Impact of Nonlinear Barotropic Tides in the North Pacific

University Of Hawaii, Honolulu

Investigators

Abstract

The daily surface tides in the ocean, forced by gravitational perturbations arising from the motions of Earth and its Moon, exert a profound influence on the biology, chemistry, geology and circulation in both pelagic and coastal environments. As more accurate models of the tides have been developed, it has been realized that non-linear interactions among the tides in the shallow seas and continental shelves (which are all forced by the pelagic tides) result in a flow of energy back into the open ocean, but at altered frequencies and wavelengths. The impact of this energy flux can have significant consequences not only for modeling the diurnal and semi-diurnal tides themselves, but for the energetics of phenomena dependent on the tides such as abyssal mixing. This study explores two lines of research. The first is to determine how much the coastal non-linear tides "contaminate" what is generally considered to be the linear diurnal and semi-diurnal tides in the pelagic ocean. The second addresses the question of whether the flux of coastal nonlinear tide energy dominates the entire suite of surface gravity wave motions in the pelagic ocean at periods shorter than the tides, even as short as 15 minutes, a possibility that has ramifications both for the ocean's energy budget and other geophysical phenomena (e.g., Earth's "seismic hum"; fragmentation of ice shelves) that are now known to depend on these waves. The project involves training of one graduate student. The broad objectives of this study are to elucidate, using observational tools, (i) the nature (e.g., amplitude, structure) of the pelagic, barotropic, nonlinear tides, especially within the diurnal and semi-diurnal tide bands; (ii) the specific source locations for the energy in the pelagic, barotropic, nonlinear tides; (iii) the flow of energy from the principal, linear barotropic tides to gravity waves in other frequency bands; (iv) the nature (e.g., isotropic or not) and sources of the gravity wave energy at periods of 15 minutes to 2 hrs. These objectives will be accomplished by analyses of existing long (> 1 year) bottom pressure records in the North Pacific that well observe these waves and tides. The methodologies include least squares harmonic analysis and a variety of auto- and cross-spectral techniques, some borrowed from seismology.