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It appears that in many (most) places on earth, a pressure oscillation of a 100-200 Pa takes place twice a day : that is, with maxima around 12AM and 12PM and minima around 6AM and 6PM.

I've found almost no information about it, except highly technical (and not freely available) papers. I think it's called "semidiurnal oscillation", and it seems to be a vertical oscillation of the full atmosphere. But I'm curious : what is the basic mechanism of such an oscillation, and what makes its period a half-day (instead of a full day)?

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Solar atmospheric tides? Some things are maximal at noon and midnight, minimal at sunrise and sunset, or vice versa, and could cause 12-hour signals. –  gerrit Oct 21 '13 at 15:52
That seems just like it! I'll try and understand that as soon as I can! –  Nicolas Oct 22 '13 at 10:22
I'm not sure if it's really it, and I don't count myself as an expert, that's why it's a comment and not an answer. But I've come across 12-hour signals in time series analysis of atmospheric components. –  gerrit Oct 22 '13 at 12:14

1 Answer 1

From measurements taken over the tropical Pacific Ocean, reported in Diurnal and Semidiurnal Variations of the Surface Wind Field over the Tropical Pacific Ocean (Deser and Smith, 1997), observe that

The observed semidiurnal near-surface wind variations are dynamically consistent with the zonally averaged semidiurnal pressure field, which is forced by the atmospheric thermal tide in the stratosphere and upper troposphere.

This 'thermal tide' is further explained in the paper Diurnal and semidiurnal variations in global wind and divergence fields (Dai and Deser, 1999), which state that solar heating and regional forcings play a major role in the pressure variations.

The article Solar Semidiurnal Tides in the Troposphere: Detection of Radar Profilers (Whiteman and Bian, 1996), state that the variation is far more pronounced in the tropics than at mid-latitudes and beyond. The authors confirm that the semi-diurnal oscillation is indeed a solar tide, as their periodicity

is an exact harmonic of the solar period

But from their observations deduce that the pressure variations are solar tides wind perturbations in the troposphere.

The authors also state that there are similarities between the solar tides and the lunar tides on the ocean, in that they both cause a "tidal bulge" on opposites of the planet and minima in between.

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For more on tidal bulges, see this question & answer. –  Kyle Kanos Sep 30 '14 at 15:12

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