Northern Hemisphere winter climate response to greenhouse gas, ozone, solar, and volcanic forcing

The Goddard Institute for Space Studies (GISS) climate-middle atmosphere mo del has been used to study the impacts of increasing greenhouse gases, pola r ozone depletion, volcanic eruptions, and solar cycle variability. We focu s on the projection of the induced responses onto Northern Hemisphere winte r surface climate. Changes in the model's surface climate take place largel y through enhancement of existing variability patterns, with greenhouse gas es, polar ozone depletion, and volcanic eruptions primarily affecting the A rctic Oscillation (AO) pattern. Perturbations descend from the stratosphere to the surface in the model by altering the propagation of planetary waves coming up from the surface, in accord with observational evidence. Models lacking realistic stratospheric dynamics fail to capture these wave flux ch anges. The results support the conclusion that the stratosphere plays a cru cial role in recent AO trends. We show that in our climate model, while ozo ne depletion has a significant effect, greenhouse gas forcing is the only o ne capable of causing the large, sustained increase in the AO observed over recent decades. This suggests that the AO trend, and a concurrent strength ening of the stratospheric vortex over the Arctic, are very likely anthropo genic in origin.