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I have been searching for a simple explanation for "Barotropic tidal forcing". Can anyone provide an definition or explain what it is?

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Although the moon and sun are the major forces that displace water across the earth, there are other forces that influence motion. The primary bulging of tides creates baroclines, which, influenced by seafloor topography sets up gradient potentials that can lead to periodic internal waves and tidal currents. This secondary effect is barotropic tidal forcing.

A very good and classic reference that describes the various forces involved in tidal motion is "The Oceans" by Sverdrup, Johnson and Fleming.

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In general, the term barotropic means motion that occurs in phase over the entire water column, i.e., motion at the surface occurs at the same time as motion at depth, although not necessarily at the same amplitude (surface amplitudes are higher). Baroclinic means motions that have vertical structure and may not have any surface expression at all. Internal waves are an example of baroclinic motion.

The main component of tides is barotropic with the motions in phase with the gravitational influence of the moon and sun (the M2 tide is the largest). These components are the ones typically displayed in tide charts. The amplitudes of the motions are influenced by the ocean boundaries (coastlines and depth) and so can vary a lot from location to location. For the M2 tide, say, the bulging of the water nearest the moon, as the Earth rotates, forms in a sense a 'tidal wave' which causes water to slosh around the boundaries, giving rise to raising and lowering water levels and ebbing and flowing currents.

When the water being forced by the barotropic tide encounters undersea topography, the water is diverted vertically where it can encounter a different surrounding density. It then responds as a gravity wave, seeking to get back to its own density level. This generates a big internal wave, being forced at the tidal frequency. The particular wavelength, frequency and amplitude of the waves depend on the actual conditions. The propagating energy represents a baroclinic tide.

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