The Role of the Stratosphere in Setting the Timescales of the Tropospheric Circulation Response to Increasing Greenhouse Gases

One common characteristic of global climate models is that, in response to an increase in greenhouse gas concentrations, the atmospheric circulation expands poleward: that is, the descending branch of the Hadley circulation shifts poleward toward midlatitudes and the midlatitude storm tracks shift toward the poles, particularly in the Southern Hemisphere (SH). It is commonly assumed that these circulation responses are directly linked to warming tropospheric temperatures, either through horizontal temperature gradients at the surface or in the upper troposphere or through vertical temperature gradients (static stability). In this presentation, we provide evidence that stratospheric temperatures, which cool in response to increasing greenhouse gases, may also be important in governing the tropospheric circulation response.

Key aspects of the tropospheric circulation equilibrate substantially faster to increasing greenhouse gases than tropospheric temperatures. One interesting example is the poleward shift of the Hadley cell edge and storm track in the SH. The SH Hadley cell edge and storm track shift poleward on the timescale of the increasing global-mean surface temperature during austral summer and fall (as would be expected), but on a much faster timescale during austral winter and spring (which is quite unexpected). This behavior is pervasive across almost all CMIP5 models. To understand this seasonality, we partition the circulation response into the components associated with the direct radiative effects of CO₂ and rising sea surface temperatures (SSTs). The direct radiative effects of CO₂ enhance the meridional temperature gradient in the upper troposphere-lower stratosphere (UTLS) during all seasons via the cooling of the polar lower stratosphere. In contrast, rising SSTs only substantially enhance the UTLS meridional temperature gradient during summer and fall months. Consequently, the direct radiative effects of the CO₂ forcing have a greater influence on the SH circulation response during winter and spring months, contributing to its faster timescale.