Low-frequency Vibrations in the Coupled Ocean - Ross Ice Shelf - Atmosphere System

A part of the Southern Ocean, the Ross Sea, together with the Ross Ice Shelf and the atmosphere over the region represent a coupled system with respect to the low-frequency (with the periods between 1 and 11 hours) wave processes observed in the three media. We study interconnections between them using a unique combination of geophysical sensors: hydrophones measuring pressure variations on the bottom of the open ocean, seismographs measuring displacements of the surface of the Ross Ice Shelf, and the Jang Bogo Dynasonde system measuring wave parameters at the altitudes of the lower thermosphere. Analysis of a year-long data sets from Ross Ice Shelf-based instruments reveals presence in their average power spectra of the peaks in the 2-11 hours period range that may be associated with the low-order resonance vibrations of the system. These peaks are absent in the average power spectra of pressure fluctuations measured on the seafloor. This is the first direct observation of the resonance peaks in vibrations of the Ross Ice Shelf. Our results demonstrate the key role of the resonances of the Ross Ice Shelf in maintaining the wave activity in the entire coupled system. Some of the resonance peaks are close to higher-order harmonics of the ocean tide. Based on this observation we suggest that the ocean tide is a major source of excitation of the Ross Ice Shelf's resonances. The ice shelf vibrations may also be supported by the energy transfer from wind and infragravity wave energy that couples with the ice shelf. Overlapping 6-month-long data sets reveal a significant linear correlation between the spectra of the vertical shifts of the Ross Ice Shelf and of the thermospheric waves with the periods of about 2.1, 3.7, and 11.1 hours. This result corroborates earlier lidar observations of persistent atmospheric wave activity over McMurdo. We propose a theory that describes the nexus between the harmonics of the ocean tide and the resonance vibrations of the Ross Ice Shelf. It complements the theoretical model of the process of generating the atmospheric waves by the resonance vibrations of the Ross Ice Shelf published by us earlier.