Zonal and Seasonal Variations in Future Predictions of the Northern Hemisphere Mid-latitude Circulation

Global climate models predict that future increases in greenhouse gas concentrations will be accompanied by changes in the Northern Hemisphere mid-latitude circulation. It is often concluded, based on a zonal mean analysis, or basin wide analysis, that a general feature of the mid-latitude circulation response to climate change is a poleward shifting of the eddy driven mid-latitude jet streams and associated storm tracks. Here, the CMIP-5 models will be used to assess the extent to which this is true when considering the full three-dimensional circulation along with seasonal variations.

It is found that the mid-latitude circulation response to climate change varies considerably with season and longitude. Various features are identified that do not resemble a poleward shifting of the climatological circulation. For example, in the winter season, the Pacific jet stream shifts equatorward on the Eastern side of the basin with implications for the climate of the west coast of the USA. On the Eastern side of the Atlantic basin, the zonal wind anomalies represent more of an extension of the jet stream rather than a poleward shift, with implications for Mediterranean climate. These features are robust across the model simulations and were also present in the older CMIP-3 ensemble.

The momentum budget of these circulation changes is examined to identify which features of the mid-latitude circulation change are associated with altered transient eddy momentum fluxes and which represent changes in the stationary circulation patterns driven by other processes, with a view to understanding the dynamics behind Northern Hemisphere mid-latitude climate change.