco and Cascais, however, would be attributed presumably to coherence in atmospheric forcing if the coherence is found in a frequency range where atmospheric signals are present.
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From page 70... ...
FREQUENCY - CYCLES/YERR 99 ss 90 82 50 10. FIGURE 3.5 Cross spectra between sea level at San Francisco and Cascais using 90 yr of data beginning in 1884.
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From page 71... ...
ACKNOWLEDGMENTS During the course of this work, I have benefited from discussions with many people as well as comments from readers of a previous version of this manuscript, including Tim Barnett, John Blaha, Allan Clarke, Nelson Hogg, Jim McCullough, and others. The spectral analysis programs were written by Chris Evans.
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From page 72... ...
Rossby wave analyses of W STURGES subsurface temperature fluctuations along the Honolulu-San Francisco great circle, J
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