Bulletin of the American Meteorological Society 2197

1. Introduction

a. Aims and rationale

The North American monsoon, variously known as

the Southwest United States monsoon, the Mexican

monsoon, or the Arizona monsoon, is experienced as

a pronounced increase in rainfall from an extremely

dry June to a rainy July over large areas of the southwestern United States and northwestern Mexico. These

summer rains typically last until mid-September when

a drier regime is reestablished over the region. The

North American monsoon (NA monsoon) has been

studied since the early part of this century, and a fairly

extensive research literature has developed that deals

with a range of meteorological and climatological aspects of the phenomenon. Surprisingly, no comprehensive review of the NA monsoon literature has been

presented. Several papers have briefly summarized the

basic climatology of the NA monsoon (e.g., Maddox

et al. 1995), but to date, no single contribution has

sought to integrate the wide range of research relating

to this notable feature of the atmospheric circulation

over North America.

In this paper, we provide a synthesis of major climatological and meteorological research themes regarding the NA monsoon: we outline the past debate

and emerging consensus on mechanisms and moisture

sources; we assess the state of knowledge pertaining

to the extent and variability of the NA monsoon, both

spatially and temporally; and we include an overview

of research on the severe storm environment and modeling efforts associated with the NA monsoon. We also

address two additional issues. First, considering the

The North American Monsoon

David K. Adams and Andrew C. Comrie

Department of Geography and Regional Development, The University of Arizona, Tucson, Arizona

ABSTRACT

The North American monsoon is an important feature of the atmospheric circulation over the continent, with a research literature that dates back almost 100 years. The authors review the wide range of past and current research dealing with the meteorological and climatological aspects of the North American monsoon, highlighting historical development and major research themes. The domain of the North American monsoon is large, extending over much of the

western United States from its region of greatest influence in northwestern Mexico. Regarding the debate over moisture

source regions and water vapor advection into southwestern North America, there is general agreement that the bulk of

monsoon moisture is advected at low levels from the eastern tropical Pacific Ocean and the Gulf of California, while the

Gulf of Mexico may contribute some upper-level moisture (although mixing occurs over the Sierra Madre Occidental).

Surges of low-level moisture from the Gulf of California are a significant part of intraseasonal monsoon variability, and

they are associated with the configuration of upper-level midlatitude troughs and tropical easterly waves at the synoptic

scale, as well as the presence of low-level jets, a thermal low, and associated dynamics (including the important effects

of local topography) at the mesoscale. Seasonally, the gulf surges and the latitudinal position of the midtropospheric

subtropical ridge over southwestern North America appear to be responsible for much spatial and temporal variability in

precipitation. Interannual variability of the North American monsoon system is high, but it is not strongly linked to El

Niño or other common sources of interannual circulation variability. Recent mesoscale field measurements gathered

during the South-West Area Monsoon Project have highlighted the complex nature of the monsoon-related severe storm

environment and associated difficulties in modeling and forecasting.

Corresponding author address: Dr. Andrew C. Comrie, Department of Geography and Regional Development, The University

of Arizona, Tucson, AZ 85721.

E-mail: comrie@climate.geog.arizona.edu

In final form 19 May 1997.

©1997 American Meteorological Society
2198 Vol. 78, No. 10, October 1997

importance of the NA monsoon within the atmospheric circulation over North America, and despite

several recent research papers, misunderstandings of

NA monsoon dynamics and influence are still surprisingly widespread within the broader meteorological

and climatological community. Second, this is an area

of research that has become quite active, particularly

during the last few years with the South-West Area

Monsoon Project (SWAMP), and it offers many theoretical and applied avenues for further investigation.

We intend this paper to be a timely contribution that

will critically summarize the state of knowledge and

stimulate further inquiry on the NA monsoon.

Pulling together the diverse array of NA monsoon

studies into a cohesive body of literature is a challenging task. We have chosen to divide the literature into

several categories, which by nature are somewhat arbitrary and partially overlapping. Many of the articles

reviewed cover a number of these categories, and thus

a particular study may appear in each of several appropriate sections. We review papers found primarily

in the major meteorological and climatological journals, including those that focus principally on the NA

monsoon as well as those less directly connected but

that nonetheless provide useful insights. Also included

are a small number of books and, where necessary, less

easily available scientific reports. Some Spanish-language references have been included for completeness,

but the reader is referred to Reyes et al. (1994) for a

more complete Spanish-language reference list.

b. Climatological setting

We use the term “North American monsoon” in

order to be geographically all-encompassing. It is important to note that most of the NA monsoon literature has a strong geographical bias toward the southwestern United States, especially the state of Arizona,

necessitating frequent mention of these areas in this

review (and in our suggestions for future research directions). We wish to emphasize that this phenomenon

influences larger areas of the southwestern United

States and that, in fact, it is centered over northwestern Mexico. A discussion of areal extent is presented

later in the paper, but we introduce the broader monsoon region here to provide background on important

physiographic characteristics.

Figure 1 highlights the major physical and topographic features of southwestern North America that

relate to the NA monsoon. The region is bounded to

the west by the Pacific Ocean, including the Gulf of

California. Summertime sea surface temperatures

(SSTs) at middle and subtropical latitudes along the

Pacific Coast are mainly cool on average (< 25°C), but

SSTs in the Gulf of California and tropical eastern

Pacific (farther south) are warm (> 28°C). To the east,

the region is bounded by the Gulf of Mexico, with

warm SSTs (> 26°C), and by the central plains of the

United States. The interior of the region is characterized by several large upland areas: the Colorado Plateau extends north and east to the Rocky Mountains

from Arizona’s Mogollon Rim; the north–south

aligned mountain ranges of the basin and range province dominate through Nevada, southwestern Arizona,

and northwestern Sonora; the Mexican Altiplano (plateau) is defined by the Sierra Madre Occidental and

Oriental to the west and east, respectively. In addition,

the peninsular ranges of southern California and Baja

California along with the Sierra Nevada play important climatological roles for the interior deserts, limiting penetration of marine moisture from the Pacific

Ocean. There are also two lowland areas associated

with the NA monsoon: the lower Colorado River valley and neighboring low desert areas, which play a

critical role in the formation of the thermal low; and

the coastal lowlands of Sonora and Sinaloa, between

the Gulf of California and the western flanks of the

Sierra Madre Occidental. Most of the lower elevation

zones of the monsoon region, particularly the Sonoran

Desert, receive up to 80% or 90% of possible summer

FIG. 1. Important physiographic features, states, and place

names mentioned in the text: Empalme (E), Mazatlán (M),

Phoenix (P), and Yuma (Y).
Bulletin of the American Meteorological Society 2199

insolation, and summer surface temperatures can be

extremely hot, often exceeding 40°C.

