High-Q Spectral Peaks and Nonstationarity in the Deep Ocean Infragravity Wave Band: Tidal Harmonics and Solar Normal Modes

Infragravity waves have received the least study of any class of waves in the deep ocean. This paper analyzes a 389-day-long deep ocean pressure record from the Hawaii Ocean Mixing Experiment for the presence of narrowband (less than or similar to 2 mu Hz) components and nonstationarity over 400-4,000 mu Hz using a combination of fitting a mixture noncentral/central chi(2) model to spectral estimates, high-resolution multitaper spectral estimation, and computation of the offset coherence between distinct frequencies for a given data segment. In the frequency band 400-1,000 mu Hz there is a noncentral fraction of 0.67 +/- 0.07 that decreases with increasing frequency. Evidence is presented for the presence of tidal harmonics in the data over the 400- to 1,400-mu Hz bands. Above similar to 2,000 mu Hz the noncentral fraction rises with frequency, comprising about one third of the spectral estimates over 3,000-4,000 mu Hz. The power spectrum exhibits frequent narrowband peaks at 6-11 standard deviations above the noise level. The widths of the peaks correspond to a Q of at least 1,000, vastly exceeding that of any oceanic or atmospheric process. The offset coherence shows that the spectral peaks have substantial (p=0.99-0.9999) interfrequency correlation, both locally and between distinct peaks within a given analysis band. Many of the peak frequencies correspond to the known values for solar pressure modes that have previously been observed in solar wind and terrestrial data, while others are the result of nonstationarity that distributes power across frequency. Overall, this paper documents the existence of two previously unrecognized sources of infragravity wave variability in the deep ocean. Plain Language Summary Infragravity waves (IGWs) have arguably received the least study of any class of waves in the deep ocean. This paper contains the analysis of a 389 day long deep ocean pressure record from northwest of Hawaii for the presence of narrowband (less than or similar to 2 Hz) components and nonstationarity over 400-4000 mu Hz. Evidence is presented for the presence of tidal harmonics in the data over the 400-1400 mu Hz band, possibly originating in the Gulf of Alaska. The power spectral density exhibits frequent narrowband peaks at 6-11 standard deviations above the noise level. Some of the peak frequencies correspond to the known values for solar normal modes that have previously been observed in solar wind, seismic, geomagnetic, barometric and ionospheric data, while others are the result of nonstationarity that distributes power across frequency. Overall, this paper documents the existence of two heretofore unrecognized sources of IGW variability in the deep ocean.