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  • View in gallery

    (a) Variable grid employed for the model simulations; (b), (c) close-ups over North America and Mexico. Heavier dots in (c) represent the typical rain gauge distribution on a given day

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    Onset of the North American monsoon as represented by the Jun to Jul changes in (a) observed precipitation and (b) SGGCM-HR simulated precipitation. Units are mm day−1

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    Vertically integrated moisture flux convergence for (a) Jul and (b) the Jun to Jul change

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    Time series of area-averaged observed daily precipitation for the four main regions indicated in the corresponding insets: (a) northwestern Mexico, (b) southern Great Plains, (c) Arizona, and (d) northern Great Plains. The heavy line is a 5-day running average, and the shaded bars in (a) and (b) denote the onset of the monsoon in northwest Mexico. Units are mm day−1

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    As in Fig. 4 for the 3-h SGGCM-HR simulated precipitation. Units are mm day−1

  • View in gallery

    As in Fig. 5 for the vertically integrated moisture flux convergence. Units are mm day−1. The horizontal lines represent the time mean for the period defined by the length of the line.

  • View in gallery

    As in Fig. 6 for the wind divergence at 200 hPa. The running average of vertical velocity (omega) at 500 hPa has been added with dashed heavy line. Divergence has been multiplied by 106 and its units are s−1. Vertical velocity units are Pa s−1.

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    Longitude–pressure section of potential temperature averaged for the 25°–30°N band for (a) Jul and (b) the Jun to Jul change. Longitude–potential temperature cross section of potential vorticity for (c) Jul and (d) the Jun to Jul change. (e), (f) As in (c), (d) for the meridional wind. The gray band in (c)–(f) represents the dynamic tropopause (PV = 2–2.5 PVU), and values below ground were masked out. All sections are averaged over the same latitude band

  • View in gallery

    Moisture flux at 925 hPa for Aug 1993, as estimated from (a) NCEP–NCAR global reanalyses, (b) the SGGCM-LR simulation, and (c) the SGGCM-HR simulation. Units are g kg−1 m s−1

  • View in gallery

    Gulf of California cross section of the meridional moisture flux at 25° and 30°N estimated from the two model configurations: (a) SGGCM-LR at 25°N, (b) SGGCM-HR at 25°N, (c) SGGCM-LR at 30°N, and (d) SGGCM-HR at 30°N. Units are g kg−1 m s −1

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    Time series of meridional moisture flux anomalies for the layer 875–925 hPa for (a) the Great Plains LLJ and (b) for the Gulf of California LLJ. Eigenvalues representing the diurnal cycle and synoptic-scale variability of the meridional moisture flux for (c) the Great Plains LLJ and (d) for the Gulf of California

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    (a) Surge and (b) no-surge composites of moisture flux at 950 hPa; contours and shades identify the region with largest moisture flux magnitude. (c), (d) Corresponding cross sections of moisture flux at 30°N; (e), (f) percentage of total precipitation for the corresponding composites

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Multiscale Diagnosis of the North American Monsoon System Using a Variable-Resolution GCM

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  • 1 Department of Meteorology, University of Maryland, College Park, Maryland
  • | 2 Earth System Science Interactions Center, University of Maryland, College Park, and Data Assimilation Office, NASA GSFC, Greenbelt, Maryland
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Abstract

The onset and evolution of the North American monsoon system during the summer of 1993 were examined from regional to large scales using the National Aeronautics and Space Administration (NASA) Goddard Earth Observing System (GEOS) stretched-grid GCM. The model's grid spacing for the dynamical core ranges from 0.4° × 0.5° in latitude–longitude over the United States to about 2.5° × 3.5° at the antipode, and the physical package is solved on an intermediate 1° × 1° uniform grid. A diagnostic analysis of the monsoon's onset reveals the development of a positive potential temperature (θ) anomaly at the surface that favors a lower-level cyclonic circulation, while a negative potential vorticity (PV) anomaly below the tropopause induces an upper-level anticyclonic circulation. Ignoring diabatic effects, this pattern is consistent with the superimposition of idealized PV and θ anomalies as previously discussed in the literature. The inclusion of the smaller-scale features of the core monsoon in the model simulation helps represent the continental out-of-phase relationship between the monsoon and the southern Great Plains precipitation, giving additional support to earlier results that highlight the strong nature of the link. A pattern of increased precipitation over the core monsoon is consistently associated with increases of moisture flux convergence and ascending motions, and the development of upper-level wind divergence. On the other hand, the southern Great Plains have a simultaneous decrease of precipitation associated with a change from convergence to divergence of moisture flux, decreased ascending motions, and a development of upper-level wind convergence.

The Gulf of California low-level jet (LLJ) was inspected with a multitaper method spectral analysis, showing significant peaks for both the diurnal cycle and synoptic-scale modes, the latter resulting from the recurrent passage of Gulf surges. Those modes were then separated with a singular spectrum analysis decomposition. Compared with the Great Plains LLJ, the Gulf of California LLJ has a weaker diurnal cycle amplitude and a smaller ratio of diurnal cycle to synoptic-scale amplitudes. Additionally, the 1993 southwestern U.S. monsoon was analyzed by constructing composites of surge and no-surge cases. Given the particular characteristics of 1993 that include the effect of Hurricane Hilary, the extension of these results to other years needs to be assessed. Surges are associated with a strong Gulf of California LLJ and increased moisture flux from the Gulf into Arizona, and they accounted for 80%–100% of the simulated precipitation over Arizona, western New Mexico, and southern Utah. As distance from the Gulf is increased, there is a rapid decay of this percentage so that northern Utah and eastern New Mexico precipitation is almost unrelated to the surges. The results from this research show that the model's regional downscaling results in a realistic representation of the monsoon-related circulations at multiple scales.

Corresponding author address: Ernesto Hugo Berbery, Department of Meteorology, 3427 Computer and Space Sciences Building, University of Maryland, College Park, MD 20742-2425. Email: berbery@atmos.umd.edu

Abstract

The onset and evolution of the North American monsoon system during the summer of 1993 were examined from regional to large scales using the National Aeronautics and Space Administration (NASA) Goddard Earth Observing System (GEOS) stretched-grid GCM. The model's grid spacing for the dynamical core ranges from 0.4° × 0.5° in latitude–longitude over the United States to about 2.5° × 3.5° at the antipode, and the physical package is solved on an intermediate 1° × 1° uniform grid. A diagnostic analysis of the monsoon's onset reveals the development of a positive potential temperature (θ) anomaly at the surface that favors a lower-level cyclonic circulation, while a negative potential vorticity (PV) anomaly below the tropopause induces an upper-level anticyclonic circulation. Ignoring diabatic effects, this pattern is consistent with the superimposition of idealized PV and θ anomalies as previously discussed in the literature. The inclusion of the smaller-scale features of the core monsoon in the model simulation helps represent the continental out-of-phase relationship between the monsoon and the southern Great Plains precipitation, giving additional support to earlier results that highlight the strong nature of the link. A pattern of increased precipitation over the core monsoon is consistently associated with increases of moisture flux convergence and ascending motions, and the development of upper-level wind divergence. On the other hand, the southern Great Plains have a simultaneous decrease of precipitation associated with a change from convergence to divergence of moisture flux, decreased ascending motions, and a development of upper-level wind convergence.