The combination of seasonally warm land surfaces

in lowlands and elevated areas together with atmospheric moisture supplied by nearby maritime sources

is conducive to the formation of a monsoonlike system. With the exception of Ramage (1971), most authors regard this circulation as a “true” monsoon, based

on seasonal reversal of pressure and wind patterns,

energy and mass transfers, and characteristic regimes

of rainfall and temperature (e.g., Bryson and Lowry

1955a; Krishnamurti 1971; Tang and Reiter 1984;

Douglas et al. 1993). Two points are worth noting with

respect to our coverage of the NA monsoon: first, seasonal wind direction changes over the North American Arctic might technically be considered a monsoon,

but we exclude this feature; second, the term “North

American monsoon” has been applied to a more extensive phenomenon that we do not explicitly cover,

which includes the wet season over Central America

and even the seasonal displacement of the intertropical convergence zone (ITCZ) over the eastern Pacific

Ocean, for example, in the Pan-American Climate

Studies program (PACS 1996).

2. Historical perspective

Historically, research interest in the NA monsoon

has its roots in the peculiar distribution of summer

precipitation in this arid and semiarid region of North

America. Figure 2 illustrates monthly rainfall distributions for various sites across the region, many of

which display the characteristic NA monsoon mid- to

late summer rainfall maximum following extremely

dry conditions in May and June. Early researchers

noted that July and August thunderstorms were abundant in the uplands of Arizona and western New

Mexico, and relatively infrequent to the west in southern California (Campbell 1906; Beals 1922; Blake

1923). However, the origin, frequency, and distribution of this warm season convective activity were not

seen as necessarily linked to some larger-scale regional

control. In fact, it was this curious distribution of warm

season precipitation (nearly absent in southern California and quite frequent in elevated zones of central

Arizona) that spurred much of the original research

that would attempt to clarify the general character of

the summer circulation of the desert Southwest (Reed

1933, 1937, 1939). For example, the infrequent summer thunderstorms of the California deserts, sometimes referred to as “Sonora storms” (Campbell 1906),

were first thought to have their origin in the Mexican

state of Sonora, from where they would travel northwestward to southern California. The proximity of

Sonora to Arizona made this northward movement

plausible for convective storms in Arizona, but the

distance to California and the localized nature of the

storms once in California seemed to negate this possibility (Blake 1923). Another explanation suggested

that the intensification and movement of the semipermanent thermal low pressure over the Colorado River

valley were responsible for widespread rain in Arizona

and California (Beals 1922). Others, too, believed that

a relationship existed between the intense thermal low

and summer thunderstorms in the region (Ward 1917;

Blake 1922; Willett 1940). It was suspected that the

resulting pressure gradient would draw Gulf of California moisture northward, enhancing instability and

increasing convective activity in the mountains and

deserts of the southwestern United States. Remarkably, the notion of the Gulf of California as a moisture source for NA monsoon precipitation would not

appear prominently in the literature again until several

decades later.

FIG. 2. Seasonal distribution of precipitation across southwestern

North America. Note that northwestern Mexico shows the

strongest monsoon signal, which diminishes through Arizona,

New Mexico, and Nevada. Northeastern Mexico and Texas

display early summer–late fall precipitation peaks, while the West

Coast has a dry summer Mediterranean distribution (vertical axis

of all graphs represents 180 mm with 20 mm increments). Areas

south of the broken line receive greater than 50% of their annual

rainfall in July, August, and September (after Douglas et al. 1993).
2200 Vol. 78, No. 10, October 1997

Following the advent of a network of upper-air

soundings over the western United States, Reed (1933)

recognized that the distribution of summer rains in this

region was, in fact, part of a larger-scale phenomenon.

The position of the “high-level anticyclone” east or

west of the Continental Divide, he proposed, dictated

the occurrence of rainfall events by stabilizing (westward position) or destabilizing (easterly position) the

lapse rate. Moreover, he believed advection of lowlevel moisture along the anticyclonic trajectory was

not necessary for destabilization because sufficient

water vapor was presumably present throughout the

region. Widespread convective activity was dependent, supposedly, on upper-level instability alone.

The first comprehensive climatology of the NA

monsoon phenomenon was presented by Bryson and

Lowry (1955a, b). Although these authors were not the

first to recognize the monsoonal nature of the circulation regime (e.g., Ives 1949), the prominence of their

work ensured that the NA monsoon would be recognized primarily as an Arizona summer feature, hence

the later predominance of Arizona in the literature.

Bryson and Lowry (1955a, b) provided the dynamic

explanation for the sudden appearance of the summer

thunderstorms that was lacking in Reed (1933). The

rapid onset of rainfall in the Arizona

summer monsoon was defined as a

“singularity” (the recurrence of an atmospheric phenomenon near a particular calendar date), and it was found

to be related to a sudden shift and readjustment in the midtroposphere

hemispheric circulation. This shift in

circulation, resulting from the westward and northward expansion of the

Bermuda high sometime around the

end of June, causes dry southwesterly

midlevel winds from the Pacific

Ocean to be replaced by moist southeasterly winds (Fig. 3). This sequence

of synoptic patterns places Arizona

under a light southeasterly flow,

which seemingly implied water vapor

advection solely from the Gulf of

Mexico. Consequently, the Gulf of

Mexico was viewed as the unique

moisture source for the NA monsoon—a view that continued to appear

in the literature for many years (Reitan

1957; Jurwitz 1953; Green 1963;

Green and Sellers 1964; Hastings and

Turner 1965) and that became a central theoretical issue in much subsequent research.

3. Moisture source debate

Reitan (1957) and, later, Rasmusson (1967) raised

several important points that seemed to contradict

Bryson and Lowry’s (1955a,b) conclusions. Although

not directly investigating moisture sources, Reitan

(1957)1

 concluded that the greatest amount of precipitable water vapor during the Arizona monsoon is found

below 800 mb. Considering the fact that the topography between Arizona and the Gulf of Mexico rises

above this level, the Gulf of Mexico appeared to be

an unlikely source for such low-level moisture. Similarly, Rasmusson (1967), in examining annual vapor

FIG. 3. Pentads of 500-mb geopotential height (dam) from 1979–88 showing the

transition from southwest flow over the NA monsoon region in early June to east or

southeast flow in July. Note rapid northward shift in height contours during last week

of June and initiation of light easterly flow over northern Mexico (after Bryson and

Lowry 1955b).

