What is located immediately above the tropopause?
The tropopause is the boundary layer in the Earth's atmosphere from the troposphere below and the stratosphere above.
It is the Troposphere. The tropopause separates the troposphere (the lowest level of the atmosphere) from the stratosphere. It can be identified by the temperature of the air -- while in the troposphere, temperature decreases as you gain altitude; in the stratosphere, temperature increases as you gain altitude.
The reason the tropopause is lower above the poles than the equator is... The time it takes air to cool. The surface air-temperature at the equator is much higher than at the poles, meaning that the air rises further to reach the equilibrium required at the tropopause. Likewise, the air at the poles is cooler, and does not require as much room to rise.