Water level oscillations in Monterey Bay and Harbor

Abstract

Abstract. Seiches are normal modes of water bodies responding to geophysical forcings with potential to significantly impact ecology and maritime operations. Analysis of high-frequency (1 Hz) water level data in Monterey, California, identifies harbor modes between 10 and 120 s that are attributed to specific geographic features. It is found that modal amplitude modulation arises from cross-modal interaction and that offshore wave energy is a primary driver of these modes. Synchronous coupling between modes is observed to significantly impact dynamic water levels. At lower frequencies with periods between 15 and 60 min, modes are independent of offshore wave energy, yet are continuously present. This is unexpected since seiches normally dissipate after cessation of the driving force, indicating an unknown forcing. Spectral and kinematic estimates of these low-frequency oscillations support the idea that a persistent anticyclonic mesoscale gyre adjacent to the bay is a potential mode driver, while discounting other sources.

Continuous seiche in bays and harbors

“…The 2 min mode is a good example of where a distinct sinusoidal oscillation in period is found throughout the record. This behavior is also observed at Monterey (Park et al, 2015), Hilo, and Honolulu where high-resolution (1 Hz) data were available, but is not shown in Fig. 2.…”

“…Breaker et al (2008) noticed continuously present seiches in Monterey Bay, leading Breaker et al (2010) to consider several possible forcing mechanisms (edge waves, long-period surface waves, sea breeze, internal waves, microseisms, and small-scale turbulence) and to question whether or not "the excitation is global in nature". Subsequent analysis by Park et al (2015) confirmed continuous oscillations in Monterey Bay over a 17.8-year record, and presented kinematic analysis discounting potential forcings of internal waves and microseisms while suggesting that a persistent mesoscale gyre situated outside the bay would be consistent with a jet-like forcing. However, jet-like currents are not a common feature along coastlines and could not be considered a global excitation of continuous oscillations.…”

“…The period of a shallow water wave resonance supported by a fixed-free boundary condition is expressed in Merian's formula for an ideal open basin as T = 4L/ √ gd, where L is the shelf width corresponding to λ/4, and d the basin depth (Proudman, 1953). Similarly, a shelf resonance is dynamically supported when the shelf width is approximately equal to gα/ ω 2 − f 2 , where g is the gravitational acceleration, α the shelf slope, ω the frequency of oscillation, and f the Coriolis parameter (Clarke and Battisti, 1981).…”

“…The harbor modes (Fig. 3) have been associated with resonances between breakwaters, and are amplified by wave energy, whereas the bay modes are weakly dependent on wave forcing (Park et al, 2015).…”

“…Modal length scales (λ) are taken from Table 3, amplitudes (h) are from bandpass filtering the 17.8-year water level record at the NOAA tide gauge, and Q from 1 Hz water level power spectra (95 % CI 2.6 dB) modal means over 120 h win- dows over 63 days (Park et al, 2015). The alongshore extent of the modes, L A , is estimated from a regional ocean modeling system (ROMS) implementation in Monterey Bay (Shchepetkin and McWilliams, 2005) as reported in Breaker et al (2010).…”