1

Several citations of this source have been misleading. Reitan

(1957) did not suggest the Gulf of California as an important

source of low-level moisture during the summer in Arizona. He,

in fact, reiterates the findings of Bryson and Lowry (1955a) that

the moisture comes from the Gulf of Mexico. After examining his

conclusions, however, other authors raised the question of how

Gulf of Mexico moisture could be advected over the Continental

Divide and still provide low-level moisture in Arizona.
Bulletin of the American Meteorological Society 2201

flux in North America for a 2-yr period, discovered a

strong northward vapor flux into the southwestern

United States from the Gulf of California and a much

weaker inflow from east of the Rocky Mountains.

Taking these observations into account, the Gulf of

California and eastern tropical Pacific arose again as

possible contributors to increases in moisture and concomitant rains during the NA monsoon.

a. The gulf surge phenomenon

Hales (1972) and Brenner (1974) argued that the

eastern tropical Pacific off the Mexican coast, and not

the Gulf of Mexico, provides the greatest amount of

water vapor for the NA monsoon west of the Continental Divide. To substantiate this hypothesis, they

offered a mechanism by which atmospheric moisture

is transported northward through the Gulf of California, occasionally as far inland as the Great Basin

Desert. The transport mechanism was described as a

“surge” of moist tropical Pacific air that is channeled

up the Gulf of California as a result of a low-level atmospheric pressure gradient. This pressure gradient is

created as the thermal equilibrium between the tropical Pacific and the Gulf of California becomes disrupted, primarily due to the development of a cloudy,

showery air mass at the mouth of the Gulf. The thermal and, hence, pressure contrasts between the cooler,

showery air mass to the south and the intensely heated

surface in the deserts of Arizona and eastern California drive the moisture surge. A gulf surge is often

manifest as a strong shift to south–southeast winds

(particularly at Yuma, Arizona) accompanied by lower

temperatures and higher dewpoints, which are detectable at both upper-air stations (Empalme and

Mazatlán) adjacent to the Gulf of California. For the

most part, the gulf surge is a lower tropospheric feature, with all but the strongest surges typically confined below the 700-mb level. The northward influx

of low-level maritime tropical air into the Sonoran

Desert and the uplands of Sonora and Arizona is believed to augment convective activity. Many other investigations have subsequently supported this moisture surge hypothesis (Hales 1974; Houghton 1979;

Jáuregui and Cruz 1981; Tang and Reiter 1984;

Carleton 1985, 1986; Hasimoto and Reyes 1988;

Reyes and Cadet 1988; Carleton et al. 1990; BadanDangon et al. 1991; Rowson and Colucci 1992; Douglas et al. 1993; Douglas 1995; Maddox et al. 1995;

McCollum et al. 1995; Stensrud et al. 1997).

Due to their apparent role in the convective environment of northwestern Mexico and the desert Southwest, gulf surges and more generally the lower atmosphere over the Gulf of California have been a focus

of recent research (Douglas 1995; Douglas and Li

1996; Stensrud et al. 1997). The actual mechanisms

for surge initiation and their relationship to large-scale

circulation patterns have been unclear (Hales 1972;

Brenner 1974). Recent evidence from Stensrud et al.

(1997) again points to the role of westward-moving

tropical disturbances south of the mouth of the Gulf

of California (approximately 20°N) in the development of major gulf surges. However, their findings

from mesoscale model simulations for the gulf region

indicate that large-scale features can indeed be linked

to major surges. They contend that major surges occur when a midlatitude trough passes through western North America (around 40°N) preceding, by 1 or

2 days, the passage of an easterly wave across the same

longitudinal belt (Fig. 4). The increased subsidence

FIG. 4. Conceptualization of the gulf surge phenomenon. Major

surges are associated with easterly waves crossing south of the

Gulf of California, out of phase with an upper-level midlatitude

trough. Minor surges are associated with mesoscale convective

systems drifting over the northern half of the Gulf (see text for

details).
2202 Vol. 78, No. 10, October 1997

behind the trough over the northern end of the gulf

apparently creates conditions in surface pressure and

low-level temperature fields conducive to surges. The

actual initiation of major surges may result from cold

outflows caused by deep convection over the Gulf of

California, or it may simply result from the low-level

convergence associated with the easterly wave.

Minor gulf surges, those typically confined to the

northern half of the Gulf (Hales 1972), can be triggered by the passage of a mesoscale convective complex passing from the mainland over the Gulf, by

backdoor cold fronts approaching from the north to

northeast as opposed to the typical west to northwest

direction (R. Maddox 1996, personal communication;

Hales 1972) or by the passage of an easterly wave

without the preceding upper-level trough associated

with major surges (Stensrud et al. 1997), as conceptualized in Fig. 4. The weather effects for Sonora and

Arizona of these minor surges are similar to major

surges; however, the former go undetected in the

Empalme and Mazatlán soundings, making forecasting these surges more difficult (R. Maddox 1996, personal communication).

The gulf surge does not, in itself, necessarily account for the total Gulf of California–eastern Pacific

moisture transport into the desert Southwest. Field

studies (Badan-Dangon et al. 1991; Douglas 1995) describe mean southerly flow at the northern end of the

Gulf of California during July and August, with a lowlevel jet as a notable characteristic (see Fig. 5 and related discussion in section 4a, also). Mesoscale observations from SWAMP (35 days of pilot balloon soundings and several research aircraft measurements) document the nature of the low-level jet. It is present under various synoptic configurations, exhibiting an

early morning maximum over the northern Gulf of

California, with the jet appearing to be strongest under surge conditions (Douglas 1995). The persistence

of this low-level flow suggests significant northerly

transport of water vapor from the Gulf of California.

For the Sonoran Desert region, a recent analysis

(Schmitz and Mullen 1996) of atmospheric water vapor flux (calculated for the time-mean wind during

July and August) for the Gulf of California shows that

time-mean moisture transport is dominant over that of

transient flux (deviations from the mean), although the

role of transient flux may be more critical for actual

precipitation episodes over the region.

b. A dual moisture source

Despite the fact that the more recent literature has

tended to view the tropical eastern Pacific–Gulf of

California as the dominant moisture source for the NA

monsoon, the Gulf of Mexico is generally still viewed

as an important contributor. Most investigations of the

NA monsoon subscribe to the idea of moisture advection in some combination from both the eastern tropical Pacific–Gulf of California and the Gulf of Mexico

(Hales 1974; Brenner 1974; Mitchell 1976; Carleton

1985, 1986; Carleton et al. 1990; Harrington et al.

