The semiarid southwestern United States

Both the mechanisms and patterns of the climate in areas dominated by the subtropical high-pressure cells are not well documented. The inhospitable nature of these arid regions inhibits data collection, and yet the study of infrequent meteorological events requires a close network of stations maintaining continuous records over long periods. This difficulty is especially apparent in the interpretation of desert precipitation data, because much of the rain falls in local storms irregularly scattered in both space and time. The climatic conditions in the southwestern United States serve to exemplify this

Figure 10.22 The moisture balances at Berkeley, California, and Halifax, Nova Scotia.

Source: After Thornthwaite and Mather (1955).

Figure 10.22 The moisture balances at Berkeley, California, and Halifax, Nova Scotia.

Source: After Thornthwaite and Mather (1955).

Figure 10.23 The ratio of actual/potential evaporation for North America determined using the Thornthwaite/Mather (1955) methods. Source: From Thompson et al. (1999). Courtesy of the US Geological Survey.

climatic type, based on the more reliable data for the semi-arid margins of the subtropical cells.

Observations at Tucson (730 m), Arizona, between 1895 and 1957 showed a mean annual precipitation of 277 mm falling on an average of about forty-five days per year, with extreme annual figures of 614 mm and 145 mm. Two moister periods in late November to March (receiving 30 per cent of the mean annual precipitation) and late June to September (50 per cent) are separated by more arid seasons from April to June (8 per cent) and October to November (12 per cent). The winter rains are generally prolonged and of low intensity (more than half the falls have an intensity of less than 5 mm per hour), falling from altostratus clouds associated with the cold fronts of depressions that are forced to take southerly routes by strong blocking to the north. This occurs during phases of equatorial displacement of the Pacific subtropical high-pressure cell. The reestablishment of the cell in spring, before the main period of intense surface heating and convectional showers, is associated with the most persistent drought episodes. Dry westerly to southwesterly flow from the eastern edge of the Pacific subtropical anticyclone is responsible for the low rainfall in this season. During one twenty-nine-year period in Tucson, there were eight spells of more than 100 consecutive days of complete drought and twenty-four periods of more than seventy days. The dry conditions occasionally lead to dust storms. Yuma records nine per year, on average, associated with winds averaging 10-15 m s-1. They occur both with cyclonic systems in the cool season and with summer convective activity. Phoenix experiences six to seven per year, mainly in summer, with visibility reduced below 1 km in nearly half of these events.

The period of summer precipitation (known in Arizona as the summer 'monsoon') is quite sharply defined. The southerly airflow regime at the surface and 700 mb (see Figures 7.4 and 7.9) often sets in abruptly around 1 July and is therefore recognized as a singularity. Figure 10.24 shows that southeastern Arizona and southwestern New Mexico receive over 50 per cent of their annual rainfall during July to September. Further south over the Sierra Madre Occidentale and the southern coast of the Gulf of California, this figure exceeds 70 per cent. The American southwest forms only the northern part of the area of the Mexican or North American monsoon.

Precipitation occurs mainly from convective cells initiated by surface heating, convergence or, less commonly, orographic lifting when the atmosphere is destabilized by upper-level troughs in the westerlies. These summer convective storms form in mesoscale clusters, the individual storm cells together covering less than 3 per cent of the surface area at any one time, and persisting for less than an hour on average. The storm clusters move across the country in the direction of the upper-air motion. Often their motion seems to be controlled by low-level jet streams. The airflow associated with these storms is generally southerly along the southern and western margins of the Atlantic (or Bermudan) subtropical high. The moisture at low levels in southern Arizona is derived mainly from the Gulf of California during 'surges' associated with the south-southwesterly low-level Sonoran jet (850 to 700 mb). Moisture from the Gulf of Mexico reaches higher elevations in Arizona-New Mexico with southeasterly flows at 700 mb.

Precipitation from these convective cells is extremely local (see Plate 11), and is commonly concentrated in the mid-afternoon and evening. Intensities are much higher than in winter, half the summer rain falling at more than 10 mm per hour. During a twenty-nine-year period, about a quarter of the mean annual precipitation fell in storms giving 25 mm rain or more per day. These intensities are much less than those

Figure 10.24 The contribution (per cent) of JAS precipitation to the annual total in the southwestern United States and northern Mexico. Area greater than 50 per cent tinted and greater than 70 per cent hatched.

Source: After M.W. Douglas et al. (1993, p.1667, fig. 3). Courtesy of the American Meteorological Society.

associated with rainstorms in the humid tropics, but the sparsity of vegetation in the drier regions allows the rain to produce flash-floods and considerable surface erosion.

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  • Jennifer Mills
    How is the climate in the southwestern states?
    5 years ago

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