The annual cycle in lower stratospheric temperatures revisited

Lower stratospheric temperatures show a prominent annual cycle. The cycles in the southern and northern extratropics are out of phase, whereas the tropics are in phase with the northern hemisphere. In an elegant and influential paper, Yulaeva, Wallace and Holton (1994) explained the observed pattern as a direct consequence of hemispheric asymmetries in the forcing of the stratospheric circulation. They showed that in MSU-4 (weighting centered in the lower stratosphere) data, the combined extratropical and the tropical temperature cycle nearly cancel. The near perfect compensation was taken as indication that the diabatic residual circulation can be approximated by Newtonian cooling with time-constant radiative equilibrium temperature, and constant radiative relaxation timescale and buoyancy frequency. Here, we show that the near-compensation of temperature variations is an artefact of the vertical weighting function of the MSU-4 channel which suppresses the amplitude in the tropics due to the higher tropopause there. We show that dynamically induced ozone variations lead to changes in radiative equilibrium temperature in particular in the tropics, where they amplify the temperature variability by about a factor 2. Latitudinal variations in thermal stratification are found to be important as well. The feedback of dynamically-induced tropical ozone variations on temperatures is a leading order process and must be included in simplified mechanistic models of the UT/LS. Implications for interpretation of trends in stratospheric temperatures and dynamics are discussed.