[1] A consistent poleward and upward shift and intensification of the storm tracks is found in an ensemble of 21st century climate simulations performed by 15 coupled climate models. The shift of the storm tracks is accompanied by a poleward shift and upward expansion of the midlatitude baroclinic regions associated with enhanced warming in the tropical upper troposphere and increased tropopause height. The poleward shift in baroclinicity is augmented in the Southern Hemisphere and partially offset in the Northern Hemisphere by changes in the surface meridional temperature gradient. The poleward shift of the storm tracks also tends to be accompanied by poleward shifts in surface wind stress and precipitation, and a shift towards the high index state of the annular modes. These results highlight the integral role that the storm tracks play in the climate system, and the importance of understanding how and why they will change in the future.

SUMMARYIs the transport of heat northward by the Gulf Stream and North Atlantic Drift, and its subsequent release into the midlatitude westerlies, the reason why Europe's winters are so much milder than those of eastern North America and other places at the same latitude? Here, it is shown that the principal cause of this temperature difference is advection by the mean winds. South-westerlies bring warm maritime air into Europe and northwesterlies bring frigid continental air into north-eastern North America. Further, analysis of the ocean surface heat budget shows that the majority of the heat released during winter from the ocean to the atmosphere is accounted for by the seasonal release of heat previously absorbed and not by ocean heat-ux convergence. Therefore, the existence of the winter temperature contrast between western Europe and eastern North America does not require a dynamical ocean. Two experiments with an atmospheric general-circulation model coupled to an ocean mixed layer con rm this conclusion. The difference in winter temperatures across the North Atlantic, and the difference between western Europe and western North America, is essentially the same in these models whether or not the movement of heat by the ocean is accounted for. In an additional experiment with no mountains, the ow across the ocean is more zonal, western Europe is cooled, the trough east of the Rockies is weakened and the cold of north-eastern North America is ameliorated. In all experiments the west coast of Europe is warmer than the west coast of North America at the same latitude whether or not ocean heat transport is accounted for. In summary the deviations from zonal symmetry of winter temperatures in the northern hemisphere are fundamentally caused by the atmospheric circulation interacting with the oceanic mixed layer.

The dynamical simulation of the latest version of the Community Atmosphere Model (CAM3) is examined, including the seasonal variation of its mean state and its interannual variability. An ensemble of integrations forced with observed monthly varying sea surface temperatures and sea ice concentrations is compared to coexisting observations. The most significant differences from the previous version of the model [Community Climate Model version 3 (CCM3)] are associated with changes to the parameterized physics package. Results show that these changes have resulted in a modest improvement in the overall simulated climate; however, CAM3 continues to share many of the same biases exhibited by CCM3.At sea level, CAM3 reproduces the basic observed patterns of the pressure field. Simulated surface pressures are higher than observed over the subtropics, however, an error consistent with an easterly bias in the simulated trade winds and low-latitude surface wind stress. The largest regional differences over the Northern Hemisphere (NH) occur where the simulated highs over the eastern Pacific and Atlantic Oceans are too strong during boreal winter, and erroneously low pressures at higher latitudes are most notable over Europe and Eurasia. Over the Southern Hemisphere (SH), the circumpolar Antarctic trough is too deep throughout the year.The zonal wind structure in CAM3 is close to that observed, although the middle-latitude westerlies are too strong in both hemispheres throughout the year, consistent with errors in the simulated pressure field and the transient momentum fluxes. The observed patterns and magnitudes of upper-level divergent outflow are also well simulated by CAM3, a finding consistent with an improved and overall realistic simulation of tropical precipitation. There is, however, a tendency for the tropical precipitation maxima to remain in the NH throughout the year, while precipitation tends to be less than indicated by satellite estimates along the equator.The CAM3 simulation of tropical intraseasonal variability is quite poor. In contrast, observed changes in tropical and subtropical precipitation and the atmospheric circulation changes associated with tropical interannual variability are well simulated. Similarly, principal modes of extratropical variability bear considerable resemblance to those observed, although biases in the mean state degrade the simulated structure of the leading mode of NH atmospheric variability.

Prescribed SST experiments are performed using the National Center for Atmospheric Research's Community Climate Model version 3 general circulation model to isolate the contribution of the tropical SSTs reconstructed by the Climate, Long-range Investigation, Mapping and Prediction study (CLIMAP) to the modeled global atmospheric circulation anomalies at the Last Glacial Maximum (LGM). The changes in tropical SST patterns cause changes in tropical convection that force large (Ͼ300 m in 500-mb geopotential height) changes in Northern Hemisphere wintertime circulation. These midlatitude circulation changes occur despite the small (1ЊC) change in the mean tropical SST between the present and the CLIMAP reconstruction. In fact, the midlatitude circulation changes due to the difference in the tropical SST pattern between the present and the CLIMAP reconstruction are greater than the circulation changes due to a uniform tropical SST cooling of 3ЊC or those due to the presence of the LGM ice sheets. The circulation anomalies due to the change in tropical SST patterns result in a wintertime warming (cooling) of 8ЊC (8ЊC) over the Laurentide (Fennoscandian) ice sheet and a decrease (increase) in annual mass balance of over 1000 mm yr Ϫ1 (800 mm yr Ϫ1) along the southern margin of the ice sheet. These results demonstrate that detailed knowledge of tropical SST patterns is needed in order to produce reliable simulations of LGM climate. In the appendix, it is shown that the lion's share of the midlatitude circulation changes are due to SST gradients in the northern Tropics, and that the physics involved in the teleconnection between tropical SST forcing and midlatitude circulation changes is rich and highly nonlinear. * Joint Institute for the Study of the Atmosphere and Ocean Contribution Number 699.