Northern Westerlies during the Last Glacial Maximum: Results from CMIP5 Simulations

Based upon simulations from phase 5 of the Coupled Model Intercomparison Project (CMIP5), the vertical and regional characteristics of the northern westerlies during the Last Glacial Maximum (LGM) are investigated in this study. At the Northern Hemispheric scale, all nine available models simulate a poleward shift of the 200-hPa jet, and eight models simulate an equatorward shift of the 850-hPa jet compared to the preindustrial period; these shifts are of approximately 2°–3° latitude for the arithmetic multimodel mean. The upper-tropospheric cooling in the tropics, possibly due to reduced latent heat release, is expected to account for the poleward shift of the 200-hPa jet through the thermal wind relationship. Changes in the midlatitude baroclinic instability in response to the amplified polar cooling are associated with the jet stream in the lower troposphere through anomalous eddy activity. In particular, the types of predominant baroclinic eddies are regionally dependent. The behavior of the 850-hPa jet over the North Pacific is steered by transient eddies and characterized by a southward displacement during the LGM. By contrast, the remarkable enhancement of the North Atlantic jet stream throughout the troposphere is associated with the notably increased stationary eddy momentum convergence, presumably due to the presence of the Laurentide ice sheet over North America. In comparison with the proxy records, although there is no observational evidence explicitly indicating changes of the upper-level northern westerlies, the simulated LGM 850-hPa westerly wind field is indirectly concordant with the reconstructed moisture conditions over the Mediterranean region and southwestern North America.