An explanation for the apparent gradient of tidal amplitude with latitude?

[Doug Richard]

I teach marine biology and have been presenting the traditional model of the equilibrium tidal theory (2 humps on rotating earth) as still presented in most basic texts, but have not been able to find the theory or presumed explanation for the apparent generally increasing amplitude in tidal fluctuation with increasing latitude. (i.e. minimal flux in Caribbean to progressive increases moving up the North American coast to Fundy, etc.). I know Fundy has some special geographic effects enhancing tides, but the New England coast tides are obviously of greater magnitude than those of Florida and the Caribbean, and similar gradient seems evident on the west coast of North America. Can anyone give me some help here??

 

[D H]

The tidal bulge model (and I certainly hope you are not using the nonsensical centrifugal/centripetal explanation) is just that -- a model. That model would be quite accurate if the Earth had no land masses, bit then the maximal height of the tides would be a bit over a foot. This would occur at the sub-moon point and its antipode.

Because the Earth *does* have land masses, the moving (700 mph!) tidal bulges reflect off those land masses. The superpositions that result from these reflections are part of what is responsible for the pattern depicted below:

http://svs.gsfc.nasa.gov/stories/topex/images/TidalPatterns_hires.tif
Source=http://svs.gsfc.nasa.gov/stories/topex/tides.html

Here the color represents the amplitude of the M2 component of the tides (the semidiurnal component of the tides caused by the Moon) and the white cotidal lines represent the timing of the high and low tides. The places where the cotidal lines converge to a point are amphidromic points; the M2 component of the tides is zero at these points.

Note that the highest tides occur at the shore. If it wasn't for those continents getting in the way, tides would be fairly small (a half-wave height of 14 inches or so, max). Tsunami are pretty dang small mid-ocean but build up to incredible heights when they reach the coast. The same phenomena magnifies the effects of the tides.