1992; Watson et al. 1994a; Stensrud et al. 1995;

Schmitz and Mullen 1996). Much recent work states

that low-level moisture is largely attributable to the

Gulf of California and that upper-level water vapor is

transported from the Gulf of Mexico. The debate is

more than academic because source regions have implications for seasonal forecasts (Schmitz and Mullen

1996) and for the development of the convective environment (Stensrud et al. 1995).

Carleton (1986) identifies contributions of both

sources by examining a set of thermodynamic parameters for several Mexican and United States upper-air

stations. Although the importance of the sources relative to one another is not evaluated, he argues that rapid

FIG. 5. Schematic of selected dynamic features during the NA

monsoon. The typical 850-mb late-afternoon height configuration

(solid dark lines) includes heat lows over the elevated interior,

which act to draw in moisture by way of low-level jets (LLJ)

originating over the Gulf of California and the Gulf of Mexico

(large arrow). The early morning Gulf of California LLJ core is

shown via shaded isotachs (3, 5, and 7 m s−1

) and streamlines at

450 m (1500 ft) above ground level (adapted in part from Douglas

1995).
Bulletin of the American Meteorological Society 2203

lower-level moisture surges moving up the Gulf of

California and slower high-level moisture from the

Gulf of Mexico work in concert during monsoon

“bursts” (his term “burst” has the same connotation as

that for the Southwest Asian monsoon—i.e., large

surges of moisture and concomitant precipitation).

Carleton et al. (1990) reaffirm this conclusion in their

examination of interannual variability of Arizona rainfall during the NA monsoon. This recognition of two

distinct moisture sources for the NA monsoon raises

important questions about the relative contributions of

the Gulf of Mexico and eastern tropical Pacific–Gulf

of California during monsoon bursts, as well as the

possible spatial and temporal variations in the relative

contribution from each source at different heights in

the atmosphere. Maddox et al. (1995) claim that this

reduced role of the Gulf of Mexico as moisture source

is still too great, but the results of Schmitz and Mullen

(1996), focusing on the Sonoran Desert, suggest that

the time-mean upper-level flow (700 mb to 200 mb)

has its origin over the Gulf of Mexico, while low-level

flow is attributable to the Gulf of California. Clearly,

this question has not been completely resolved.

One further complicating factor in the moisture

source debate is the role of the Sierra Madre Occidental in moistening the midlevels of the atmosphere.

Recent empirical and modeling studies (Maddox et al.

1995; Stensrud et al. 1995; Schmitz and Mullen 1996)

highlight the importance of low-level Gulf of California and eastern Pacific moisture being drawn inland

over the Sonoran and Sinaloan lowlands and up the

western flanks of the Sierra Madre Occidental due to

topographic heating. The resulting convection redistributes moisture to higher levels. This moistened air

may then mix with elevated Gulf of Mexico moisture

from the east and can subsequently be transported into

the southwestern United States via flow around the

anticyclone. As a result, the simple Gulf of California–Gulf of Mexico moisture source dichotomy becomes much more difficult to unravel.

4. Geographic extent

a. Spatial domain

It is quite apparent that Arizona has been the principal geographic study area for NA monsoon research,

particularly in relation to summer precipitation

(Bryson and Lowry 1955a,b; Reitan 1957; Bryson

1957a,b; Green 1963; Carleton 1985, 1986, 1987;

Moore et al. 1989; Smith and Gall 1989; Adang and

Gall 1989; Carleton et al. 1990; Rowson and Colucci

1992; Dunn and Horel 1994a,b; McCollum et al. 1995;

Maddox et al. 1995). This may be a result of the fact

that the monsoon phenomenon was first recognized in

the literature as the “Arizona summer monsoon”

(Bryson and Lowry 1955a).2

 Furthermore, it has been

stated that, in the United States, the NA monsoon is

most visible in Arizona (Trewartha 1981; Adang and

Gall 1989; Carleton et al. 1990). Nevertheless, several

investigators have expanded or shifted the geographic

focus of NA monsoon influence. Reyes and Cadet

(1988) and Douglas et al. (1993) have shown that the

“Arizona monsoon” is merely an extension of a more

pronounced Mexican phenomenon centered over the

western foothills of the Sierra Madre Occidental (Fig.

6). New Mexico has also been recognized as greatly

influenced by the NA monsoon (Tuan et al. 1973;

Mitchell 1976; Trewartha 1981; Harrington et al.

1992; Douglas et al. 1993; Bowen 1996), and one index of monsoon influence has shown that New Mexico

is the state most affected by the NA monsoon (Douglas et al. 1993). For example, note the similarities in

precipitation regime between Tucson and Alamogordo

in Fig. 2. The NA monsoon moisture and attendant

thunderstorms even extend into the lower Colorado

River valley and the Mojave Desert, and farther west

to the transverse and peninsular ranges of southern

California, with some regularity (Tubbs 1972).

On a much broader scale, Tang and Reiter (1984)

and Reiter and Tang (1984) demonstrate that the western United States and northern Mexican plateau act to

create a true monsoonal circulation pattern over the

North American continent, comparable to that of the

Tibetan Plateau. This expanded region of summer

monsoon influence, which they designate as the “western plateau,” ranges longitudinally from the Sierra

Nevada in the west to the Colorado and Wyoming

Rockies in the east. The northernmost limit runs from

Oregon along the Idaho–Utah border into Wyoming.

This limit, according to Tang and Reiter (1984), coincides with an anticyclonic shear line that marks the

northernmost extension of summer convective activity. The Mexican Plateau along the international border serves as their southern boundary. Tang and Reiter

(1984) postulate a low-level jet containing tropical

moisture drawn up the Gulf of California into the Great

2

Though this phenomenon was dubbed the Arizona monsoon,

Bryson (1957b) clearly recognized that this was also a Mexican

feature and suggested that it should be properly called the

“Sonoran summer monsoon.”
2204 Vol. 78, No. 10, October 1997

Basin desert of Nevada and Utah as a result of monsoonal pressure gradients. This corresponds closely to

ideas put forth by Brenner (1974), Hales (1974), and

Houghton (1979). Likewise, moisture is transported

east of the Continental Divide by a low-level jet from

the Gulf of Mexico. The moisture is moved up the

Continental Divide as a result of interior plateau heatlows, thereby increasing convective activity (Fig. 5).

