The University of Arizona

Regional Variability

By Melanie Lenart | The University of Arizona | September 14, 2008

Human-induced climate change makes its mark on an ever-shifting background of natural climate variability. More so than temperature, precipitation rises and falls in sync with a variety of factors. The variability of precipitation involves how much rain or snow falls in a given season or year compared to previous seasons or years.1 A few of the most important influences on precipitation variability in the Southwest are:

Graph of precipitation trends in AZ and NM

Figure 1. Average monthly precipitation for Arizona and New Mexico.
| Enlarge This Figure |
Credit: Western Regional Climate Center

  • The summer monsoon, which reigns from about late June through mid-September
  • Remnant tropical storms from the East Pacific hurricane season, which runs through summer and autumn
  • El Niño fluctuations, particularly influential on winter and spring patterns
  • The Pacific Decadal Oscillation (PDO), which influences winter precipitation patterns for decades at a time

The summer monsoon

The spring months leading up to the monsoon tend to be the driest time of year in the Southwest. New Mexico’s driest months include March and April, while Arizona’s are May and June (Figure 1).

During these months, average rainfall hovers around half an inch or less. In some years, entire months can pass with no measurable rainfall. The dry heat of spring helps create conditions that usher in the summer monsoon.

During the local monsoon, winds from the south bring clouds and associated humidity to the Southwest from three major sources: the Gulf of Mexico, the Gulf of California, and the Pacific Ocean.

Sea surface temperatures in each of these regions can vary from year to year. Subtle variations in these temperatures and their influence on local highs and lows in air pressure combine in myriad ways that make monsoon rainfall variable from year to year.

In the Southwest region subject to the North American Monsoon, average seasonal rainfall varies greadtly, ranging from less than 2 inches in 1924 in to almost 14 inches in 1964.

Although monsoon variability remains challenging to predict in time and space, recent advances are showing improvements in seasonal predictions of how this summer rainfall pattern will fare in relation to typical seasons.

It remains difficult, however, to predict how the monsoon will respond to changing climate. The heating of land and sea could influence this climate pattern over the long term, although no overall trend in monsoon precipitation has been detected so far.

Satellite image of Hurricane Hernan in the Pacific Ocean

Hurricane Hernan off the coast of Baja California, Mexico on September 2nd, 2002.
Credit: NASA

Tropical storms

Remnant hurricanes and other tropical storms, mainly from the East Pacific, also contribute to summer and fall rainfall tallies.

Interested in the hurricanes and tropical storms in the Southwest? Read CLIMAS' articles East Pacific hurricanes bring rain to the Southwest and Hurricane intensity rises with sea surface temps.

Albuquerque received about 20 inches of rain and Tucson saw about 12 inches from remnant tropical storms passing over the Southwest between 1992 and 2004, according to analyses by University of Arizona climatologist Elizabeth Ritchie. This amounts to a typical year’s rainfall for Tucson and two year’s average rainfall for Albuquerque.

While hurricane intensity appears likely to increase as sea surface temperatures rise, projections also suggest the frequency and paths of these tropical cyclones could change in ways that are not fully predictable.2 So, as with many other climate factors, debate continues over how climate change will affect hurricanes and their impact on the Southwest.

Along with climate change and other factors, hurricane variability can be influenced by El Niño. Known more in the Southwest for its influence on winter precipitation, El Niño also tends to suppress the formation of hurricanes in the Atlantic while promoting their formation in the East Pacific Ocean. Most tropical storms funneling moisture into the Southwest come from the Pacific Ocean, but eastern New Mexico sometimes receives rainfall from Atlantic storms moving inland from the Gulf of Mexico.

El Niño

The El Niño pattern originates in the tropical Pacific and extends its reach through many corners of the globe, including the Southwest. When waters in the eastern Pacific Ocean warm, the subtropical jet stream is more likely to dip south and bring precipitation into the Southwest (Figure 2).

Illustration of El Niño and La Niña circulation patterns

Figure 2. El Niño and La Niña patterns bring different winter precipitation and climate patterns to North America.
| Enlarge This Figure |
Credit: Smithsonian Institution

The opposite climate pattern, known as La Niña, occurs when waters in the eastern tropical Pacific are cool. In that case, the jet stream tends to stay further north, delivering precipitation to the northwestern rather than southwestern United States (Figure 2).

Atmospheric variations accompany both patterns involved in these ocean temperature changes. The longer name for this fluctuating pattern, the El Niño Southern Oscillation (ENSO), acknowledges the airborne oscillations in wind and pressure patterns.

El Niño events tend to promote more precipitation in spring and winter in the Southwest, while La Niña events tend to bring drier conditions. If an El Niño event continues to pull the jet stream south in the summer, however, it can interfere with the expansion of the tropical conditions that usher in the monsoon.

El Niño has proven to be one of the most reliable climate patterns for making seasonal predictions. Like many climate patterns, though, El Niño is subject to change in time and space, particularly as global warming alters climate. Its future will have implications for the Southwest and the rest of the world.

Global climate models have progressed in their ability to simulate El Niño patterns, but challenges remain. Because El Niño’s influence extends across the globe, it’s particularly important to assess its future changes.

“In many regions far from eastern equatorial Pacific, accurate projections of climate change in the 21st Century depend upon the accurate projections of changes to El Niño,” stated a 2008 report by the U.S. Climate Change Science Program.3

Along with precipitation, El Niño fluctuations also can influence temperature in the Southwest and around the globe. These fluctuations and other climate patterns add variability to the global trend toward increasing temperatures.

The Pacific Decadal Oscillation (PDO)

Another ocean temperature pattern in the Pacific Ocean, dubbed the Pacific Decadal Oscillation (PDO) impacts southwest climate at the decadal scale. Along with other longer-term climate patterns, it helps explain why southwestern climate often lingers in drought mode for decades at a time.

An analysis of the instrumental record suggests the PDO relates in roughly equal parts to three climatic factors that account for its variations at the decadal scale:4

  • fluctuations between El Niño and La Niña
  • changes in the atmospheric low-pressure pattern known as the Aleutian Low
  • changes in the Kuroshio-Oyashio current that swirls through the northern Pacific Ocean

The PDO pattern tends to influence drought throughout the West, along with the Atlantic Multidecadal Oscillation (AMO), another variation that has been observed in the Atlantic Ocean.5

These patterns of climate variability can alternately moderate or exacerbate global warming and related climate changes. Either way, they’re sure to keep precipitation fluctuating to some degree in the Southwest.

References

  1. Lenart, M., et al. 2007. Global warming in the Southwest: projections, observations and impacts. University of Arizona, Climate Assessment for the Southwest, Tucson, Arizona.
  2. Arblaster, J., et al. 2007. Summary for policymakers. In Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M.Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.
  3. Bader, D.C., et al. 2008. Climate models: An assessment of strengths and limitations. A Report by the U.S. Climate Change Science Program and the Subcommittee on Global Change Research. Department of Energy, Office of Biological and Environmental Research, Washington, D.C.
  4. Schneider, N. and B.D. Cornuelle. 2005. The forcing of the Pacific decadal oscillation. Journal of Climate, 18:4355–4373.
  5. McCabe, G.J., M.A. Palecki and J.L. Betancourt. 2004. Pacific and Atlantic Ocean influences on multidecadal drought frequency in the United States. Proceedings of the National Academy of Sciences, 101(12):4136–4141.