"Long" ocean waves, with frequencies between 0.2 and xo cycles per hour (cph), have been recorded simultaneously at La Jolla on the California coast and at San Clemente Island, about rno km seaward. The spectral power is greatest at the lowest frequencies but remains fairly uniform (about 5 x 10-2 cm2/cph) between 0.7 and 10 cph. Comparison of the Island and shore records shows that at the lowest frequencies (below 0.7 cph) the two records are consistently in phase and are highly coherent, as might be expected; above 0.7 cph they are out of phase, and the coherence is low....
Snodgrass, Frank E., Walter H. Munk, and Gaylord R. Miller. 1962. "Long-period waves over California's continental borderland Part I. Background spectra." Journal of Marine Research 20, (1). https://elischolar.library.yale.edu/journal_of_marine_research/970