Connections between upper tropospheric and lower stratospheric circulation responses to increased CO2 (in revision)


There are a myriad of ways atmospheric circulation responds to increased CO2. In the troposphere, the region of tropical upwelling narrows, the Hadley Cells (HC) expand, and the upper level subtropical zonal winds that comprise the subtropical jet (STJ) strengthen. In the stratosphere, the tropical upwelling narrows and strengthens, enhancing the Brewer-Dobson Circulation (BDC). Despite the robustness of these projections, the physical mechanisms for these responses remain unclear and it’s difficult to discern any dynamical coupling between the features. In this study, we analyze output from the NASA Goddard Institute for Space Studies (GISS) Model E coupled climate model to examine any connection between the upper tropospheric and lower stratospheric circulation by considering the features' seasonality, hemispheric asymmetry, and time scale in response to CO2. We find that a narrowing and strengthening of upper tropospheric upwelling occurs with a strengthening of the subtropical jet. These coupled features are distinct from the poleward expansion of the HC edge. Also, a narrowing and strengthening of lower stratospheric upwelling is related to an equatorward shift in critical latitude for wave breaking and the associated strengthening of the subtropical upper troposphere and lower stratosphere (UTLS) zonal winds. Ultimately, the responses of the stratospheric features display different seasonal, hemispheric, and transient patterns than those in the troposphere, indicating independent circulation changes between the two domains.

Weather and Climate Dynamics