These ideas are in agreement with the postulated dual

moisture source for the NA monsoon, and they indicate that the Continental Divide may be important in

determining low-level moisture trajectories.

b. Distribution of air masses

Several of the aforementioned studies have also

described the NA monsoon as a contrast between dry

and moist air masses in the southwestern United States

and northwestern Mexico. The moist air mass, characterized as a moist tongue moving north from

Mexico into the southwestern United States, is

bounded to the north and west by drier air from the

Pacific subtropical high (Bryson and Lowry 1955a,b).

The zone of interaction between the air masses has

been viewed as a definite, but shifting, boundary that

exists between the moist air mass brought in by the

westward extension of the Bermuda high and the drier

eastern-Pacific air (Bryson and Lowry 1955a,b;

Mitchell 1976; Moore et al. 1989; Adang and Gall

1989; Watson et al. 1994a). For example, while the

transition from dry to moist air masses at the onset of

the monsoon season is not always rapid, it has been

shown to occur in as short a period as a few hours

(Nastrom and Eaton 1993). In addition, gradients in

temperature, water vapor content, and winds similar

to those of a quasi-stationary, midlatitude front between the moist tongue and dry air have also been

observed (Moore et al. 1989; Adang and Gall 1989).

The boundary of this moist tongue has been described

as shifting spatially in agreement with the westwardexpanding Bermuda high (Bryson 1957a; Trewartha

1981; Tang and Reiter 1984; Harrington et al. 1992),

from eastern New Mexico in June to western New

Mexico and central and eastern Arizona by July. Eastern California and western Arizona are under its influence by August. Climatologically, this boundary

FIG. 6. Frequency (in percent of total hours) of cloud-top

temperatures less than −38°C for June, July, and August of 1985–

92, derived from averaged hourly infrared GOES imagery

(adapted from Douglas et al. 1993). Shaded regions delineate areas

of systematic deep convection.
Bulletin of the American Meteorological Society 2205

has been defined in terms of contrasting equivalent

potential temperature (Mitchell 1976). These studies

of the NA monsoon boundary do not, however, examine the specific contributions provided by the Gulf

of Mexico or the Gulf of California for the development of the moist tongue.

5. Spatial and temporal variability

The NA monsoon region, like other arid and semiarid locales, experiences great climatic variability on

a range of spatial and temporal scales. Difficulties in

understanding the variability of summertime convective activity in the southwestern United States and

northwestern Mexico result from the complex interactions between synoptic and mesoscale circulation

features and the extremely varied topography. Although larger-scale atmospheric motions control the

broad distribution of water vapor and the general stability or instability in the atmosphere, local topographic effects in the western United States and northwestern Mexico are critical to the geographic and even

temporal distribution of convective activity. In fact,

dealing with these factors has been at the heart of many

modeling and forecasting efforts in the region, as we

discuss in section 6b.

a. Regional-scale spatial variability

Spatial variability of the NA monsoon has been

examined almost exclusively in terms of precipitation.

The geographic influence of NA monsoon rains is primarily centered in the western foothills of the Sierra

Madre Occidental in northwestern Mexico, specifically in the Mexican states of Nayarit, Sinaloa, and

Sonora (Hales 1972; Douglas et al. 1993). This region

receives up to 70% of its annual rainfall in the months

of July, August, and September (Mosiño and Garcia

1974; Douglas et al. 1993; Reyes et al. 1994). The intensity of convection over the Sierra Madre is apparent from satellite imagery (Douglas et al. 1993; Negri

et al. 1993; Stensrud et al. 1995), as illustrated in Fig.

6. The distinctness of the July through September

maximum decreases rapidly as one moves northward

through Sonora toward the international border. The

NA monsoon precipitation regime also becomes less

notable toward the eastern flank of the Sierra Madre

Occidental and onto the Mexican Altiplano. Much of

northeastern Mexico actually experiences a June and

September rainfall maximum with a minimum in July

and August. Similarly, NA monsoon rains diminish

rapidly to the west, across the Gulf of California along

the Baja Peninsula and northward into California. The

greater influence of subsidence under the Pacific subtropical high in these latter areas has been suggested

as a reason for the steep decrease in rainfall (Hastings

and Turner 1965). Yet the drop-off takes place over a

relatively short distance (as compared to the scale of

subsidence); a concise, supported explanation for this

westward decrease is not found in the literature. North

of about 26°N on the Pacific side of the peninsula, the

cool waters of the California current and strong subsidence stifle summertime convection, and the winter

precipitation regime of the West Coast dominates

(Hastings and Turner 1965). These patterns of precipitation seasonality are illustrated in Fig. 2.

Within the United States, southern and eastern Arizona, much of New Mexico, and south-central Colorado receive precipitation related to the NA monsoon,

but rainfall amounts are much more variable than in

northwestern Mexico. For example, the Mogollon Rim

of central Arizona and the White Mountains of eastern Arizona receive a much greater percentage of rainy

days during the monsoon than the lower desert to the

southwest, despite generally higher dewpoints in this

latter region (although relative humidities are lower).

This is not unexpected considering that convection is

also greater over the elevated regions of Arizona.

Across the NA monsoon area, regional-scale spatial

variability in precipitation is closely linked to the penetration of moisture into the interior regions (e.g.,

Hales 1972; Brenner 1974), but synoptic-scale controls are believed to be equally important for the regional variability of NA monsoon precipitation in

Arizona (Carleton 1986). Although not directly part

of the NA monsoon, decaying tropical cyclones also

contribute to the regional variation of total summer

precipitation (Reyes 1988; Hereford and Webb 1992),

particularly over Sinaloa, Sonora, and the southern tip

of Baja California.

b. Local-scale variability

Much of the early NA monsoon research focused

on the spatial and temporal variability of Arizona summer rainfall (McDonald 1956; Bryson 1957a; Green

1960). Considering the nature of air mass thunderstorms, it is not surprising that there is a great spatial

variability in precipitation and a variable geographic

distribution of summer thunderstorms. In Arizona,

only small-to-moderate interstation correlations exist

between rainfall events (McDonald 1956). Yet, diurnal variability in summer thunderstorms does show a
2206 Vol. 78, No. 10, October 1997

marked geographic dependence, particularly in Arizona (McDonald 1956; Ackerman 1959; Sellers 1958;

