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Report Series, CL1-99
Paul R. Sheppard, Andrew C. Comrie, Gregory D. Packin, Kurt Angersbach, and Malcolm K. Hughes
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This paper summarizes the current state of knowledge concerning the climate of the Southwest. Low annual precipitation, clear skies, and year-round warm weather over much of the Southwest 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 variability. El Niño, which is an increase in sea surface tem-perature of the eastern equatorial Pacific Ocean with an associated shift of the active center of atmospheric convection from the western to the central equatorial Pacific, has a well developed teleconnection with the Southwest, usually resulting in wet winters. La Niña, the opposite oceanic case of El Niño, usually results in dry winters for the Southwest. Another important 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 major feature that sets climate of the Southwest 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 occurs 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 in.] while that of New Mexico is 340 mm [13.4 in.], and mean annual temperature of New Mexico is cooler (12°C [53°F]) than Arizona (17°C [62°F]). As instrumental meteorological records extend back only about 100-120 years throughout the Southwest, they are of limited utility for studying climate phenomena at the multi-decadal to century or longer time frames. Hence, there is a need to extend the measured meteorological 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 years or more in various forests of the Southwest, and integrate well the influences of both temperature and precipitation, 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 variable 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 precipitation 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 three occurrences of a multi-decadal variation of alternating dry (below average PDSI) to wet (above average PDSI). The amplitude of this multi-decadal variation seems to have increased since the 1700s. Should this pattern persist into the future, then perhaps the American Southwest will next enter an extended dry period. The most obvious feature of the temperature record is its current increase to an extent unprecedented in the last four hundred years. Because this warming trend is outside the variation of the natural archives, it is possible that anthropogenic impacts are playing a role in climate of the Southwest. Accordingly, this pattern merits further research in search of its cause or combination of causes.