The Gulf of California low-level jet (LLJ) was inspected with a multitaper method spectral analysis, showing significant peaks for both the diurnal cycle and synoptic-scale modes, the latter resulting from the recurrent passage of Gulf surges. Those modes were then separated with a singular spectrum analysis decomposition. Compared with the Great Plains LLJ, the Gulf of California LLJ has a weaker diurnal cycle amplitude and a smaller ratio of diurnal cycle to synoptic-scale amplitudes. Additionally, the 1993 southwestern U.S. monsoon was analyzed by constructing composites of surge and no-surge cases. Given the particular characteristics of 1993 that include the effect of Hurricane Hilary, the extension of these results to other years needs to be assessed. Surges are associated with a strong Gulf of California LLJ and increased moisture flux from the Gulf into Arizona, and they accounted for 80%–100% of the simulated precipitation over Arizona, western New Mexico, and southern Utah. As distance from the Gulf is increased, there is a rapid decay of this percentage so that northern Utah and eastern New Mexico precipitation is almost unrelated to the surges. The results from this research show that the model's regional downscaling results in a realistic representation of the monsoon-related circulations at multiple scales.

Corresponding author address: Ernesto Hugo Berbery, Department of Meteorology, 3427 Computer and Space Sciences Building, University of Maryland, College Park, MD 20742-2425. Email: berbery@atmos.umd.edu

1. Introduction

Monsoonal circulations are the result of seasonally varying differences in the atmospheric forcing by the land and the ocean at low latitudes. While the Asian–Australian monsoon is the most conspicuous of such systems, other regions in North and South America and Africa show similar warm season circulations (Trenberth et al. 2000; Rodwell and Hoskins 2001; Qian et al. 2002). The increasing interest in understanding the North American monsoon system (NAMS) has led to many studies that can be broadly grouped according to the spatial scales that are being addressed.

The core region of the North American monsoon has mesoscale circulations that have been discussed using observational datasets (e.g., Douglas et al. 1993), numerical simulations (e.g., Stensrud et al. 1995), and regional analysis diagnostics (Berbery 2001). These articles and others highlighted the importance of the Gulf of California, its sea surface temperature (SST) fields significantly different from nearby Pacific Ocean SSTs, and the presence of a low-level jet (LLJ) that flows northward along the Gulf. The mesoscale circulations and the diurnal cycle are crucial for describing the moisture transports associated with the monsoonal precipitation: typically, increased moisture resulting from evaporation over the Gulf of California is transported toward the western slopes of the Sierra Madre Occidental by an afternoon sea breeze favored by the sloping terrain; then, the convergence of moisture flux that occurs over the mountain slopes is followed by heavy precipitation (Douglas et al. 1993; Stensrud et al. 1995; Berbery 2001). During nighttime and early morning, the circulation reverses and clouds and rain develop over the Gulf (Negri et al. 1993; Berbery 2001).

The continental-scale features of the monsoonal precipitation depict an out-of-phase relationship between northwestern Mexico and the Great Plains (Douglas and Englehart 1996); farther east, precipitation becomes again in phase with the monsoon (Higgins et al. 1997, 1999). The evolution of the out-of-phase precipitation pattern was also found in other dynamical components, and particularly, in the associated tropospheric circulation and diabatic heating (Barlow et al. 1998). This latter research stressed that the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) and European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis products have a good agreement in the rotational flow associated with the monsoon, but less reliability was found in the divergent flow and derived quantities, like diabatic heating or moisture flux convergence. One reason is that global analyses, even at a grid spacing of 1.125°, do not resolve local terrain features (Schmitz and Mullen 1996). Hence, they can miss the structure and diurnal march of the moisture flux convergence fields over the Gulf of California and slopes of the Sierra Madre Occidental, which are essential for the correct monsoon representation (Berbery 2001).

The typical dilemma in regional climate studies is whether to use global model products with their current low resolutions, or use regional limited-area models with higher resolution but subject to boundary conditions that might sometimes prevent proper interactions with the larger scales. In general, it is desirable to avoid lateral boundary condition problems while still maintaining a grid spacing that resolves the mesoscale features. Variable resolution global models with fine resolution over an area of interest provide a unique opportunity for studying regional events while preserving interactions with the larger/global-scale phenomena (e.g., Paegle 1989; Deque and Piedelievre 1995; Fox-Rabinovitz et al. 1997). Nevertheless, those interactions will be reliable as long as the stretching factors to control computational dispersion effects, and thus to provide a monotonic solution, are limited in magnitude (Vichnevetsky 1987; Fox-Rabinovitz 1988; Côté et al. 1993).

The purpose of this study is to examine the North American monsoon system circulations from regional to continental scales by using a stretched-grid variable-resolution GCM, and in doing so to evaluate its capability to simulate the system at different relevant scales. Particular attention is given to the monsoon's onset and evolution, including the regional circulations over the Gulf of California, and their links to the Great Plains precipitation. Specifically, our interest in the mesoscales is to investigate the role of moisture surges on the southwestern United States precipitation. On continental scales, it is to explore further the out-of-phase relationship between the core monsoon and the southern Great Plains. Although of great interest, this article will not discuss the effect of the continental and oceanic boundary conditions on the monsoon. The model and experimental configurations are discussed in section 2. The onset and evolution of the regional components of the monsoon system are addressed in section 3, while in section 4 the mesoscale circulations and the LLJ over the Gulf of California are investigated. A summary and conclusions are presented in section 5.

2. Model and data products

a. The uniform grid model

The stretched-grid general circulation model employed in this study is based on the National Aeronautics and Space Administration (NASA) Goddard Earth Observing System (GEOS) general circulation model, known as GEOS GCM. The model and the physics parameterization have been described extensively in Takacs et al. (1994), and only a brief description is given here. The model's momentum equations are solved with a fourth-order energy and potential enstrophy conserving scheme (Arakawa and Lamb 1981), while the thermodynamic and moisture equations are written in flux form to ensure potential temperature and moisture conservation. The vertical differencing scheme is described in Arakawa and Suarez (1983). A complete description of the numerical scheme can be found in Suarez and Takacs (1995). The relaxed Arakawa–Schubert cumulus convective parameterization and the reevaporation of falling rain are based upon the work of Moorthi and Suarez (1992) and Sud and Molod (1988) respectively; the longwave and shortwave radiation are parameterized following Chou and Suarez (1994); the planetary boundary layer and the upper-level turbulence parameterizations are based on a level-2.5 closure model (Helfand and Labraga 1988); last, the orographic gravity wave drag parameterization follows Zhou et al. (1996). The uniform grid model is the basis of the data assimilation system known as GEOS DAS (Schubert et al. 1993; Pfaendtner et al. 1994), and is also used for seasonal simulations (Schubert et al. 2002).

b. Configuration of the stretched-grid version

The stretched-grid design is similar to that introduced by Staniforth and Mitchell (1978); here, the area of interest is a spherical rectangle covering approximately 20°–50°N and 126°–72°W, where the grid has a uniform fine resolution (Fig. 1a). This region covers the United States, central and northern Mexico, southern Canada, and part of the nearby ocean areas. Figures 1b,c present close-ups over North America and Mexico. The grid intervals outside the area of interest stretch as a geometric progression with a constant factor. A stretching factor limited to 1.05–1.10 values, and a filter adjusted to variable resolution, are used to reliably control computational problems that may emerge due to grid irregularity (Côté et al. 1993; Fox-Rabinovitz 1988; Fox-Rabinovitz et al. 1997; Takacs et al. 1999).

