The second phase of the North American Monsoon Experiment (NAME) Model Assessment Project (NAMAP2) was carried out to provide a coordinated set of simulations from global and regional models of the 2004 warm season across the North American monsoon domain. This project follows an earlier assessment, called NAMAP, that preceded the 2004 field season of the North American Monsoon Experiment. Six global and four regional models are all forced with prescribed, time-varying ocean surface temperatures. Metrics for model simulation of warm season precipitation processes developed in NAMAP are examined that pertain to the seasonal progression and diurnal cycle of precipitation, monsoon onset, surface turbulent fluxes, and simulation of the low-level jet circulation over the Gulf of California. Assessment of the metrics is shown to be limited by continuing uncertainties in spatially averaged observations, demonstrating that modeling and observational analysis capabilities need to be developed concurrently. Simulations of the core subregion (CORE) of monsoonal precipitation in global models have improved since NAMAP, despite the lack of a proper low-level jet circulation in these simulations. Some regional models run at higher resolution still exhibit the tendency observed in NAMAP to overestimate precipitation in the CORE subregion; this is shown to involve both convective and resolved components of the total precipitation. The variability of precipitation in the Arizona/New Mexico (AZNM) subregion is simulated much better by the regional models compared with the global models, illustrating the importance of transient circulation anomalies (prescribed as lateral boundary conditions) for simulating precipitation in the northern part of the monsoon domain. This suggests that seasonal predictability derivable from lower boundary conditions may be limited in the AZNM subregion.

A fully closed zonal momentum budget is decomposed to explain the occurrence of zonal mean easterlies at subtropical latitudes in July. Eddy momentum fluxes from stationary eddies, most prominently the western sector of the Indian monsoon Tibetan High, are the primary mechanism governing the negative tendency of zonal mean momentum near 20°N–30°N. This strengthening of the zonal mean easterlies in July is significantly correlated with the concurrent strengthening of the North Atlantic Subtropical High (NASH) and the rainfall deficit in the western North Atlantic (WATL). Interannual variations of the Indian monsoon reflect changes in the strength of these zonal mean easterlies, with downstream teleconnections on the westward displacement of the NASH and precipitation in the WATL. An increase in rainfall in India from June to July corresponds to a decrease in rainfall in the WATL.

The effects of a progressively enhanced Asian summer monsoon on the mean zonal wind are examined in a series of experiments using the Community Atmosphere Model version 4 (CAM4). The response of the barotropic mean zonal wind varies in a linear fashion with the forcings of 5, 10, and 20 W m−2 in net radiation over South Asia. The authors increase the strength of the monsoon by making the South Asian land surface hotter (via lower soil albedo). This leads to an enhanced Rossby wave source region over the Balkan Peninsula at 45°N, northwest of the upper-level Tibetan high (TH). Equatorward propagation of Rossby waves causes stationary eddy momentum flux divergence (SEMFD) to the south of this source region. This local area of SEMFD produces easterly tendencies of the barotropic part of the mean zonal wind in the subtropics. As the easterly mean flow strengthens, so do low-level easterlies across the subtropical Atlantic, leading to a westward displacement of the North Atlantic subtropical high (NASH) on its equatorward flank. The western intensification of the NASH causes drying in the west Atlantic and neighboring land masses primarily because of near-surface wind divergence in the anticyclone. These modeling results confirm the mechanisms deduced in the authors’ recent observational analysis of the mean seasonal cycle’s midsummer drought.

This paper describes a new intermediate global atmosphere model in which synoptic and planetary dynamics including the advection of water vapor are explicit in 10 layers, the time‐mean flow is centered near a realistic state through the use of carefully calibrated time‐independent 3‐D forcings, and temporal anomalies of convective tendencies of heat and moisture in each column are represented as a linear matrix acting on the anomalous temperature and moisture profiles. Currently, this matrix is Kuang's [] linear response function (LRF) of a cyclic convection‐permitting model (CCPM) in equilibrium with specified atmospheric cooling (i.e., without radiation or WISHE interactions, so it conserves column moist static energy exactly). The goal of this effort is to cleanly test the role of convection's free‐tropospheric moisture sensitivity in tropical waves, without incurring large changes of mean climate that confuse the interpretation of experiments with entrainment parameters in the convection schemes of full‐physics GCMs. When the sensitivity to free‐tropospheric moisture is multiplied by a factor ranging from 0 to 2, the model's variability ranges from: (1) moderately strong convectively coupled Kelvin waves with speeds near 20 m s−1; to (0) similar but much weaker waves; to (2) similar but stronger and slightly faster waves as the water vapor field plays an increasingly important role. Longitudinal structure in the model's time‐mean tropical flow is not fully realistic, and does change significantly with matrix‐coupled variability, but further work on editing the anomaly physics matrix and calibrating the mean state could improve this class of models.

Here we demonstrate that changes of the North Atlantic subtropical high and its regional rainfall pattern during mid‐Holocene precessional changes and idealized 4xCO2 increase can both be understood as a remote response to increased land heating near North Africa. Despite different sources and patterns of radiative forcing (increase in CO2 concentration versus changes in orbital parameters), we find that the pattern of energy, circulation, and rainfall responses in the Northern Hemisphere summer subtropics are remarkably similar in the two forcing scenarios because both are dominated by the same land‐sea heating contrast in response to the forcing. An increase in energy input over arid land drives a westward displacement of the coupled North Atlantic subtropical high‐monsoon circulation, consistent with increased precipitation in the Afro‐Asia region and decreased precipitation in the America‐Atlantic region. This study underscores the importance of land heating in dictating remote drying through zonal shifts of the subtropical circulation.