Collaborative Research: Forcing, Energy Flow and Impacts of Oceanic Infragravity Waves

Woods Hole Oceanographic Institution, Woods Hole MA

Investigators

Abstract

This project will examine oceanic surface gravity waves with periods from 30 seconds to an hour, the ‘infragravity’ waveband. Existing observations of ocean bottom pressure, geomagnetic field, atmospheric pressure, and ocean gravity waves will be analyzed with spectral techniques to examine the mechanisms (sources) by which energy is used to generate infragravity waves (IGW). Sources include the interactions of tsunami and swell with coastlines, ocean surface tides, and energy from the solar normal modes that have previously been observed in solar wind, seismic, geomagnetic, barometric, and ionospheric data. IGWs can be important connectors between disparate phenomena, connecting source variability to the excitation of earth’s normal modes, flexing of ice shelves in Antarctica, contribution to mixing in the abyssal ocean, and adding ‘noise’ in altimeter data. If understood and modeled appropriately as IGW, such noise could be minimized, to improve such altimetry data. In addition, this project will contribute to the education of a graduate student in physical oceanography building the next generation of scientists capable of exploring our planet for a more environmentally secure future. As technology has expanded the availability of observations of terrestrial and extra-terrestrial variables, such as the solar wind and pressure at the bottom of the oceans, new information has yielded insight into how highly connected disparate phenomena can be. Of longstanding interest are so-called infragravity waves (IGWs; oceanic surface gravity waves at periods of 30 seconds to an hour) due to their excitation by tsunamis and swell (gravity wave periods less than 30 seconds) at the coasts. IGWs forced at the coasts are a factor that enhances flooding and erosion by tsunamis and swell. In the open ocean, IGWs are weaker, but have been found to be unexpected pathways for energy that engender remarkable phenomena, such as the continuous excitation of Earth's normal modes, signals that are now regularly used to explore the details of Earth's structure and composition. But understanding is limited regarding the various sources of energy for the IGWs, and the mechanisms this energy contributes to infragravity waves (i.e., the energy pathways). The principal investigators recently demonstrated the existence of two new energy sources: ocean surface tides and solar normal modes. This project will: (1) acquire more observational evidence of the tidal and solar energy pathways into the IGWs; and, (2) quantify the contributions of these two sources as a function of frequency, time period (e.g., season; 11-yr solar cycle) and location (e.g., geomagnetic latitude; marginal seas vs. open ocean). The relative contributions of these sources to IGWs, and especially their spatial distribution, are crucial characteristics for understanding the impacts of IGWs and their connections to the other phenomena noted above. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.