The stretched-grid version of NASA's GEOS GCM (hereafter called SG GCM for brevity) has a stretched-grid dynamical core with realistic orography as described by Fox-Rabinovitz et al. (1997, 2000). To avoid potential complications of treating the model physical parameterizations on a variable-resolution grid, they are computed on an intermediate uniform-resolution grid. Such an approach is justified by the assumption that the model physics and dynamics can be treated at different temporal and spatial resolution (see Lander and Hoskins 1997). Fox-Rabinovitz et al. (2001) found that following this approach the model physics captures the finer-scale patterns produced by the model dynamics on the stretched grid.

The orographic forcing is applied directly on the stretched grid as an integral part of the model dynamics (Fox-Rabinovitz et al. 2000). As a result, the regional fine-resolution orographic forcing and its gradients have a significant impact and can lead to better-resolved regional mesoscale climate features. Orography is calculated directly on the stretched grid by averaging, within each grid box, the U.S. Navy ⅙° × ⅙° surface elevation dataset available from NCAR. NCEP's weekly analyses of SST, snow, and sea ice distributions, and monthly analyses of soil moisture are used for the surface boundary forcing. The SST fields at either 2° × 2.5° or 1° × 1° grid spacing are used depending on the model configuration. Other surface forcing components (the snow, sea ice, and soil moisture distributions) are available only at 2° × 2° grid spacing and are interpolated onto a 2° × 2.5° or 1° × 1° grid for calculating the model physics, also depending on the stretched-grid version.

A stretched-grid data assimilation system, called SG DAS, was developed from the uniform grid GEOS DAS system (Fox-Rabinovitz 2000). In this case, the SG GCM provides the first guess or 6-h forecast fields. Analyses are then produced using an analysis algorithm called Physical-space Statistical Analysis System (PSAS). It is based on the concept of minimizing the variance of analysis error through an appropriate combination of observation and background fields (e.g., Cohn et al. 1998). Initial imbalance problems are controlled using the incremental analysis update described by Bloom et al. (1996). At this stage, the SG GCM within the SG DAS is combined with the uniform 2° × 2.5° or 1° × 1° grid spacing PSAS (depending on the system's version) to avoid complications related to variable-resolution aspects of the statistical models employed by the system. A detailed discussion can be found in Fox-Rabinovitz (2000).

c. Experimental setup

This study uses the simulation results of a special mode of integration introduced and discussed by Fox-Rabinovitz (2000) to examine the 1988 drought and the 1993 summer floods. The integration mode was designed for participation in the Project to Intercompare Regional Climate Simulations (PIRCS; Takle et al. 1999). The nested grid models participating in PIRCS are driven by lateral boundary condition forcing obtained from the NCEP global reanalyses. The SG GCM special mode of integration is used to imitate the nested grid framework: the SG DAS is run withholding all observational data within the area of enhanced regional resolution, while outside SG DAS analyses are still calculated ingesting all available global observational data. Consequently, within the region of enhanced resolution, where no data are assimilated, the SG GCM is run continuously producing first guesses or 6-h forecasts with initial conditions not affected by regional data. No reinitialization within the region of interest is used for the special mode of integration, but the information obtained from analyses outside the region can flow freely to the area of enhanced resolution during the SG GCM simulation.

Here, the same simulation for 1993 employed by Fox-Rabinovitz (2000) is used to inspect aspects of the North American monsoon. This year was not a typical monsoon year, in the sense that it was one of the latest onsets registered over Arizona and New Mexico (Higgins and Shi 2000). However, the onset over the core monsoon was delayed by 10 days only; in addition, inspection of another year (1988) with an early monsoon onset showed that the features discussed in this paper are not unique to 1993.

The SG GCM experiments for 1993 are started from initial conditions at 1200 UTC 15 May and continued through 31 August. (Because of the ending time, the demise of the monsoon will not be discussed.) The initial conditions were obtained from the SG DAS runs that started 2 weeks earlier, on 1 May, to avoid any initial spinup effects. The dynamical fields are stored at 6-h intervals, while physical parameters like precipitation are available at 3-h intervals.

Two different model configurations of the stretched-grid model were employed in this study. The first one consists of a redistribution of 360 × 181 grid points that results in a regional grid spacing of 0.4° × 0.5° in latitude–longitude, or about 40 km, over the United States and northern Mexico. For this grid the local stretching factors are moderate, of about 3% and 2%, so that the maximum grid intervals at the antipode are about ∼2.5° and ∼3.5° for latitude and longitude, respectively. The physical parameterizations were computed on an intermediate uniform 1° × 1° grid. There are 48 sigma levels in the vertical, of which 11 lie below 700 hPa and 21 below 100 hPa. Most of this article will focus on this model setup, which will be called SGGCM-HR (HR referring to higher resolution).

In the second configuration, the stretched-grid results from the redistribution of 144 × 91 grid points, with a regional spacing of 0.65° × 0.8125° in latitude–longitude, or approximately 60–70 km, over the United States and northern Mexico. The corresponding local stretching factors are ∼5% and ∼7%; latitude–longitude grid intervals at the antipode are about 4.5° and 5.5°. In this case, the physical parameterizations were computed on a 2° × 2.5° grid. The model has 70 sigma levels, of which 14 are found below 700 hPa and 30 below 100 hPa. This model setup will be called SGGCM-LR (LR meaning lower resolution).

d. Observations

The model-simulated products are complemented with two datasets of observed precipitation prepared at NCEP's Climate Prediction Center (CPC); the first one consists of daily rain gauge precipitation interpolated to a 0.25° × 0.25° grid that covers the United States. The dataset and methodology employed to develop it are explained by Higgins et al. (2000). The second is a dataset of Mexican rain gauge daily precipitation interpolated to a 1° × 1° grid. The typical distribution of stations on a given day used to create the gridded datasets is presented in Fig. 1c. The United States has reasonable coverage, but Mexico has large data-void regions, in particular over that country's complex terrain. The North American Monsoon Experiment (NAME) will study the sources and limits of predictability of warm season precipitation; in particular its field campaign during 2004 will seek to fill in the data-void regions of the Sierra Madre Occidental to help improve the model simulation of the monsoon. (Further information about NAME can be found online at http://www.joss.ucar.edu/name/.)

