GSWM Sample Output: April Zonal Semidiurnal Tide

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Apr Zon Semi: Alt v. Lat
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Contour plot of the zonal (eastward) semidiurnal tide in April as a function of altitude, latitude, and local time. Dashed lines and blue colors indicate westward winds. This cross section is independent of longitude. The phase velocity is downward, though the tide propagates upward.

The semidiurnal zonal tide is characterized at equinox by symmetric peaks at +/- 60 degrees latitude, and a weaker peak over the equator, at 115km altitude. The amplitude diminishes rapidly down to 90km, below which there is virtually no 12-hr oscillation. The wavelength of the semidiurnal tide is approximately 50 km.

The phase of the tide is the same across the equator, so the maxima at a given altitude in the northern and southern hemispheres occur in the same direction at the same time.

Contrast the semidiurnal zonal tide with the semidiurnal meridional tide in either an altitude vs. latitude cross section (January case), or a latitude vs. longitude "top view" at 100km (April).

Also, compare the semidiurnal zonal tide with the diurnal zonal tide (April), which exhibits a shorter wavelength, peaks nearer the equator, and symmetry across the equator.