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 intrase...

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The North American Monsoon

This paper summarizes the current state of knowledge of the climate of southwest USA (the 'Southwest'). Low annual precipitation, clear skies, and year-round warm climate over much of the South- west are due in large part to a quasi-permanent subtropical high-pressure ridge over the region. However, the Southwest is located between the mid-latitude and subtropical atmospheric circulation regimes, and this positioning relative to shifts in these regimes is the fundamental reason for the region's climatic vari- ability. Furthermore, the Southwest's complex topography and its geographical proximity to the Pacific Ocean, the Gulf of California, and the Gulf of Mexico also contribute to this region's high climatic vari- ability. El Nino, which is an increase in sea-surface temperature of the eastern equatorial Pacific Ocean with an associated shift of the active center of atmospheric convection from the western to the central equa- torial Pacific, has a well-developed teleconnection with the Southwest, usually resulting in wet winters. La Nina, the opposite oceanic case of El Nino usually results in dry winters for the Southwest. Another im- portant oceanic influence on winter climate of the Southwest is a feature called the Pacific Decadal Oscillation (PDO), which has been defined as temporal variation in sea-surface temperatures for most of the Northern Pacific Ocean. The effects of ENSO and PDO can amplify each other, resulting in increased annual variability in precipitation over the Southwest. The major feature that sets the climate of the South- west apart from the rest of the United States is the North American monsoon, which in the US is most noticeable in Arizona and New Mexico. Up to 50% of the annual rainfall of Arizona and New Mexico oc- curs as monsoonal storms from July through September. Instrumental measurement of temperature and precipitation in the Southwest dates back to the middle to late 1800s. From that record, average annual rainfall of Arizona is 322 mm (12.7''), while that of New Mexico is 340 mm (13.4''), and mean annual tem- perature of New Mexico is cooler (12°C (53°F)) than Arizona (17°C (62°F)). As instrumental meteorolog- ical records extend back only about 100 to 120 yr throughout the Southwest, they are of limited utility for studying climate phenomena of long time frames. Hence, there is a need to extend the measured meteo- rological record further back in time using so-called 'natural archive' paleoclimate records. Tree-ring data, which provide annual resolution, range throughout the Southwest, extend back in time for up to 1000 yr or more in various forests of the Southwest, and integrate well the influences of both temperature and pre- cipitation, are useful for this assessment of climate of the Southwest. Tree growth of mid-elevation forests typically responds to moisture availability during the growing season, and a commonly used climate vari- able in paleo-precipitation studies is the Palmer Drought Severity Index (PDSI), which is a single variable derived from variation in precipitation and temperature. June-August PDSI strongly represents precip- itation and, to a lesser extent, temperature of the year prior to the growing season (prior September through current August). The maximum intra-ring density of higher elevation trees can yield a useful record of summer temperature variation. The combined paleo-modern climate record has at least 3 occurrences of multi-decadal variation (50 to 80 yr) of alternating dry (below average PDSI) to wet (above average PDSI). The amplitude of this variation has increased since the 1700s. The most obvious feature of the temperature record is its current increase to an extent unprecedented in the last 400 yr. Because this warming trend is outside the variation of the natural archives, it is possible that anthropogenic impacts, such as increased atmospheric concentrations of greenhouse trace gases, are playing a role in climate of the Southwest. Accordingly, this pattern merits further research in search of its cause or combination of causes.

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The climate of the US Southwest

This paper reviews the current understanding of moist orographic convection and its regulation by surface-exchange processes. Such convection tends to develop when and where moist instability coincides with sufficient terrain-induced ascent to locally overcome convective inhibition. The terrain-induced ascent can be owing to mechanical (airflow over or around an obstacle) and/or thermal (differential heating over sloping terrain) forcing. For the former, the location of convective initiation depends on the dynamical flow regime. In “unblocked” flows that ascend the barrier, the convection tends to initiate over the windward slopes, while in “blocked” flows that detour around the barrier, the convection tends to initiate upstream and/or downstream of the high terrain where impinging flows split and rejoin, respectively. Processes that destabilize the upstream flow for mechanically forced moist convection include large-scale moistening and ascent, positive surface sensible and latent heat fluxes, and differential advection in baroclinic zones. For thermally forced flows, convective initiation is driven by thermally direct circulations with sharp updrafts over or downwind of the mountain crest (daytime) or foot (nighttime). Along with the larger-scale background flow, local evapotranspiration and transport of moisture, as well as thermodynamic heterogeneities over the complex terrain, regulate moist instability in such events. Longstanding limitations in the quantitative understanding of related processes, including both convective preconditioning and initiation, must be overcome to improve the prediction of this convection, and its collective effects, in weather and climate models.