During the summer of 1993, special radiosonde measurements were taken as part of the Southwest Area Monsoon Project (SWAMP-93) field experiment. A simultaneous field experiment over northwestern Mexico also contributed upper-air observations, and results from these field campaigns were discussed by Douglas and Li (1996). This dataset was not employed directly in our study, but it was used in others, like Douglas and Li (1996), that we used for verifying the simulations.

3. Onset and maintenance of the monsoon

a. June to July changes

The year 1993 was characterized by record-setting floods in the northern Great Plains, and, consequently, much effort was devoted to understand the mechanisms that forced the anomalously large precipitation observed during that summer. Research examined both local feedbacks through evaporation and soil moisture anomalies (e.g., Paegle et al. 1996; Bosilovich and Sun 1999; Xue et al. 2001), and remote effects acting through dynamical mechanisms that manifest as teleconnection patterns (Trenberth and Guillemot 1996; Liu et al. 1998). The focus of this study is on the monsoon system during 1993, which had many features typically found in the climatology (despite the delay in its onset). Hence, its analysis should provide information that is useful for other years as well.

Figure 2a depicts the June to July changes in the observed precipitation field as a measure of the changes that follow the onset of the monsoon. Increased precipitation is found over the slopes of the Sierra Madre Occidental in northwestern Mexico and southwestern United States, but also over the northern Great Plains (centered over Kansas and Missouri), and to the north along the border with Canada. A June to July decrease of precipitation is observed over the eastern coast of Mexico and the southern Great Plains (consistent with the results of Higgins et al. 1997 and Barlow et al. 1998). Those studies showed that a climatological negative precipitation anomaly band extends from the Great Plains to the north, and then to the northwest surrounding the area of the monsoon precipitation. In the particular case of 1993, the band is also present, but it is interrupted by a large increase of precipitation over Kansas. [Note, from Higgins et al. (1997), that Kansas is the region where the climatological band of decreased precipitation is weakest.]

The June to July precipitation change in the SGGCM-HR simulation (Fig. 2b) has most aspects in common with the observations, like the large increase of precipitation over northwestern Mexico and the northern Great Plains, and the reduced precipitation over the southern Great Plains. Even smaller-magnitude changes, such as those over the coastal areas of the Gulf of Mexico, northern U.S. tier, and western region, are all reproduced. However, the core monsoon precipitation in northwest Mexico is more localized and intense. This does not necessarily means an incorrectly simulated feature, since the sparsity of stations over Mexico may have led to a spatially smoother pattern (Fig. 2a).

The pattern of higher-intensity precipitation concentrated on the slopes of the Sierra Madre Occidental is typical of higher-resolution satellite estimates and model simulations; the same structure is shown in Berbery (2001) for the Eta Model and Gochis et al. (2002) for the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5). Satellite estimates at 0.5° × 0.5° grid spacing (Negri et al. 1993; Berbery 2001) are also closer in structure and intensity to the regional model simulations than to the rain gauge observations. On the other hand, the observations were interpolated from a data-sparse rain gauge network to a uniform 1° × 1° grid at CPC using a Cressman approach that very possibly smoothed the magnitude of the maxima while enlarging the area covered by the precipitation. It is expected that measurements from NAME's field campaign to take place during the summer of 2004 will help better identify the structure and intensity of the core monsoon precipitation patterns.

The positive band over the northern United States that extends farther north and east over Canadian territory in the model simulation (Fig. 2b) cannot be assessed due to the absence of observations over Canada in this dataset, but is supported by coarser satellite precipitation estimates (not shown). Other minor differences between the model and observed precipitation are found: first, the model precipitation does not extend into southern Arizona as in the observations because it did not reproduce the isolated precipitation event observed in mid-July (see Figs. 4c and 5c). Second, a weak increase of precipitation is noticed both over the northern tip of Texas and near the southwest of the Great Lakes, where observations show a slight decrease in precipitation.

Barlow et al. (1998) described the evolution of the monsoon in dynamical and thermodynamical terms, finding consistent patterns of change in circulation and diabatic heating; nevertheless, the grid spacing of the global reanalyses prevented a more detailed diagnosis at regional scales. In particular, their Fig. 8 depicted the moisture flux convergence field as estimated from NCEP–NCAR and ECMWF reanalyses. Both fields showed convergence over the monsoon region and divergence to the west, over the Pacific Ocean. However, the two had appreciable differences and failed to reproduce the smaller features near the monsoon region (discussed by Berbery 2001). Figure 3 presents the moisture flux convergence for July 1993 and the June to July changes estimated from the SGGCM-HR simulation. (The region covered in the figure is larger than in Fig. 2 to allow comparison to Barlow et al.'s Fig. 8.) During July (Fig. 3a), convergence of moisture flux spreads along the intertropical convergence zone (ITCZ) and achieves the largest magnitude over the coastal areas and mountains of Colombia. Large values are also found along the west and east coasts of Mexico, as did the ECMWF field but not the NCEP–NCAR field in Barlow et al. (1998). The magnitude is larger in the higher-resolution estimate, as is typical in cases where the resolved scales get closer to the mesoscales. The June to July changes in moisture flux convergence (Fig. 3b) are consistent with the corresponding changes in precipitation discussed from Fig. 2b: there is a large increase in the convergence over the core monsoon region and the northern Great Plains, while divergence increases over the southern Great Plains and eastern-to-southern Mexico and Central America. The latter may be associated with the midsummer break of the monsoon documented by Magaña et al. (1999).

b. The time evolution

Four regions were chosen for further study: (a) northwestern Mexico (24°–30°N, 112–106°W, also called core monsoon region); (b) the southern Great Plains (28°–37°N, 102°–90°W); (c) the southwestern United States (32°–37°N, 115°–108°W); and (d) the northern Great Plains, where the floods of 1993 took place (37°–44°N, 100°–88°W). Curves in Figs. 4–7 are shown for the temporal resolution of the corresponding dataset (3 h for the model precipitation; 6 h for the model dynamical fields; 1 day for observed precipitation) and for 5-day running means to highlight the more relevant aspects. Most of the discussion will be centered on the time series of the running means.

