Regional Variability

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

The imprint of climate change makes its mark on the ever-shifting influence of natural climate variability. More so than temperature, precipitation rises and falls in synch with a variety of factors, many of which are also subject to change with climate.¹

The variability of precipitation involves how much rain or snow falls in a given season or year compared to previous seasons or years. A few of the most important influences on variability in the Southwest are:

• 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, along with the contrast between lukewarm seas and heated land, 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 varied by threefold, ranging from 3 inches in 1973 to 9.4 inches in 1990. The 52-year record compiled by climatologist Andrew Ellis, which ended in 2001, defined the monsoon season onset based on the moisture content of the atmosphere.

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.

Tropical storms

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

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.

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 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. Basically, when warm water pools in the eastern Pacific Ocean off the coast of Peru, the subtropical jet stream is more likely to dip south and bring precipitation into the Southwest (Figure 2).

The opposite climate pattern, known as La Niña, occurs when warm water pools closer to Australia. 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 sea changes. The longer name for this fluctuating pattern, the El Niño Southern Oscillation (ENSO), acknowledges the airborne oscillations in wind patterns.

El Niño events tend to promote precipitation in spring and winter in the Southwest, while La Niña events tend to bring drier conditions. If an El Niño event keeps the jet stream pulling 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.

What’s more, El Niño fluctuations contribute to a longer-lasting climate pattern that has been strongly linked to southwestern drought.

The Pacific Decadal Oscillation (PDO)

A pattern related to El Niño, the Pacific Decadal Oscillation (PDO) registers its imprint 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.