Wave Forcing of the Tropical Upwelling in the Lower Stratosphere under Increasing Concentrations of Greenhouse Gases

Two simulations from the Whole Atmosphere Community Climate Model, covering the periods 1950-2003 and 1980-2050, are used to investigate the nature of the waves that force the increase of the tropical upwelling in the lower stratosphere as the concentration of greenhouse gases increases. Decomposition of the wave field resolved by the model into stationary and transient wavenumber spectra allows attribution of trends in the Eliassen-Palm (EP) flux and its divergence to specific wave components. This analysis reveals that enhanced dissipation of stationary planetary waves is the main driver of trends in the tropical upwelling in the lower stratosphere. The contribution of transient waves is smaller and is responsible mainly for trends in wave forcing in the subtropics and middle latitudes, which, however, provide only minor contributions to the mean tropical upwelling. Examination of individual wave structures shows that the stationary waves are tropical Rossby waves trapped in the upper troposphere and lower stratosphere, whereas the transient components are synoptic waves present in the subtropics and middle latitudes. The authors also present evidence that trends in resolved wave forcing in the lower stratosphere are due to both changes in wave transmissivity and changes in wave excitation, with the first mechanism dominating the behavior of the simulation during the last half of the twentieth century, while the second is clearly more important in the simulation during the first half of the twenty-first century.