The Climate Response to Small Solar Perturbations: The Importance of the Background State

We examine the steady-state climae response to small perturbations in total solar irradiance (TSI) using the Community Atmosphere Model (CAM) version 3.0 coupled to a mixed layer ocean. We find that the climate response to 0.1% increase in the solar constant is highly dependent on the background state. Specifically, a small solar perturbation to a present-day climate (0.1%S) produces an equatorward shift of the Southern Hemisphere (SH) midlatitude jet. Meanwhile, the same perturbation applied to a doubled-CO2 climate (0.1%S(2x)) produces a poleward shift of the SH jet. In many other respects the 0.1%S and 0.1%S(2x) experiments produce climate responses that are qualitatively opposite to each other: they produce contrasting changes in sea level pressure over the North Pacific and North America, and contrasting changes in precipitation over most of the globe. We perform long enough integrations to ensure that these contrasting responses are statistically significant.

We perform additional experiments that demonstrate that the pattern pf SST change--and not the global mean change in SST--is responsible for producing these contrasting responses to solar forcing. This means that, even for present-day values of CO2, small biases in a model's ckimate might lead to a very different response to changes in TSI. This also has important implications for understanding the influence of the 11-year solar cycle on climate. Finally, we perform integrations in which we impose 2% increases in the solar constant. The responses in these cases do not depend much on the background state. This shows that the response to solar forcing is nonlinear, and that studying the response to large solar forcing, as many past studies have done, does not necessarily provide information about the response to small solar forcing.

Note: This material has not been previously published or presented.