The Eastern Bering Sea Shelf : oceanography and resources / edited by Donald W. Hood and John A. Calder easternberings00hood Year: 1981 62 Physical oceanography function of frequency. When plotted the spectrum gives a concise picture of the frequency distribution of the kinetic energy (variance). Of course, the characteristics that showed clearly on other plots should also be evident in the spectral plots. Calculating the spectrum for one series (autospec- trum) can be useful, but this can also be done for two records (cross spectrum). Normalizing such a spec- trum by the variances produces
The Eastern Bering Sea Shelf : oceanography and resources / edited by Donald W. Hood and John A. Calder easternberings00hood Year: 1981 62 Physical oceanography function of frequency. When plotted the spectrum gives a concise picture of the frequency distribution of the kinetic energy (variance). Of course, the characteristics that showed clearly on other plots should also be evident in the spectral plots. Calculating the spectrum for one series (autospec- trum) can be useful, but this can also be done for two records (cross spectrum). Normalizing such a spec- trum by the variances produces a 'coherence' as a function of frequency. The coherence is the frequen- cy-dependent counterpart of the correlation function between two series as a function of time lag; instead of showing the time lags at which records are corre- lated, it shows the frequencies at which records are phase locked (the calculation also yields the corres- ponding phase spectrum, a frequency-dependent time difference). We plot the square of coherence because, like the squared correlation coefficient, the squared coherence corresponds to the percent of the variance at the frequency that is 'explained' by a linear relationship. Coherence is always positive, but phase information (not shown) conveys sign. For instance, two sine waves of identical frequency but opposite sign have a coherence of and a phase of 180°. We illustrate with examples that summarize the frequency distribution of kinetic energy over the shelf. Autospectra for BC-15C, 20 m and 34 m depth, from summer 1977 are plotted in Fig. 5-5. The vertical axis is kinetic energy density, , per unit frequency, (cm/sec)^ /CPD (CPD is cycles per day). In this presentation the vector series have been decomposed into clockwise (an ticy clonic) and counterclockwise (cyclonic) rotating components, instead of into north-south and east-west compo- nents. When there is no strong local orientation (, a shoreline or strong bathymetric gradie
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