Time series of area-averaged observed and model-simulated precipitation for each of these regions are presented in Figs. 4 and 5, respectively. The onset of the monsoon over northwestern Mexico occurs at the end of June in observations and simulation (Figs. 4a, 5a). Once it is established, precipitation rates range between 3 and 6 mm day−1. The 1 August peak in observations, if real, may be a one-station event due to a particularly heavy storm. Such feature would not be captured in a simulation with a grid spacing of 40 km. The onset of the monsoon is accompanied by an almost simultaneous decrease of precipitation over the southern Great Plains (Figs. 4b, 5b), which from then on never recovers the magnitude achieved during late spring. Over Arizona the monsoon begins near 1 August (Figs. 4c, 5c), about 1 month after the onset over northwestern Mexico. This difference between the two regions is consistent with that reported by Higgins et al. (1999) for the climatology, only that the whole 1993 onset was delayed by nearly 2–4 weeks. The model precipitation has an overall evolution that resembles the observations, although the magnitude and timing of some monsoon bursts do not match the observations. Over the northern Great Plains the observations and model simulation agree in showing the largest values during July (Figs. 4d, 5d), at the time when the floods took place, with rainfall intensities of about 10–15 mm day−1. The performance of the model over this region was discussed by Fox-Rabinovitz (2000).

Figure 6 presents the area-averaged vertically integrated moisture flux convergence for each of the four previously mentioned regions. It is suggested that the out-of-phase pattern between the monsoon region and the southern Great Plains can be better identified here because the crucial mesoscale circulations over the core monsoon region are taken into account. Over northwestern Mexico, and before the monsoon's onset, moisture flux convergence (Fig. 6a) has alternating positive and negative values (convergence vs divergence). Nevertheless, a steady increase is also present so that during June it becomes convergent at most times, peaking during the monsoon's onset. Afterward, moisture flux convergence prevails until the end of August. Associated with the reduction in precipitation over the southern Great Plains, the convergence of moisture flux (Fig. 6b) has a steady decrease beginning in mid-June; it becomes negative around 1 July, and remains mostly divergent for the rest of the summer. These changes are also captured in the time mean moisture flux convergence before and after the onset, as represented by the horizontal lines: over northwestern Mexico, the mean moisture flux convergence increases from about 1 to about 5 mm day−1, while over the southern Great Plains the change is of about the same magnitude but of the opposite sign, thus having mean divergence once the monsoon is established. [The divergence of moisture flux over the Great Plains during summer is a known feature of the atmospheric hydrological cycle; e.g., see Rasmusson (1968); Berbery and Rasmusson (1999).]

The convergence of moisture flux over Arizona (Fig. 6c) is weaker than that over northwestern Mexico and remains close to zero until almost mid-August. Once the monsoon is established, it has maxima and minima that are consistent with monsoon bursts and breaks (Figs. 4c, 5c). The large moisture flux convergence (Fig. 6d) during June and July over the northern Great Plains corresponds well with the large precipitation, and only becomes divergent at the end of July and beginning of August, when the large precipitation events came to an end.

c. Changes in dynamical variables

The evolution of the 200-hPa wind divergence and 500-hPa vertical velocity (omega) area-averaged for each of the four regions is presented in Fig. 7. According to the continuity equation, divergence in upper levels can be associated with ascending motions and therefore increased precipitation. The consistency between precipitation changes (Figs. 4, 5) and upper-level wind divergence changes (Fig. 7) is remarkable. Over northwestern Mexico (Fig. 7a), upper-level convergence prevails preceding the monsoon, but the sign reverses with its onset, and from then onward it remains divergent. Even the increase of precipitation during the last 2 weeks of August (related to Hurricane Hilary, see section 4c) is associated with a simultaneous increase of upper-level wind divergence. The vertical velocity confirms this evolution as it also shows negative values (ascending motion) at the time of the monsoon's onset and onward. Note that before the onset there are times of ascending motions as well, but other factors like moisture availability (see, e.g., Fig. 6a) may be preventing the production of precipitation.

According to Fig. 7b, much of the wind divergence over the southern Great Plains is positive before the onset of the monsoon; later, it decays and becomes slightly negative (convergence), suggesting that convection may be inhibited by the large-scale descending motions; again, this is consistent with the observed reduction of precipitation. After the monsoon's onset, omega over the southern Great Plains remains slightly negative (ascending motion) much of the time, but its values are smaller in magnitude—and at times the change of the sign implies descending motions. Again, the contrasting behavior of the upper-level divergent circulation and vertical motion fields before and after the monsoon's onset is captured by the corresponding time means in both Figs. 7a and 7b.

The wind divergence time series over Arizona (Fig. 7c) is similar to that over northwestern Mexico, although smaller in magnitude: prior to the monsoon, upper-level wind convergence prevails despite some positive values during July, and it is only after August that the sign of the upper-level divergence remains always positive. The time evolution of omega over Arizona does not show as clear an evolution as over northwestern Mexico, but a trend toward increased ascending motion (negative omega) is noticed after the monsoon's onset. The northern Great Plains have upper-level wind divergence and ascending motions from the beginning of June to almost the end of July (Fig. 7d), occurring simultaneously with the largest precipitation that contributed to the floods.

In order to investigate the dynamical structure of the monsoon, we further analyze the June to July changes representing its onset. A zonal cross section of the potential temperature averaged for the band 25°–30°N is used to describe the thermal structure of the core monsoon region. During July (Fig. 8a) the potential temperature has a positive anomaly in the lower levels of about 5–8 K with respect to neighboring regions, due to the intense heating of the land. However, the June to July potential temperature change actually shows a reduction in the intensity of the anomaly of about 1–2 K over the western slopes of the Sierra Madre Occidental. This is because during July the development of the monsoon clouds and precipitation prevents a further increase of the surface sensible heating. Note that in regions of no precipitation, like the top and eastern parts of the Sierra Madre Occidental, the July potential temperature is still larger than during June. A similar effect is noted over the peninsula of Baja California. A June to July warming in upper levels is observed in association with the monsoon's onset, while above the tropopause the temperature change is again negative. This is the typical structure of an anticyclonic anomaly in upper levels, as discussed in Hoskins et al. (1985).

Potential vorticity (PV) is a variable that condenses much of the dynamical information of a system and its anomalies imply a contribution from the absolute vorticity and from the stability term (Hoskins et al. 1985). It is defined as PV = −g(f + ζθ)/(∂p/∂θ) where g is the gravity, f the Coriolis parameter, ζθ the relative vorticity computed on isentropic surfaces, p the pressure, and θ the potential temperature. The computation requires first a conversion to isentropic coordinates. Diabatic heating effects were discussed by Hoskins (1991) in general, and by Barlow et al. (1998) for the North American monsoon in particular.

Figure 8c presents the cross section of PV in isentropic coordinates (masked-out values in Figs. 8c–f correspond to regions below the ground and a thin layer above it). Lower values of PV over the monsoon region—with respect to neighboring regions—are noted in the upper levels and most prominently at 350 K below the dynamic tropopause, which is defined here as the band of 2–2.5 PV units (PVU). As seen in Fig. 8e, this negative anomaly of PV induces an anticyclonic circulation during July with northward wind in its western sector (∼111°W) and southward wind on its eastern portion (∼102°W). Figure 8e also shows at lower levels near 99°W the upper part of the Great Plains LLJ; according to Rodwell and Hoskins (2001), this LLJ is an essential component to maintain Sverdrup vorticity balance [βυf(∂ω/∂p)] of the North American monsoon system.