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Moist Orographic Convection: Physical Mechanisms and Links to Surface-Exchange Processes

Marked intraseasonal variations in the intensity of the south-west U.S. summer precipitation singularity (the ‘Monsoon’) are identified for the years 1980–1982 from a previously developed satellite cloud climatology. These ‘bursts’ and ‘breaks’ are examined synoptically by compositing sea level pressure and 500 mb height data. Both types of events are controlled by the upper-level flow. This typically involves, for bursts, the intrusion into subtropical latitudes of a trough in the westerlies with attendant cold air advection. In conjunction with intense surface heating, this produces widespread atmospheric destabilization over the South-west. Monsoon breaks are characterized by enhanced ridging of the Bermuda and North Pacific subtropical anticyclones with associated subsidence over southern Arizona/California. Between-year differences in these composite patterns are identified, and it is suggested that wetter summers may be characterized by larger within-season variations in 500 mb heights and isobaric curvature compared with drier summers. Several 500 mb synoptic types associated with burst events are determined subjectively, and composite patterns are derived. These indicate that bursts occasionally occur with anticyclonic conditions aloft, but the latitude of the ridge seems critical in determining the extent of moist air penetration from the south.

Twice-daily aerological soundings at five stations in the South-west and northern Mexico are also used to determine the thermodynamic characteristics of bursts and breaks and the dominant source of monsoon moisture. Monsoon bursts in Arizona are evidently a combination of moisture from the Gulf of Mexico, transported on a light south-easterly flow, and more rapid lower-level ‘surges’ originating in the Gulf of California. Moisture from the latter source is transported rapidly into the Great Basin on a low-level wind-speed maximum of marked directional constancy. This study has implications for longer-term summer rainfall-synoptic circulation variations in the American South-west.

Synoptic-dynamic character of `bursts' and `breaks' in the South-West U.S. summer precipitation singularity

Summertime convection over Arizona typically begins in the early afternoon and continues into the night. This suggests that the evolution of the daytime planetary boundary layer is important to the development of Arizona convection. If numerical models are to provide useful guidance for forecasting convection during the monsoon, then the planetary boundary layer must be simulated as accurately as possible through utilization of the appropriate physical parameterizations. This study examines the most appropriate Pennsylvania State University‐National Center for Atmospheric Research fifth-generation Mesoscale Model (MM5) planetary boundary layer parameterization(s) for deterministic and ensemble modeling of the monsoon. The four MM5 planetary boundary layer parameterizations tested are the Blackadar, Burk‐Thompson, Eta, and medium-range forecast (MRF) schemes. The Blackadar and MRF planetary boundary layer schemes correctly predict the development of the deep, monsoon planetary boundary layer, and consequently do a better job of predicting the convective available potential energy and downdraft convective available potential energy, but not the convective inhibition. Because the convective inhibition is not accurately predicted, it is possible that the MM5’s ability to initiate or ‘‘trigger’’ convection might be a limiting factor in the model’s ability to produce accurate quantitative precipitation forecasts during the monsoon. Since the MM5 planetary boundary layer predicted by the Burk‐Thompson and Eta schemes does not accurately reproduce the basic structure of the monsoon planetary boundary layer, their inclusion in a mixed physics ensemble is discussed.

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The Sensitivity of the Numerical Simulation of the Southwest Monsoon Boundary Layer to the Choice of PBL Turbulence Parameterization in MM5

The diurnal variability of thunderstorm occurrence in Arizona is complex and related to terrain elevations. Generally, thunderstorms occur in the higher mountains during the afternoon with activity primarily of the nocturnal nature in the adjacent desert valleys, most noticeably the Phoenix area. Aircraft temperature probes have found cooling in cumulus areas. This cooling can frequently advect with the steering wind to destabilize the desert air mass and increase nighttime thunderstorms. The same destabilization process could contribute to nocturnal thunderstorms in the High Plains.

On the Relationship of Convective Cooling to Nocturnal Thunderstorms at Phoenix

Intense thunderstorms, which frequent the desert Southwest of the United States in the summer months, have been known by several different names: chubasco, haboob, and Sonora storm. Prior to the advent of satellites and radar, the sparsity of observations in the desert Southwest precluded any determination of where these storms developed, as well as information on their areal coverage and life cycle. A particularly severe and long-lasting thunderstorm occurred 19–20 August 1973. This storm was followed through its life cycle by means of radar, satellite, and surface observations. This particular storm was noteworthy for its very strong winds, locally heavy rain, and the magnitude of the pressure jump associated with it. The converging of two separate mesohighs is believed to be the cause of the very intense storm that moved westward across the Imperial Valley of Southern California.

A Severe Southwest Desert Thunderstorm: 19 August 1973

Tornado events of 1984 are reviewed. Significant and interesting aspects of the 907 reported tornadoes are noted. Synoptic patterns associated with four noteworthy tornado days are examined.

John E. Hales

Papers

On the Relationship of Convective Cooling to Nocturnal Thunderstorms at Phoenix

A Severe Southwest Desert Thunderstorm: 19 August 1973

The Tornado Season of 1984