Why is the tropopause at a higher altitude at the equator?

Question

This site says that the tropopause is at a higher altitude at the equator because:

1. Gravitational force from a point is higher closer to it and reduces with increasing distance from it. Because the Earth itself is broader at equator, the equator experiences less gravity allowing air to reach bigger heights. Poles are closer to gravitational center and experience higher gravity.

2. Earth is rotating at a rate of 24 hours per spin. Not just the ground, but also the atmosphere is spinning with it. The gas molecules at poles are closer to this rotational axis while those near the equator are farther away on a larger radius. Therefore, air at equator experience a greater centrifugal force and moves farther away from Earth.

3. Earth's orientation in space allows equator to be more closer to the Sun. Due to this higher gravity, atmosphere deforms slightly towards the sun while draining a bit more air from the poles.

4. Regions near equator receives more sunlight than the poles making them hotter and less air dense. So equatorial gases reaches greater heights to exert the same pressure as at the poles.

5. Tidal effect by the moon Just like the tides, Moon's gravity causes the atmosphere to deform. Since the moon orbits close to the equator, equatorial thickness is increased.

These all seem like plausible factors, but I'm not sure what the main reasons are. Which factors contribute the most, and how much?

To me reasons 1, 2 and 4 seem like the main causes. Do tidal forces really matter? Does distance to the Sun really matter? (Seems ridiculous because the Sun is 150 million kilometers away from Earth, so a few extra thousand kilometers shouldn't matter at all)

Is there any main biggest factor that makes much more of a difference than the other factors?

Answer 1

There are a couple of ways the atmosphere can be viewed as being "thicker" at the equator than at the poles. One sense is the mass per unit area of all of the air overhead, from sea level to outer space, at the equator vs at the North Pole. Atmospheric pressure at sea level, being essentially a potential, is on average pretty much constant worldwide when averaged out over the course of years.

Since atmospheric pressure is the weight per unit area of all of the air overhead, those places where acceleration due to gravity is low must necessarily support more air in terms of mass to achieve the same pressure than areas where gravitational acceleration is high. Gravitational acceleration at sea level is a bit less at the equator than at the poles, making the atmosphere at the equator a tiny bit "thicker" than the atmosphere over the North Pole, by about half a percent.

This is a tiny effect. It is not what people mean when they say the atmosphere is thicker at the equator. The tropopause is considerably higher over equatorial regions (more specifically, over the intertropical convergence zone, or ITCZ) than it is over polar regions. The tropopause can 20 km above sea level or higher at the ITCZ and only 4 km above sea level at the poles. The boundary between the troposphere and stratosphere is essentially non-existent at the South Pole during parts of the winter.

The small decrease in gravitation (including centrifugal acceleration) from the poles to the equator accounts for only a tiny bit of that large change in troposphere height. In fact, Of the five reasons listed in the question, none is primary. The key reason for the tropopause height reaching a maximum at the ITCZ is the general circulation of the atmosphere, with the ITCZ itself playing a key role.

The ITCZ is where the moisture-laden northern and southern hemisphere trade winds meet at the surface, are driven to great heights due to convection and latent heat, and finally separate to flow northward and southward near the top of the troposphere. It is the strong convection and associated very strong thunderstorms created by this meeting of these atmospheres that pushes the tropopause to great heights at the ITCZ. Tropical cyclones and CAPE-driven thunderstorms elsewhere can also push the troposphere to great heights, but these are transient local effects compared to globe-spanning ITCZ.

The polar regions generally have very little, if any convection. Oftentimes they suffer descending rather than rising air, pulling the tropopause downward. The tropopause is also pulled downward at the jet streams, particularly the polar jet.