The magnitude of the PV anomaly is better represented in the June to July change (Fig. 8d), where a negative maximum of about 0.5 PVU is observed below the tropopause at 350–360 K. Above the tropopause positive PV deviations are noticed. The June to July change in the wind structure (Fig. 8f) is similar to the one for July (Fig. 8d), suggesting that the structure evolved during July. The tropospheric and stratospheric southward wind anomalies toward the west (115°–120°W) correspond to the easternmost boundary of the subtropical anticyclone over the Pacific Ocean (not discussed here). In summary, the structure described here is similar to a combination of the idealized patterns resulting from an upper-level negative PV anomaly and a positive potential temperature anomaly at the surface, as discussed by Hoskins et al. (1985).

4. Mesoscale circulations over the Gulf of California

a. The low-level jet

The core monsoon region includes the Gulf of California and the Sierra Madre Occidental, where circulation and precipitation have distinctive mesoscale features that are usually not properly represented by the typical resolutions of global models. This is exemplified in Fig. 9, which presents the moisture flux at 925 hPa for August 1993 estimated from the NCEP–NCAR global reanalyses, and the two stretched-grid model configurations described here (SGGCM-LR and SGGCM-HR).

Figure 9a shows that the global reanalyses represent the southern Great Plains LLJ; but do not resolve the circulations over the Gulf of California; therefore, the regional moisture flux appears as weak and undistinguishable from the large-scale flux. Figure 9b presents the same field but now computed from the SGGCM-LR simulation. Recall that the physics of this configuration is computed on a 2° × 2.5° latitude–longitude grid, thus similar to the global reanalyses. However, the dynamical fields are computed on the variable grid, which at this latitude translates to a grid spacing of about 60–70 km. In this case, southerly moisture flux is found along the western coast of Mexico, and also inland over the slopes of the Sierra Madre Occidental at ∼27°N. Still, no distinct flux is found over the northern part of the Gulf of California and the southwestern United States.

When the SGGCM-HR configuration (1° × 1° lat–lon for the model physics and about 0.4° × 0.5° lat–lon of grid spacing for the dynamical core) is used, the resulting moisture flux has a well-defined direction along the Gulf of California (Fig. 9c). This is consistent with the observational study of Douglas et al. (1998). Part of the flux spans toward the slopes of the Sierra Madre Occidental, but another portion continues to the north into Arizona, where it can supply moisture for the development of the local monsoonal precipitation (Douglas et al. 1993; see also the review article by Adams and Comrie 1997). These remarkable differences among the estimates at different grid spacing highlight the need for higher resolutions than those of typical reanalyses or GCMs in studies of the North American monsoon.

To further address this issue, Fig. 10 presents cross sections of the northward moisture flux across the Gulf of California at 25°N and 30°N. At 25°N (Fig. 10a) the SGGCM-LR shows a relatively shallow layer of northward moisture flux over and around the Gulf of California and southward flux over the Pacific Ocean. The latter is associated with the eastern boundary of the subtropical anticyclone. The 25°N cross section from the SGGCM-HR (Fig. 10b) suggests a deeper structure, up to 600 hPa. Note the lateral shift toward the east that is more in concordance with other studies, like Stensrud et al. (1995) and Berbery (2001). The more remarkable differences between the two resolutions are found on the northern part of the Gulf of California, at 30°N. At the lower resolution (Fig. 10c) there is no LLJ structure over the Gulf, but the higher-resolution simulation (Fig. 10d) depicts a well-defined one. Interestingly, the differences between the two model simulations are not restricted to the lower levels: the SGGCM-HR cross section over the Gulf at 30°N shows a secondary northward maximum at 700 hPa that is consistent with observations and regional analyses (Rasmusson 1967; Berbery 2001). On the other hand, the lower-resolution simulation depicts moisture flux of the opposite direction. The reason is that the northward winds and fluxes up to 600 hPa do not extend as far north at the lower resolution as in the higher resolution, and, because of this limited extent, a southward tongue can develop. This is hinted from the cross sections at 25°N, but is confirmed by the analysis of the wind field at 700 hPa (not shown).

b. The diurnal cycle

The moisture fluxes that are perpendicular to the Gulf of California's eastern coastline have a large-amplitude diurnal cycle due to a strong sea breeze further enhanced by the topography of the Sierra Madre Occidental (as stressed earlier and discussed in Berbery 2001). This circulation is mostly in the zonal direction, and does not resemble a LLJ. In contrast, the wind along the Gulf is known to have a LLJ structure and a diurnal cycle with maximum intensity during nighttime (Douglas 1995). The associated moisture transport in the northern Gulf is mostly northward and has been linked to precipitation over Arizona and neighboring regions. Therefore, the Great Plains and the Gulf of California have in common a northward LLJ that supplies moisture to higher latitudes. On the other hand, Fig. 10b shows that the maximum flux of the Gulf of California LLJ tends to be near the surface, unlike the Great Plains LLJ whose core is elevated from the surface (see, e.g., Figs. 12c,d). Because of the overall differences in the lower boundaries (land vs water; vegetation, topography, etc.), it can be expected that the two jets will have other dissimilar characteristics. Some of them, as seen in the SGGCM-HR meridional moisture flux during the 1993 summer season, will be discussed here.

Figures 11a,b present the time series of meridional moisture flux at the core of each jet with the corresponding time mean removed. At the Great Plains LLJ core (30°N, 101°W; 875–925 hPa) the everyday presence of the diurnal cycle can be noticed (Fig. 11a), which agrees with earlier studies (see, e.g., Schubert et al. 1998). On the other hand, the moisture flux at the Gulf of California LLJ (30°N, 113.5°W; 875–925 hPa) is weaker and its diurnal cycle, although present, is not as marked (Fig. 11b). (The layer 875–925 hPa is above the core of the Gulf of California LLJ, which is closer to the surface, but we chose it for comparison to the Great Plains. Choosing lower levels does not change the results reported here.)

The differences between the diurnal cycles and synoptic-scale variabilities of the two jets are analyzed with the singular spectrum analysis (SSA) technique (Vautard et al. 1992), which is a valuable method to study the dominant oscillatory modes of variability in short-term series. The time series cover the period 15 May–31 August, which at 6-h intervals correspond to 436 values; because the length of the time series is only 3.5 months, the results are valid for timescales not beyond the synoptic scales. A spectral analysis [multitaper method (MTM)] was performed using the SSA-MTM Toolkit of Dettinger et al. (1995), showing that the diurnal cycle and a band of frequencies representing the synoptic scales have significant spectral peaks (not shown). SSA decomposition of the time series shows that the first nine eigenvalues account for about 88% of the Great Plains LLJ total variance, and for about 82% of the Gulf of California LLJ total variance.

