I think your best bet is speed, and lots of it.
If you look at pictures of Jupiter and Saturn (and, to a lesser extent, Uranus and Neptune), you would notice belts and zones across the surface. On Jupiter, these are especially prominent because the belts (the dark bands, which fall) and the zones (the light bands, which rise) are composed of different gasses.
To get those belts and zones, you'd need a planet with a very short day (Jupiter's is on the order of 10 hours). Bear in mind that this will give you very high winds; you will have to consider what this will do to your biosphere. This also gives you the "opportunity" to create massive storms (a la the Great Red Spot) on the boundaries that last for years, if not centuries, creating all kinds of story potential.
One potential effect of the high winds: In large equatorial oceans, the winds might be going fast enough to whip the sea into a froth, effectively erasing the boundary between sea and sky. This might be what creates the tropical zone, as opposed to the polar belts.
In order to keep up your planet's speed, you will need to limit tidal forces (so no or very small moons) and internal drag (so no or very small equatorial landmasses). A bigger planet might also help, lending rotational inertia to the system.