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Why does atmospheric pressure on Mars change with the seasons?

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Mars
Mars An especially serene view of Mars (Tharsis side), a composite of images taken by the Mars Global Surveyor spacecraft in April 1999. The northern polar cap and encircling dark dune field of Vastitas Borealis are visible at the top of the globe. White water-ice clouds surround the most prominent volcanic peaks, including Olympus Mons near the western limb, Alba Patera to its northeast, and the line of Tharsis volcanoes to the southeast. East of the Tharsis rise can be seen the enormous near-equatorial gash that marks the canyon system Valles Marineris.

The atmospheric pressure on Mars undergoes significant seasonal variations because carbon dioxide—which makes up the vast majority of the Martian atmosphere—cycles between the atmosphere and the polar caps.12

During the winter in a given hemisphere, temperatures drop low enough for carbon dioxide to freeze and deposit onto the surface at the poles, essentially "snowing out" of the atmosphere. Conversely, as spring arrives, this frozen carbon dioxide returns directly to a gaseous state through a process called sublimation, re-entering the atmosphere.1

Because the southern polar cap is more extensive than the northern one, the total atmospheric pressure reaches its annual minimum during the southern winter when the largest amount of carbon dioxide is locked away in the ice cap. Overall, the planet's surface pressure varies by approximately 26 percent annually as trillions of metric tons of carbon dioxide move into and out of the atmosphere.1