Inspection of the modes for each region reveals that two of them hold the diurnal cycle information. For the Great Plains, these are the second and third modes, while for the Gulf of California they are the eighth and ninth modes. In each jet, the two modes are about the same magnitude, as should correspond to an oscillatory signal (Vautard and Ghil 1989). The two modes representing the diurnal cycle of the Great Plains LLJ account for about 25% of the total variance, while those representing the diurnal cycle of the Gulf of California LLJ account for about 5% of the corresponding total variance. Figures 11c,d depict the eigenvectors representing the diurnal cycle, and the reconstructed time series without the diurnal cycle (i.e., only the modes representing the synoptic-scale variability). First to be noticed is that the SSA technique successfully separates the two bands, but several other features are evident as well. Not only large differences between the diurnal cycle amplitudes of both LLJs are found, but also their ratio with respect to the synoptic-scale variability is different. In the Great Plains' case, the diurnal cycle has an amplitude that is about 76% of the amplitude of all the other modes under consideration; meanwhile, the same ratio over the Gulf of California indicates that the amplitude of the diurnal cycle is only about 25% of that of the remaining combined modes. [The following is not shown for brevity: A similar analysis at other latitudes along the Gulf of California reveals the same ratio as in the north. In addition, if the technique is applied to the zonal component of the moisture flux at lower levels, it shows the strong effect of the diurnal cycle in that direction, as shown in Berbery (2001).]

c. Moisture surges

The comparatively larger effect of the synoptic-scale modes over the Gulf of California is due to the recurrent passage of surges that supply moisture and instability to the southwestern United States storms (Hales 1972; Brenner 1974; Stensrud et al. 1997). They are of an episodic nature with wind along the Gulf of California developing more prominently during July and August. Surges do not simply reorganize the precipitation patterns; rather, they are a substantial part of the mechanisms favoring the development of monsoon precipitation over Arizona. This section describes the particular effects that surges had during the 1993 season, and they may in part reflect the passage near and over the Gulf of California of Hurricane Hilary (category 3) toward the end of August. At this time the model produced the largest wind magnitude of 30 m s−1 at the south of the Gulf. In general, and consistent with measurements reported in Douglas and Li (1996) for the summer of 1993, and Douglas et al. (1998), typical values decrease toward the northern sector, where winds of 5 m s−1 with peaks of about 10 m s−1 were common.

The relevance of the northward moisture flux associated with surges at the northern end of the Gulf of California is manifested in the composites presented in Fig. 12. The left column presents a composite of the cases when the meridional moisture flux equals or exceeds the mean plus 75% of the standard deviation. The threshold is arbitrary, but composites with larger and smaller thresholds do not differ qualitatively from the ones presented here. In addition, this definition does not take into account persistence or other typical features of the Gulf surges. However, an analysis of the time series of wind and specific humidity suggests that these are indeed surge events. Thus, this case is called surge composite. A climatology of Gulf surges based on observations and global reanalysis is discussed in Douglas and Leal (2003).

The right column is the composite of all the cases that do not comply with the previous condition (no-surge composite). The composites are limited to cases during July and August, since the interest is to see their impact in the southwestern U.S. monsoon. This is consistent with earlier studies (Fuller and Stensrud 2000) that have shown that the number of surges in June is smaller than those during July and August. The differences between the two composites are remarkable. Figure 12a shows that the surge composite has a pattern of large northward moisture flux all along the Gulf of California, not only over the northern sector for which the index was defined. Spreading over the slopes of the Sierra Madre Occidental can also be noticed, and this is the net effect of the sea breeze/mountain slope effect discussed in Berbery (2001).

The moisture fluxes associated with the surge cases along the northern Gulf have a LLJ structure (Fig. 12c) whose magnitude is more than double the mean value (Fig. 10b). More important, these surges are associated with about 80%–100% of the total precipitation over Arizona, southern Utah, and western New Mexico (Fig. 12e). The percentage of precipitation associated with the surges decays rapidly away from these areas to about 20%, and as distance is further increased, the partition settles at about 50%–50% probably reflecting the lack of significance of the composites; therefore, it is expected that only very large or very small values will have physical meaning.

In contrast, the no-surge moisture flux composite (Fig. 12b) does not show northward moisture flux over the northern sector of the Gulf of California, and therefore the supply of moisture to the southwestern United States is not discernible. In our composite, the corresponding LLJ-like vertical structure of the moisture fluxes is also absent, as suggested in Fig. 12d. However, in the southern part of the Gulf the direction is like that in the surge case, including the spreading over the slopes of the Sierra Madre Occidental. The no-surge cases represent not more than 0–20% of the total precipitation over Arizona, southern Utah, and New Mexico (Fig. 12f). The large percentages detected over other regions, noticeably over eastern New Mexico, suggest that precipitation processes unrelated to Gulf surges are acting. Notice that the Great Plains LLJ in the no-surge composite is stronger than in the surge composite, suggesting that the out-of-phase relationship discussed in section 3 might also involve the intensity of the LLJs. The results reported here are based on the summer of 1993; therefore this subject remains open to research.

5. Summary and conclusions

The purpose of this study has been to assess a stretched-grid GCM's capabilities for studies of the North American monsoon system, and to provide further insight into key components of the monsoon. Two different model configurations were tested: (a) the SGGCM-HR, with a grid spacing of 0.4° × 0.5° latitude–longitude over the United States for the dynamical core, and a uniform 1° × 1° grid for the model physics, and (b) the SGGCM-LR, with a grid spacing of 0.65° × 0.8125° for the dynamical core and 2° × 2.5° for the model physics. The higher-resolution configuration was found to handle a wide range of spatial scales associated with the monsoon system from mesoscale features, such as the Gulf of California LLJ, to larger-scale characteristics like the simultaneous onset of the monsoon and the decay of precipitation over the Great Plains. The lower resolution, while still being able to reproduce features on intermediate to large scales, failed to simulate the LLJ over the northern part of the Gulf of California, therefore missing a necessary element associated with the development of the southwestern U.S. precipitation. Grid spacing was changed simultaneously for the dynamical core and the physics; therefore, no attempt was made here to evaluate their relative impacts. There are several issues that deserve the interest of future studies, like the role of topography in triggering the convective precipitation, and the partition between sensible and latent heat fluxes. It is expected that the NAME field campaign will generate datasets that combined with model products will help clarify those issues.

A composite analysis of surge and no-surge cases shows that most of the model-simulated precipitation over the southwestern United States was related to Gulf surges, but this relationship decays rapidly as distance is increased. The surge cases also show a strong influx of moisture into the southwestern United States via the LLJ over the Gulf of California; when the surge cases are removed from the total sample (no-surge cases) the low-level jet structure was not found. The realism of this feature still needs further evaluation, since other numerical studies (Anderson et al. 2000) have suggested that LLJs can be found even under weak synoptic forcing. Our study was based on composites during the summer of 1993, which may have been heavily influenced by the presence of Hurricane Hilary at the end of August. Anderson et al.'s study covers the summers of 1990 and 1995, therefore, it is also possible that interannual variations accounted for the differences.

