Basin mode internal tides

Basin Mode Internal Tides Jenny Thomas, Jim Lerczak, Clint Winant, and Kraig Winters College of Earth, Oceanic, and Atmospheric Sciences, Oregon State University jthomas@coas.oregonstate. edu Abstract The Regional Ocean Modeling System (ROMS) is used to test the hypothesis that the equilibrium tide can excite coupled barotropic and baroclinic basin modes in large basins and result in large-amplitude internal tides, dependent on parameters such as basin-scale bathymetry and stratification. The domain is an idealized, stratified, mid-latitude rotating ocean basin with a horizontally uniform continental shelf and slope around the deep basin, with horizontally uniform stratification, forced by the equilibrium M 2 tide. Different stratifications are compared. Results show internal tide standing basin modes and coastally trapped modes. The baroclinic response is strongly dependent on stratification, with some stratifications having a near resonant response. Stratification affects internal tide wavelength scales, amplitudes and their distribution, and alongshore propagation distance of coastally trapped modes. Perhaps some of the spatial variability and temporal intermittency observed in coastal internal tides is due to spatial and temporal (from changes in stratification) variability in the basin mode response. Introduction We are all familiar with the excitation of normal modes in some way. In playing a musical instrument, normal modes of strings, surfaces, or air cavities are excited. Blowing across the opening of an empty drink bottle can excite a normal mode of the air cavity, making it whistle at a distinct frequency. And simply splashing water in a bathtub excites the normal modes of the basin. If the water is sloshed back and forth at a regular frequency and that frequency happens to be close to that of a normal mode of the tub, then a large amplitude response can occur, splashing water out of the tub. Just like the bathtub, any closed basin will have normal modes. The response of a basin to a periodic forcing can be described as an expansion over the basin’s normal modes, oscillating at the forcing frequency. A mode will have a larger response if its frequency is close to the forcing frequency and if its spatial pattern fits that of the forcing. Barotropic tides have been described as the excitation of normal modes of the world oceans in a series of papers, showing that observations of barotropic tides are consistent with some of these modes [Platzman, 1975, 1978, 1984a, 1984b; Platzman et al., 1981]. For example, the large- scale cyclonic California amphidrome, which yields the observed M 2 tide propagating northward along the west coast of North America, is directly related to the structure of an individual Kelvin wave normal mode with a 15.5-hour period that is non-resonantly forced at the M 2 tidal period [Platzman et al., 1981; Platzman, 1984b]. When a basin is stratified, its normal modes become more complicated, with coupled barotropic and baroclinic normal modes. The stratification introduces internal waves, adding smaller spatial scales to the modes, and bathymetric variability also results in variability in spatial scales of the modes. Winant [2010] shows that the excitation of basin modes can explain large amplitude VIII th Int. Symp. on Stratified Flows, San Diego, USA, Aug. 29–Sept. 1, 2016