What determines the Northern Hemisphere circulation response to increasing greenhouse gases?

Obtaining credible climate change projections in NH extratropical winter is challenging as the current generation of coupled atmosphere-ocean models shows a wide range in the Northern Annular Mode (NAM) response to increasing greenhouse gases. Previous studies have suggested that the NAM response critically depends on the stratospheric representation in climate models. In this study, we assess the influence of the stratosphere on the tropospheric circulation response to increasing greenhouse gases by comparing the response in various versions of a comprehensive atmospheric general circulation model (AGCM) without a well-resolved stratosphere (‘low-top' model), to the response in a version of the same AGCM with a well-resolved stratosphere (‘high-top' model). We show that the circulation response is more sensitive to orographic gravity wave drag (OGWD) parameter settings than to the model lid height.

The causal relationship between OGWD and changes in NH wintertime circulation response is further investigated by introducing a methodology that allows OGWD forcing fixed to its 1xCO₂ value when CO₂ is doubled. Such experiments show that the changes in GWD forcing due to CO₂ doubling have essentially no impact on the circulation response. The primary conclusion is that the OGWD influence is limited to its impact on the 1xCO₂ basic-state climatology, which defines the propagation characteristics of resolved planetary waves. It is shown that the action of planetary waves explains essentially all of the NH wintertime circulation sensitivity.

Finally, we will show preliminary results of an investigation of the sensitivity of the NH wintertime circulation response between IPCC models and its possible relation to the basic state climate.