Hales 1972, 1977; Brenner 1974; Reiter and Tang

1984; Balling and Brazel 1987; Maddox et al. 1991;

Watson et al. 1994b; Maddox et al. 1995). Diurnal

convective activity is generally at a maximum over the

Colorado Plateau and the higher elevations of the

Continental Divide during early afternoon. Over the

highlands of southern Arizona and northern Sonora,

maximum activity is characteristic of the early evening

hours. By late evening and early morning, the coastal

lowlands of northwest Mexico and the lower desert

regions of southern and central Arizona experience a

nighttime convective maximum (in contrast to the timing of convective storms elsewhere), as illustrated in

Fig. 7. Similar diurnal precipitation variation due to

topographic effects has also been noted in central New

Mexico (Bowen 1996). The specific causes of the nocturnal maximum have been explored but are still not

clearly understood. Hales (1977) states that midtropospheric cooling results from localized evaporation of

cumulonimbus towers that develop over the higher

terrain of central Arizona in the afternoon. This elevated cool layer is supposedly advected west and

southwest over the low desert toward evening, thereby

destabilizing the local environment (although no further support of this idea has been found). Ascending

air in the late evening and early morning over the

Sonoran Desert low may also support the nocturnal

convection (Douglas and Li 1996), as a result of the

larger lapse rate. With regard to the development of

severe nocturnal mesoscale convective systems (MCS)

over southern and central Arizona, McCollum et al.

(1995) state that subsynoptic scale features are also of

great import, particularly low-level moisture advection

from the Gulf of California and localized convergence

in favored areas.

c. Seasonal variability

Apart from the spatial variability that is directly

related to topographic features, there is much intraseasonal variability of NA monsoon rainfall (in terms

of intensity and areal extent), particularly in the southwestern United States. The highlands of eastern

Sinaloa and Sonora in northwestern Mexico experience shower activity almost every day during the entire NA monsoon season. Nevertheless, there are apparent periodicities in rainfall episodes in different

areas of northwestern Mexico, which may be attributable to variations in source region influence (Reyes et

al. 1994). Intraseasonal variation of precipitation totals in this region may be related to the frequency of

MCS and tropical perturbations that account for a great

portion of the yearly precipitation (Reyes et al. 1994).

Farther north, much of the intraseasonal precipitation

variability may be related to the gulf surge phenomenon described by Hales (1972) and Brenner (1974).

These surges are believed to contribute significantly

to the widespread summer rainfall events over much

of the desert Southwest. Considering that several gulf

surges can occur in this area during a given NA monsoon season, they may account for a great deal of the

variability in total summer precipitation (Brenner

1974). It is important to note here that gulf surges,

weak or strong, have been assumed to result in increased convective activity and precipitation, but to

date no study has been published in the formal literature quantifying this relationship.

A number of authors have examined changes in the

synoptic-scale circulation and its effects on intraseasonal variability of the NA monsoon (Bryson and

Lowry 1955a, b; Carleton 1986, 1987). Analyses of

composite synoptic pressure patterns have identified

atmospheric configurations that enhance or stifle

FIG. 7. Percentage of precipitation events occurring between

2000 and 0800 true solar time in Arizona for the months of July

and August [redrafted from Balling and Brazel (1987)]. Note

gradient from the Mogollon Rim and Colorado Plateau to the low

deserts of central Arizona.
Bulletin of the American Meteorological Society 2207

convective activity (bursts or

breaks) in terms of cloudiness,

precipitation, and cloudto-ground lightning (Carleton

1986, 1987; Watson et al.

1994b). Carleton (1986, 1987)

has shown that widespread convective activity in the southwestern United States is typically associated with passing

upper-level troughs in the westerlies. In addition, the northward

displacement of the subtropical

ridge and the formation of a cutoff “four corners high” also results in increased thunderstorm

activity in Arizona. A southerly

displacement of the subtropical

ridge over northern Mexico

tends to result in (drier) break

conditions over the desert Southwest. His conclusions highlight

the importance of the location of

the subtropical ridge in controlling convective activity in the

southwestern United States.

In examining cloud-to-ground

lightning, Watson et al. (1994a) found that similar synoptic configurations to those of Carleton led to bursts

and breaks in total convective activity. Again, a southerly shift of the subtropical ridge axis places Arizona

under the strong, drying effects of the westerlies,

which promote break conditions. A northerly shift of

the subtropical high is associated with increased inflow of moisture from the south and results in widespread shower activity.

Although the results of the studies by Carleton

(1986, 1987) and Watson et al. (1994a) agree on the

importance of the latitudinal position of the subtropical ridge in determining bursts and breaks, Maddox

et al. (1995) point out that the composite 500-mb burst

pattern of Watson et al. (1994a; their Fig. 18c) closely

resembles Carleton’s (1986) composite break pattern

(his Fig. 5b). Their definition of burst and breaks are

based, however, on different criteria: Carleton (1986)

on total cloudiness over a large portion of the western

United States, and Watson et al. (1994a) on cloud-toground lightning only in Arizona. Such intricacies

further demonstrate the complex nature of the relationship between convective activity and synoptic circulation during the NA monsoon.

d. Interannual variability

The sparseness of long-term instrumental precipitation records (both temporally and spatially) in the

southwestern United States and northwestern Mexico

has hindered research on NA monsoon variability at

interannual timescales. Nonetheless, several studies

have noted marked variations in variability of the NA

monsoon since the turn of the century (Green and Sellers 1964; Bradley 1976; Tenharkel 1980; Brazel and

Prasad 1984; Carleton et al. 1990; Hereford and Webb

1992).

Interannual variability in NA monsoon activity may

result from the corresponding variability of certain

synoptic-scale patterns, similar to those associated

with intraseasonal variability. Carleton et al. (1990)

have argued that latitudinal shifts in the midlevel subtropical ridge over the southwest United States, which

accounts for a great deal of within-season monsoon

variability, are also responsible for year-to-year and

decadal-scale variability. Figure 8 shows that, in general, northerly displacement of the subtropical ridge

is associated with wetter Arizona summers, while a

southerly shift coincides with decreases in summer

rainfall totals (there are exceptions; e.g., 1990 was a

FIG. 8. Composites of 500-mb geopotential height (dam), 1946–88, showing the overall

mean circulation for July and August (upper left), the means of the five wettest and driest

years in Arizona (upper right; solid contour lines correspond to wet years, and broken lines

correspond to dry years), and the wet (lower left) and dry (lower right) anomalies from the

overall mean (precipitation data are from the seven Arizona climate divisions).
2208 Vol. 78, No. 10, October 1997

very wet year even though there was a southerly shift).

