Dynamically forced upwelling in the tropical lower stratosphere: climatology, trends and response to ENSO

In the tropical lower stratosphere, the meridional circulation acts to transport air upward, inserting tropospheric air and thus trace gases into the stratosphere. The forcing mechanisms of upwelling in the tropical lower stratosphere are investigated using the E39C-A chemistry-climate model and are compared to results from ERA-Interim reanalysis data. It is shown that the strength and annual cycle in tropical upwelling can be largely explained by local resolved wave forcing. The climatological mean forcing is due to both stationary planetary scale waves that originate in the tropics and to extratropical transient synoptic scale waves that are refracted towards the equator. Using a long-term transient simulation with the CCM E39C-A, long-term trends in upwelling are investigated. Tropical upwelling is shown to increase throughout the year in a future climate, and this is due to enhanced wave driving by resolved waves. It is shown that enhanced tropical upwelling is driven by processes induced by increases in tropical sea surface temperatures (SSTs). Higher tropical SSTs cause (a) a strengthening of the subtropical jets and (b) modification of deep convection, leading to changes in the strength and location of latent heat release. While the former can modify wave propagation and dissipation, the latter affects tropical wave generation. Evidence is presented that the dominant mechanism leading to enhanced vertical wave propagation into the lower stratosphere is an upward shift of the easterly shear zone due to the strengthening and upward and equatorward shift of the subtropical jets. The mechanisms that force enhanced upwelling in response to the long-term trend in tropical SSTs are compared to the response of upwelling to ENSO. It is shown that tropical upwelling is enhanced in ENSO warm phases compared to ENSO cold phases. This modification of tropical upwelling is caused by changes in the local resolved wave driving, as it is the case for the long-term trend. The response to ENSO in the model is compared to the response in reanalysis data. The reanalysis show similar response pattern in the meridional circulation and in wave fluxes, indicating that the same mechanisms act in the model and in the reanalysis.