Low Frequency Seiche in a Large Bay

Monday, 15 December 2014
James H MacMahan1, Edward B Thornton1, Edith L Gallagher2 and Ad Reniers3, (1)Naval Postgraduate School, Monterey, CA, United States, (2)Franklin & Marshall College, Lancaster, PA, United States, (3)Delft University of Technology, Hydrolic Engineering, Delft, Netherlands
Short-term observations of sea surface elevations (η) along the 10m isobath, and long-term observations inside and outside of a large bay (Monterey Bay, CA) were obtained to describe the nodal structure of the mode 0-3 seiches within the bay and the low frequency (<346 cpd) seiche forcing mechanism. The measured nodal pattern validates previous numerical estimates associated with a northern amplitude bias, though variability exists across the modal frequency band, particularly for mode 0 and 1. Low frequency oceanic η white noise within seiche frequency bands (24-69 cpd) provides a continuous forcing of the bay seiche with a η2 (variance) amplification of 16-40 for the different modes. The temporal variation of the oceanic η white noise is significantly correlated (R2=0.86) at the 95% confidence interval with the bay seiche η that varies seasonally. The oceanic η white noise is hypothesized as being from low frequency, free, infragravity waves that are forced by short waves. This work was funded by NPS, ONR, & NSF.