Carleton et al. (1990) found that latitudinal displacement of the subtropical ridge is tied to different phases

of the Pacific–North American (PNA) teleconnection

pattern, which in turn is linked to the El Niño–Southern Oscillation (ENSO). Wetter summers are associated with the meridional phase of the PNA and positive Pacific SST anomalies that enhance ridging over

North America. Drier summers have tended to follow

zonal phases of the PNA.

e. ENSO-related studies

Relationships between ENSO and NA monsoon

variability have been examined for possible causality,

but such links have proven to be elusive. Some studies have analyzed the strength and geographic distribution of rainfall during and following ENSO events.

More frequent tropical perturbations off the southwestern coast of Mexico have tended to follow the mature

ENSO stage (Northern Hemisphere winter) when the

ITCZ is displaced farther north and there is increased

northward transport of water vapor (Reyes and Cadet

1988; Reyes and Mejía-Trejo 1991). Under these conditions, parts of northwestern Mexico (particularly the

Sonoran Desert) have experienced positive anomalies

in summer rainfall (Reyes and Mejía-Trejo 1991).

Within the southwestern United States, Andrade and

Sellers (1988) found little correlation between ENSO

and total summer rainfall in New Mexico and Arizona.

Yet Harrington et al. (1992) found a correlation between ENSO warm and cold events and the geographic

distribution of summer precipitation for the same areas. Likewise, Hereford and Webb (1992) suggest

there is some relationship between increased summer

precipitation in the Colorado plateau region and the

warm ENSO phase. Thus, relationships between

ENSO-related phenomena and the NA monsoon are

not yet well understood.

f. Paleoclimate studies

The focus of this review is on the modern NA monsoon, but a number of studies utilizing proxy data include description of summer paleoclimate in the southwestern United States (e.g., Wells 1979; van Devender

and Spaulding 1979; Davis and Shafer 1992). Briefly,

paleoenvironmental conditions consistent with a circulation similar to the modern NA monsoon were

present about 10 000 to 8000 BP, or roughly 10 000

yr after the last glacial maximum (van Devender and

Spaulding 1979; Davis and Shafer 1992). Prior to this

time, the presence of the Laurentide ice sheet over

eastern North America delayed the development of the

Bermuda high and associated summer rains west of the

Continental Divide (van Devender and Spaulding

1979). The period since the establishment of a monsoonlike circulation has included intervals considerably wetter and drier than those found in the instrumental record (Davis 1994; Petersen 1994). In general,

the atmospheric controls on the paleomonsoon are

assumed to be fundamentally the same as those for the

modern monsoon (i.e., position and strength of subtropical highs and associated latitudinal position of the

westerlies), with changes in their relative location and

intensity causing the observed long-term variability.

6. Severe storms

a. Convective environment

An important aspect of NA monsoon research deals

with severe storms and their predictability. The southwestern United States and northwestern Mexico have

experienced tremendous population growth in the last

few decades, and severe thunderstorms and related

phenomena can pose a serious hazard for residents of

this region. A range of studies have been performed,

including those on the severe storm environment

(Hales 1975; Randerson 1986; Smith and Gall 1989;

McCollum et al. 1995; Maddox et al. 1995), lightning

(Reap 1986; Watson et al. 1994a; Watson et al. 1994b),

dust storms (Idso et al. 1972; Nickling and Brazel

1984; Brazel and Nickling 1986), and flash flooding

(Randerson 1976, 1986; Maddox et al. 1980). In recent years, there has been a greater focus on the convective environment of severe storms in the NA monsoon. Much of the latest research has resulted from the

SWAMP, which is aimed at greater understanding of

convective environments in central Arizona and northwestern Mexico (Maddox 1990; Meitin 1991; Reyes

et al. 1994).

Two common approaches for severe storm analysis in the NA monsoon region have been analysis of

synoptic patterns (Maddox et al. 1980; Randerson

1976; Brazel and Nickling 1986; Maddox et al. 1995)

and of satellite imagery (Hales 1975; Smith and Gall

1989; Farfán and Zehnder 1994). Smith and Gall

(1989) describe a characteristic MCS of the NA monsoon that shares a number of characteristics with tropical squall lines. Here, MCSs typically form over the

elevated regions of southeastern Arizona and northern Sonora as the result of the merging of late afternoon thunderstorm cells. By evening, the storms tend
Bulletin of the American Meteorological Society 2209

to move in a westerly direction toward the low desert

via discrete propagation. Although their life span is

typically less than 1 day, they often produce remnant

cyclonic circulation that can stimulate convection the

next day over southern California and Baja California. Interestingly, this behavior is not too far from the

turn-of-the-century description of the “Sonora storm”

(Campbell 1906). The necessary pattern for the formation of these squall lines appears to be a 500-mb

ridge with its axis centered over the Utah–Nevada

border. This configuration promotes strong flow from

the east and northeast over the elevated terrain along

the Continental Divide. Under these conditions, dry

midtropospheric air is advected westward over moist

warm air covering the lower terrain of Sonora and

southern Arizona. If low-level wind shear is present,

squall lines may form. Work by Farfán and Zehnder

(1994) highlights the association between strong

midlevel easterlies over the Sierra Madre Occidental

in the formation of propagating MCS over northern

Mexico.

The same anomalous ridge pattern has also been

associated with severe storms in central Arizona

(Maddox et al. 1995) and dust storm generation in

Arizona (Brazel and Nickling 1986). Figures 9a and

9b illustrate an example of this pattern for a severe

storm over central Arizona. Other patterns have also

been identified as precursors to severe thunderstorm

development in central Arizona (Dunn and Horel

1994a; Maddox et al. 1995), and although these patterns are not always positioned directly over Arizona,

they do provide clues for the evolution of the central

Arizona convective environment (Maddox et al. 1995).

Yet subsynoptic features such as low-level moisture

advection from the Gulf of California and localized

convergence resulting from the complex terrain of

central Arizona are also critical for development of the

severe thunderstorm environment. As a result, forecasting of severe thunderstorms for this region is extremely difficult (Dunn and Horel 1994a,b; McCollum

et al. 1995).

b. Forecasting and modeling

Because the convective environment is tied to particular synoptic patterns, to various subsynoptic features, and to complex topography, considerable difficulties must be overcome for successful model prediction and forecasting. A major problem is that nearly

identical synoptic situations can produce days where

thunderstorms are either confined to topographically

favored locations or where thunderstorm activity

spreads to lower elevations (Dunn and Horel 1994a).

