This is the first part of a three-part paper on North American climate in phase 5 of the Coupled Model Intercomparison Project (CMIP5) that evaluates the historical simulations of continental and regional climatology with a focus on a core set of 17 models. The authors evaluate the models for a set of basic surface climate and hydrological variables and their extremes for the continent. This is supplemented by evaluations for selected regional climate processes relevant to North American climate, including cool season western Atlantic cyclones, the North American monsoon, the U.S. Great Plains low-level jet, and Arctic sea ice. In general, the multimodel ensemble mean represents the observed spatial patterns of basic climate and hydrological variables but with large variability across models and regions in the magnitude and sign of errors. No single model stands out as being particularly better or worse across all analyses, although some models consistently outperform the others for certain variables across most regions and seasons and higher-resolution models tend to perform better for regional processes. The CMIP5 multimodel ensemble shows a slight improvement relative to CMIP3 models in representing basic climate variables, in terms of the mean and spread, although performance has decreased for some models. Improvements in CMIP5 model performance are noticeable for some regional climate processes analyzed, such as the timing of the North American monsoon. The results of this paper have implications for the robustness of future projections of climate and its associated impacts, which are examined in the third part of the paper.

In part III of a three-part study on North American climate in phase 5 of the Coupled Model Intercomparison Project (CMIP5) models, the authors examine projections of twenty-first-century climate in the representative concentration pathway 8.5 (RCP8.5) emission experiments. This paper summarizes and synthesizes results from several coordinated studies by the authors. Aspects of North American climate change that are examined include changes in continental-scale temperature and the hydrologic cycle, extremes events, and storm tracks, as well as regional manifestations of these climate variables. The authors also examine changes in the eastern North Pacific and North Atlantic tropical cyclone activity and North American intraseasonal to decadal variability, including changes in teleconnections to other regions of the globe. Projected changes are generally consistent with those previously published for CMIP3, although CMIP5 model projections differ importantly from those of CMIP3 in some aspects, including CMIP5 model agreement on increased central California precipitation. The paper also highlights uncertainties and limitations based on current results as priorities for further research. Although many projected changes in North American climate are consistent across CMIP5 models, substantial intermodel disagreement exists in other aspects. Areas of disagreement include projections of changes in snow water equivalent on a regional basis, summer Arctic sea ice extent, the magnitude and sign of regional precipitation changes, extreme heat events across the northern United States, and Atlantic and east Pacific tropical cyclone activity.

Easterly waves (EWs) are prominent features of the intertropical convergence zone (ITCZ), found in both the Atlantic and Pacific during the Northern Hemisphere summer and fall, where they commonly serve as precursors to hurricanes over both basins. A large proportion of Atlantic EWs are known to form over Africa, but the origin of EWs over the Caribbean and east Pacific in particular has not been established in detail. In this study reanalyses are used to examine the coherence of the large-scale wave signatures and to obtain track statistics and energy conversion terms for EWs across this region. Regression analysis demonstrates that some EW kinematic structures readily propagate between the Atlantic and east Pacific, with the highest correlations observed across Costa Rica and Panama. Track statistics are consistent with this analysis and suggest that some individual waves are maintained as they pass from the Atlantic into the east Pacific, whereas others are generated locally in the Caribbean and east Pacific. Vortex anomalies associated with the waves are observed on the leeward side of the Sierra Madre, propagating northwestward along the coast, consistent with previous modeling studies of the interactions between zonal flow and EWs with model topography similar to the Sierra Madre. An energetics analysis additionally indicates that the Caribbean low-level jet and its extension into the east Pacific-known as the Papagayo jet-are a source of energy for EWs in the region. Two case studies support these statistics, as well as demonstrate the modulation of EW track and storm development location by the MJO.

Deep convection over the western tropical Pacific warm pool is analyzed in terms of its relation to the atmospheric Kelvin-Rossby wave, which dominates the large-scale flow during the austral summer. The study uses Doppler radar data collected by aircraft and ship radars during different time periods in the Tropical Ocean Global Atmosphere Coupled Ocean-Atmosphere Response Experiment to characterize the mesoscale circulations of organized convective cloud systems occurring throughout the season. The study focuses on convection in two contrasting phases of the wave: the ''westerly onset region'' just west of the point within the wave where low-level easterlies change to westerlies, and the ''strong westerly region'' (or ''westerly wind burst'') lying between the large-scale counterrotating gyres of the Kelvin-Rossby wave.In the westerly onset region the zonal wind component had midlevel easterlies overlying low-level westerlies. In the strong westerly region a deep layer of westerlies extended from the surface up to the upper troposphere, with a maximum of westerly component at about the 850-mb level. The different vertical shear of the zonal wind in these two regions of the wave led to different momentum transport by the mesoscale circulations that develop into very large ''super convective systems'' (cloud tops colder than Ϫ65ЊC over regions of ϳ300 km or more in lateral dimension). The super convective systems developed strong midlevel inflow jets. The direction of the jet was determined by the environmental shear, which in turn was determined by the dynamics of the large-scale wave. In the westerly onset region, the large-scale shear determined that the jet had an easterly component. In the strong westerly region, the jet had a westerly component. In both cases, the inflow intensified within the cloud system as the convective cells of the super convective system filled a broad region with a deep stratiform ice cloud, from which ice particles fell. Evidently, as the particles sublimated and melted, they cooled the air at midlevels in the cloud system. The cooling evidently modified the mesoscale pressure field in the system so as to accelerate the flow of ambient air into the system and to encourage the inflow to subside. In this way, the mesoscale inflow to super convective systems transported easterly momentum downward in the westerly onset region and westerly momentum downward in the strong westerly region, so that the mesoscale momentum feedback of the mesoscale inflow jets were negative in the westerly onset region and positive in the strong westerly region (accelerating the westerly wind burst). These momentum transports by the broad mesoscale midlevel inflow of super convective systems affected broad horizontal regions and were sometimes different in sign from the momentum transports of individual convective-scale cells in the same system. * Joint Institute for the Study of the Atmosphere and the Oceans Contribution Number 666.

This is the second part of a three-part paper on North American climate in phase 5 of the Coupled Model Intercomparison Project (CMIP5) that evaluates the twentieth-century simulations of intraseasonal to multidecadal variability and teleconnections with North American climate. Overall, the multimodel ensemble does reasonably well at reproducing observed variability in several aspects, but it does less well at capturing observed teleconnections, with implications for future projections examined in part three of this paper. In terms of intraseasonal variability, almost half of the models examined can reproduce observed variability in the eastern Pacific and most models capture the midsummer drought over Central America. The multimodel mean replicates the density of traveling tropical synoptic-scale disturbances but with large spread among the models. On the other hand, the coarse resolution of the models means that tropical cyclone frequencies are underpredicted in the Atlantic and eastern North Pacific. The frequency and mean amplitude of ENSO are generally well reproduced, although teleconnections with North American climate are widely varying among models and only a few models can reproduce the east and central Pacific types of ENSO and connections with U.S. winter temperatures. The models capture the spatial pattern of Pacific decadal oscillation (PDO) variability and its influence on continental temperature and West Coast precipitation but less well for the wintertime precipitation. The spatial representation of the Atlantic multidecadal oscillation (AMO) is reasonable, but the magnitude of SST anomalies and teleconnections are poorly reproduced. Multidecadal trends such as the warming hole over the central-southeastern United States and precipitation increases are not replicated by the models, suggesting that observed changes are linked to natural variability.