Title: What is Driving Changes in the Tropospheric Circulation? New Insights from Simplified Models
Description: This thesis seeks an improved understanding of what has been driving changes in the large scale tropospheric circulation. First, we consider the effects of stratospheric water vapor levels, which exhibit significant changes on both interannual and decadal timescales. It is shown that idealized thermal forcings mimicking increases in stratospheric water vapor produce poleward expansion of the Hadley cells (HCs) and poleward shifts of the midlatitude jets. Quantitatively, the circulation responses are comparable to those produced by increased well-mixed greenhouse gases. This suggests that stratospheric water vapor may be a significant contribution to past and projected changes in the tropospheric circulation. The second part of this thesis focuses on the response to idealized thermal forcings in the troposphere. It is found that zonally uniform warming confined to a narrow region around the equator produces contraction of the HCs and equatorward shifts of the midlatitude jets. Forcings with wider meridional extent produce the opposite effect: HC expansion and poleward shifts of the jets.

If the forcing is confined to the midlatitudes, the amount of HC expansion is more than three times that of a forcing of comparable amplitude that is spread over the tropics. This finding may be relevant to recently observed trends of amplified warming in the midlatitudes. Furthermore, a simple diffusive model is constructed to explain the sensitivity of the circulation response to the meridional structure of the thermal forcing. The final part of this thesis considers the possible influence of solar forcing on the tropospheric circulation. Of particular interest is the steady state response to a 0.1% increase in total solar irradiance (TSI), the approximate amplitude of the 11-year solar cycle. Using a comprehensive atmospheric general circulation model coupled to a mixed layer ocean, it is found that a 0.1% TSI increase produces a circulation response that has a high dependence on the background state. Specifically, a TSI perturbation applied to a present day climate produces an equatorward shift of the Southern Hemisphere (SH) midlatitude jet, while the same forcing applied to a warmer climate produces a poleward shift of the SH jet.

Opposite-signed responses are also evident in regions of the sea surface temperature, sea level pressure, and precipitation fields. These divergent responses may help to explain why earlier studies reach highly disparate conclusions about the influence of solar variations on climate.