ON THE CAUSE OF THE ANNUAL CYCLE IN TROPICAL LOWER-STRATOSPHERIC TEMPERATURES

An analysis of monthly mean, zonally averaged temperatures from the lo wer-stratospheric channel of the microwave sounding unit (MSU-4) shows that on the month-to-month time scale, there is nearly complete compe nsation between temperature changes in the tropics and in the extratro pics. For the annual cycle the MSU4 data show a similar compensation b etween temperatures in the tropics and those in high latitudes, with o nly a small residual variation in the global mean. The tropics are col dest in January and warmest in July, compensating for the warmer winte rtime temperatures in the Northern Hemisphere compared to those in the Southern Hemisphere. These out-of-phase temperature variations betwee n the tropics and extratropics are interpreted as the signature of an annual cycle in the strength of the wave-driven, Lagrangian mean merid ional circulation, which warms the high-latitude winter hemisphere and cools the tropics. The observed phase of the annual cycle in tropical lower-stratospheric temperatures is thus determined by the stronger o rographic and thermal forcing of the wintertime planetary waves in the Northern Hemisphere, which drives a stronger Lagrangian mean meridion al circulation. In the absence of fluctuations in the global mean diab atic heating, the compensation would be complete and the globally aver aged temperature would be constant. The small annual cycle in the glob ally averaged temperature in the MSU-4 data is nearly in phase with th e annual cycle,in the tropics and is consistent with the annual cycle in diabatic heating by ozone.