Seasonality in Future Tropical Lower Stratospheric Temperature Trends

Simulations by a group of comprehensive chemistry climate models show significant seasonal dependence in the zonal mean tropical lower stratospheric temperature (LST) trend in the 21st century. Sensitivity simulations with isolated forcing reveal that greenhouse gases (GHG) increases dominate the future tropical LST trend, with GHG increases leading to strongest cooling around July-September and minimal cooling in February-March. This seasonally varying trend can no longer explained by the changes in the Brewer-Dobson Circulation (BDC), the dominant driving factor for the past LST trend. The seasonality in the future is modulated by changes in the horizontal mixing associated with the enhanced subtropical wave breaking induced by GHG increases. These sensitivity simulations also confirm that the seasonality of the past LST trends analyzed in previous studies is primarily driven by the BDC changes associated with the Antarctic stratospheric ozone depletion. The ozone depletion produces an anti-correlation of the LST trend between the Antarctic and tropics, which is de-correlated in the GHG forcing dominated 21st century.