The stratopause (formerly Mesopeak) is the almost center of the atmosphere which is the numeric between two layers, stratosphere and the mesosphere. In the stratosphere the temperature increases with altitude, and the stratopause is the section where a maximum in the temperature occurs.

Instability at the Winter Stratopause: A Mechanism for the 4-Day Wave.
Dynamical response to the solar cycle: Winter stratopause and lower stratosphere
Evidence for High-Frequency Synoptic Disturbances Near the Stratopause.
The temperature dependence of the ozone concentration near the stratopause
Interaction of the quasi‐biennial oscillation and stratopause semiannual oscillation
The evolution of the stratopause during the 2006 major warming: Satellite data and assimilated meteorological analyses
A note on record‐high temperatures at the northern polar stratopause in winter 1997/98
Water vapor, molecular hydrogen, methane, and tritium concentrations near the stratopause
Consistent Simulation of Bromine Chemistry From the Marine Boundary Layer to the Stratopause
Lidar Observations of Gravity Waves and Their Spectra near the Mesopause and Stratopause at Arecibo.