The continental-scale circulation and its interactions with the core monsoon region were examined. The onset of the monsoon in northwestern Mexico, best identified in the development of the precipitation over the western slopes of the Sierra Madre Occidental, is preceded by large sensible heating over land and lower levels. Nevertheless, once precipitation begins in July the potential temperature anomaly (with respect to neighboring regions), while remaining positive, decays slightly with respect to June. The structure of the monsoon is similar to that resulting from the superimposition of a positive potential temperature anomaly at the surface and a negative anomaly of upper-level potential vorticity. The dynamical nature of the out-of-phase relationship between the monsoon in northwestern Mexico and the southern Great Plains was also analyzed. Barlow et al. (1998) found that the linkages are robust, and here we extend such assessment, as we found a consistent evolution of several other dynamical variables and even in the vertically integrated moisture flux convergence. The onset of the monsoon is accompanied by increased moisture flux convergence, large-scale ascending motions and upper-level divergence. The simultaneous decrease of precipitation over the southern Great Plains reveals the opposite behavior, with a change in the sign of moisture flux convergence (becoming divergent), a reduction in the intensity of the ascending motion (which remains close to zero) and predominant wind convergence in the upper-levels. Finally, the evolution of the same variables over the southwestern United States suggests similar features as those of the core monsoon, although of much smaller magnitude. There are indications that a similar out-of-phase relationship may exist between the Gulf of California and the Great Plains LLJs, but this subject remains open to research.

Acknowledgments

The authors are thankful to Mr. Ravi Govindaraju for providing programming support, to Drs. Wayne Higgins, Art Douglas, and Wei Shi for supplying the Mexican rainfall dataset, and to Dr. Andrew Robertson for his comments. The suggestions and clarifications by two anonymous reviewers are also appreciated. This research was supported by NASA Grant NAG511577, NOAA Grant NA76GP0291 (GCIP), and NSF Grant ATM0105839.

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Fig. 1.
Fig. 1.

(a) Variable grid employed for the model simulations; (b), (c) close-ups over North America and Mexico. Heavier dots in (c) represent the typical rain gauge distribution on a given day

Citation: Journal of Climate 16, 12; 10.1175/1520-0442(2003)016<1929:MDOTNA>2.0.CO;2

Fig. 2.
Fig. 2.

Onset of the North American monsoon as represented by the Jun to Jul changes in (a) observed precipitation and (b) SGGCM-HR simulated precipitation. Units are mm day−1

Citation: Journal of Climate 16, 12; 10.1175/1520-0442(2003)016<1929:MDOTNA>2.0.CO;2

Fig. 3.
Fig. 3.

Vertically integrated moisture flux convergence for (a) Jul and (b) the Jun to Jul change

Citation: Journal of Climate 16, 12; 10.1175/1520-0442(2003)016<1929:MDOTNA>2.0.CO;2

Fig. 4.
Fig. 4.

Time series of area-averaged observed daily precipitation for the four main regions indicated in the corresponding insets: (a) northwestern Mexico, (b) southern Great Plains, (c) Arizona, and (d) northern Great Plains. The heavy line is a 5-day running average, and the shaded bars in (a) and (b) denote the onset of the monsoon in northwest Mexico. Units are mm day−1

Citation: Journal of Climate 16, 12; 10.1175/1520-0442(2003)016<1929:MDOTNA>2.0.CO;2

Fig. 5.
Fig. 5.

As in Fig. 4 for the 3-h SGGCM-HR simulated precipitation. Units are mm day−1

Citation: Journal of Climate 16, 12; 10.1175/1520-0442(2003)016<1929:MDOTNA>2.0.CO;2

Fig. 6.
Fig. 6.

As in Fig. 5 for the vertically integrated moisture flux convergence. Units are mm day−1. The horizontal lines represent the time mean for the period defined by the length of the line.

Citation: Journal of Climate 16, 12; 10.1175/1520-0442(2003)016<1929:MDOTNA>2.0.CO;2

Fig. 7.
Fig. 7.

As in Fig. 6 for the wind divergence at 200 hPa. The running average of vertical velocity (omega) at 500 hPa has been added with dashed heavy line. Divergence has been multiplied by 106 and its units are s−1. Vertical velocity units are Pa s−1.

Citation: Journal of Climate 16, 12; 10.1175/1520-0442(2003)016<1929:MDOTNA>2.0.CO;2

Fig. 8.
Fig. 8.

Longitude–pressure section of potential temperature averaged for the 25°–30°N band for (a) Jul and (b) the Jun to Jul change. Longitude–potential temperature cross section of potential vorticity for (c) Jul and (d) the Jun to Jul change. (e), (f) As in (c), (d) for the meridional wind. The gray band in (c)–(f) represents the dynamic tropopause (PV = 2–2.5 PVU), and values below ground were masked out. All sections are averaged over the same latitude band

Citation: Journal of Climate 16, 12; 10.1175/1520-0442(2003)016<1929:MDOTNA>2.0.CO;2

Fig. 9.
Fig. 9.

Moisture flux at 925 hPa for Aug 1993, as estimated from (a) NCEP–NCAR global reanalyses, (b) the SGGCM-LR simulation, and (c) the SGGCM-HR simulation. Units are g kg−1 m s−1

Citation: Journal of Climate 16, 12; 10.1175/1520-0442(2003)016<1929:MDOTNA>2.0.CO;2

Fig. 10.
Fig. 10.

Gulf of California cross section of the meridional moisture flux at 25° and 30°N estimated from the two model configurations: (a) SGGCM-LR at 25°N, (b) SGGCM-HR at 25°N, (c) SGGCM-LR at 30°N, and (d) SGGCM-HR at 30°N. Units are g kg−1 m s −1

Citation: Journal of Climate 16, 12; 10.1175/1520-0442(2003)016<1929:MDOTNA>2.0.CO;2

Fig. 11.
Fig. 11.

Time series of meridional moisture flux anomalies for the layer 875–925 hPa for (a) the Great Plains LLJ and (b) for the Gulf of California LLJ. Eigenvalues representing the diurnal cycle and synoptic-scale variability of the meridional moisture flux for (c) the Great Plains LLJ and (d) for the Gulf of California

Citation: Journal of Climate 16, 12; 10.1175/1520-0442(2003)016<1929:MDOTNA>2.0.CO;2

Fig. 12.
Fig. 12.

(a) Surge and (b) no-surge composites of moisture flux at 950 hPa; contours and shades identify the region with largest moisture flux magnitude. (c), (d) Corresponding cross sections of moisture flux at 30°N; (e), (f) percentage of total precipitation for the corresponding composites

Citation: Journal of Climate 16, 12; 10.1175/1520-0442(2003)016<1929:MDOTNA>2.0.CO;2

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