In Arizona, the forecasting skill scores for prediction of severe storm events have been exceptionally

FIG. 9. Example of a severe thunderstorm over central Arizona showing the strong ridge over the Utah–Nevada border and associated

NE flow over Arizona, represented by (a) the 500-mb geopotential height pattern (dam) for 0000 UTC 10 August 1984 (1700 LST 9

August 1984) with matching wind barbs (m s−1

) from radiosonde observations, and (b) GOES-W infrared image for 0100 UTC 10

August 1984 (1800 LST 9 August 1984). The broken line in (a) frames the image in (b).
2210 Vol. 78, No. 10, October 1997

low in comparison to other regions of the continental

United States (Maddox et al. 1995). Likewise, numerical weather prediction models have not performed

well for summer convection in Arizona. Dunn and

Horel (1994a,b) describe the difficulties experienced

with the National Center for Environmental Prediction models (both the nested grid and Eta models) in

predicting southwest United States precipitation.

Overall, both models provided poor predictions of

heavy rain episodes over Phoenix. Several mesoscale

features prevented the Eta model from properly simulating severe convection over Phoenix. The model

does not resolve the advection of low-level moisture

from the Gulf of California into central Arizona,

which is critical for destabilization of the local environment prior to convection. This may result from the

failure of the model to produce the low-level jet above

the Gulf of California that seems to be responsible for

moisture transport into the low deserts (Douglas et al.

1993; McCollum et al. 1995). This, in turn, may result from inaccurate initial conditions for summertime

SSTs over the Gulf of California (Stensrud et al. 1995).

In addition, another critical factor in localized severe

convection is that of thunderstorm outflows (McCollum

et al. 1995), which are not included in the model’s

convection scheme. Stensrud et al. (1995) found that

many of the general features of the NA monsoon circulation could be reproduced using the PSU–NCAR

mesoscale model. They demonstrate that model initialization of mesoscale features such as the low-level

winds over the Gulf of California and the summertime

warming of the Gulf are critical for dynamically consistent simulations of mesoscale structures.

7. Summary and conclusions

The NA monsoon is one of the more complex and

interesting atmospheric circulation features over North

America, and the associated research literature dates

back to the early part of this century. Although the

circulation covers a significant portion of the continent, research on the NA monsoon (as well as its

broader understanding in the meteorological and climatological communities) has not enjoyed a matching prominence. The recent appearance of several papers from the SWAMP project has, however, somewhat improved this situation, especially in the context

of its mesoscale manifestations.

It appears that a consensus has gradually emerged

regarding major circulation mechanisms. The role of

the westward expansion of the Bermuda high was recognized early on, following the availability of upperair data. Synoptic climatological studies have provided

further details on the subtropical ridge and the effects

of its latitudinal displacement. More recently, topographic influences and mesoscale phenomena have

been highlighted as critical components of the NA

monsoon circulation. These findings have had important consequences for the debate over moisture

sources. Most investigators are now in agreement that

low-level moisture is advected from the eastern tropical Pacific–Gulf of California, while the Gulf of

Mexico may contribute to upper-level moisture. In

fact, recent work suggests that moisture from these two

sources becomes mixed over the Sierra Madre Occidental, prior to being transported northward into the

United States.

The spatial extent of the NA monsoon is large, covering much of the western United States and northwestern Mexico. Historically, most of the literature has

focused on Arizona, but the NA monsoon is actually

most spatially consistent over northwestern Mexico

with greater variability to the north. Variability at a

range of temporal scales is related to interactions between synoptic and mesoscale circulations, in particular the latitudinal position of the subtropical ridge and

the low-level moisture surges from the Gulf of California. Interannually, these relationships are partly influenced by the PNA pattern (and perhaps to some

degree by ENSO). Forecasting of the NA monsoon,

especially the ability to distinguish between weak or

strong severe storm outbreaks, has been poor because

current models do not adequately resolve the mesoscale features. Recent research from the SWAMP

project, together with mesoscale modeling, has further

highlighted the integrated roles of synoptic circulation,

topography, and mesoscale circulations in controlling

NA monsoon activity.

Many of the relationships noted above are not yet

well understood, and there are a number of important

questions for future research.

1) What are the relative roles of convective mixing

over the Sierra Madre Occidental and subsequent

northward moisture transport compared to lowlevel moisture from the Gulf of California and

upper-level moisture from the Gulf of Mexico,

with respect to widespread rainfall across the NA

monsoon region?

2) Additionally, what are the spatial and temporal

variations in each of these kinds of moisture de-
Bulletin of the American Meteorological Society 2211

livery and how are they related to seasonal and

interannual circulation changes?

3) With respect to ongoing research into the central

Arizona convective environment, what are the

causes of the nocturnal maxima in thunderstorm

activity over the lower deserts?

4) Furthermore, what are the complementary roles of

low-level moisture advection and localized convergence at the mesoscale over these same areas?

5) What conditions trigger gulf surges (at the synoptic or mesoscale), and can their relationship to convective activity and precipitation be quantified?

6) The ENSO signal seems unclear in interannual NA

monsoon variability, so what other causes are

likely to influence interannual variation?

7) For improved forecasting and modeling, a better

understanding of mesoscale features is required on

a routine basis. What are the minimum data requirements and how might the data be acquired?

Regarding this last question, Douglas and Stensrud

(1996) have called for an increased density of upperair stations, which for the NA monsoon area would help

to overcome some of the current problems with unresolved circulation features and complex topography.

Overall, there is clearly a need to better understand

the complex links between synoptic circulation, topography, gulf surges, and mesoscale features within the

NA monsoon. As mentioned above, this will most

likely be achieved via improved data availability and

modeling at the mesoscale. As with many other areas

of atmospheric research, advances in understanding

and forecasting depend to some degree on new measurements, in this case at a level of detail currently

unavailable (except in field campaigns such as

SWAMP). In conclusion, a somewhat cohesive body

of knowledge has emerged from past and current NA

monsoon research, but the picture is by no means complete. There are still many aspects of the NA monsoon

that require further investigation, and these include a

number of challenging meteorological and climatological questions.

Acknowledgments. We wish to thank Bob Maddox, Andrew

Carleton, and the reviewers for their detailed comments on early

versions of the manuscript. Also, thanks go to Sandra Brazel of

the Office of Climatology at Arizona State University for providing the 1984 satellite image.

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