Analyzing the convection within Earth’s mantle is actually very easy. The layers of Earth have been measured, and each of them is a layer of convection. Physics tell us convection of the mantle should be in the form of convection rolls. The shape of the rolls must be in accordance with the shape of the Earth. Thereby we know the basics, and can make use of our knowlegde, understanding the geology better.
Between the large scale rolls and the tectonic plates, there are two layers. Also, there is a special system for the poles, and a system extending from equator. These system intersect each other, forming a transition zone. The central axis of transition zone is 64°N. The picture above shows the central axis of 64°N. Therefore, there are 4 layers found between the large scale rolls and the tectonic plates.
The two layers from south become the lower convection rolls. The polar system is composed of the upper more convection rolls of the transition zone.
We are so lucky that the weather system follows the same rule. So those who have studied meteorolgy might see the analogy. here. Southern winds along the ground rise and head back. Northern winds also rise and find their way back. The result of the transition is also four layers, exactly at the same latitudes.