On the natural oscillations of Monterey Bay: Observations, modeling, and origins

Observations of the natural oscillations of Monterey Bay have revealed periods that range from several minutes to almost one hour. These oscillations can be excited by winter storm events, and historical data show that they were also excited by tsunamis produced by the 1989 Loma Prieta Earthquake and the 1964 Great Alaskan Earthquake. However, these oscillations also tend to be continuous in nature although low in amplitude. The four lowest frequency oscillations have periods of approximately 55, 36, 27, and 22min. The 55-min period corresponds to the first longitudinal mode of oscillation, and the 36-min period, to the first transverse mode. Numerical simulations are employed to examine the oscillating characteristics of the bay and to help ascertain their origin. The model results are consistent with earlier studies, suggesting that Monterey Submarine Canyon divides the bay into two separate oscillating basins, although water level and pressure data indicate that at least for the four lowest frequencies, these oscillations tend to be bay-wide. Spatial patterns extracted from model-generated power spectra at the four lowest frequencies show good agreement with the modal patterns predicted by . Impulsively generated seiche oscillations should be subject to relatively strong damping and consequently decay within several cycles. Thus, it is not clear why the oscillations tend to be continuous, since the natural oscillations that are observed in most basins are transient, due to the transient nature of the forcing. Model simulations further indicate that both wind and tidal forcing contribute to the oscillations. Several mechanisms that could be responsible for the natural oscillations of Monterey Bay on a continuous basis were considered but it is not clear which, if any, of these mechanisms is ultimately responsible for exciting them. However, if the source of their excitation is wide-ranging or global, then they are likely to occur elsewhere around the world as well.