Not really sure exactly what you mean but this is what I reckon.
Under normal conditions (i.e. temperature decreasing with height) if you fly a constant CAS in a climb, your TAS will increase and your Mach number will increase faster. However, the tropopause is a layer where the temperature stops falling with height, i.e. essentially an isothermal layer. Climbing through an isothermal layer, at constant CAS the TAS will still increase, but now the Mach number will increase at the same rate as the TAS since Mach number is TAS/(speed of sound) and the speed of sound is dependent on temperature. Conversely, if you are flying at a constant Mach number in the climb then your TAS would remain constant and CAS would decrease.
I bet that doesn't answer anything!