In this Section:

	What is PDO?
	What does it mean for the Southwest?
	How severe might the impacts be?
	For more information

Learn About SW Climate:
Pacific Decadal Oscillation "PDO"

What is the Pacific Decadal Oscillation?

There has been considerable press recently, arising from the annual meeting of the American Meteorological Society, about a climate phenomenon called the Pacific Decadal Oscillation (the "PDO"). While some climatologists believe we are moving into a different climate regime due to PDO conditions, others continue to reserve judgment. We probably won’t know for sure whether the shift has really happened until we have a decade's worth of data, but it’s not too early to begin thinking about the possible implications of such a shift, for the Southwest could be moving into a decade or longer dry phase.

PDO is not a ten-year La Niña. As with most things related to climate, it is more complicated than that. Basically, research has suggested that there is a fairly regular pattern of high and low pressure systems over the northern portions of the Pacific Ocean, off the coast of Alaska and Canada. The pattern operates at something on the order of a 20- to 30-year time scale, and correlates with relatively wetter or drier periods in the western portion of North America. Shifts in the PDO regime occurred in 1925, 1947, and 1977. Some climatologists believe that another shift began around 1995.

What does it mean for the Southwest?

Research conducted by Nate Mantua and colleagues at the University of Washington's Climate Impacts Group suggests that these PDO phases can combine with El Niño/La Niña conditions in certain ways to affect precipitation in the West, particularly in winter. As indicated below, the positive phase of the PDO tends to enhance El Niño conditions and weaken the effects of La Niñas, while the PDO negative phase can enhance the effects of La Niñas and weaken the effects of El Niños.

What this suggests for the Southwest is that, when the PDO is in its positive phase, as it has been since 1977, we tend to experience wetter El Niño winters, but relatively normal La Niña winters. This combination of climatic conditions may improve water supply because—even though more rain may actually fall in summer—it is winter precipitation that is typically responsible for most of the water replenishment in our region.

If, however, as some climatologists believe, conditions have shifted to the negative PDO phase, we may experience drier La Niña winters, and El Niño winters that reflect something closer to our long-term average precipitation. An extended period of, on average, drier than usual winters would likely produce a decrease in renewable water supplies. Thus, the onset of a negative PDO phase could cause the Southwest to experience generally drier conditions over the next several decades. How dry and for how long we cannot yet predict. Most of the region experienced a prolonged and severe drought during the 1950s, in the midst of the last PDO negative phase. If a drought of similar magnitude and duration occurred today, the consequences could be at least as severe.

How severe might the impacts be?

Decreases in winter precipitation associated with negative PDO conditions and intensified La Niña conditions could, for example, put stress on the urban water systems of the Southwest. The UA’s Climate Assessment Project for the Southwest (CLIMAS), operating under funding provided by the National Oceanic and Atmospheric Administration (NOAA), conducted an analysis of the effects of prolonged drought on the urban water supplies in the Phoenix and Tucson Active Management Areas (AMAs).

The research team assumed a ten-year drought of the magnitude of the one that occurred in the 1950s and demand levels projected by the Arizona Department of Water Resources for 2025. Even assuming full availability of Central Arizona Project (CAP) water, the Phoenix AMA could well exceed its renewable water supply by 39 percent. This constitutes a 15 percent increase over the 24 percent overdraft the AMA has already projected for 2025 in its recently completed Third Management Plan.

Likewise, even under normal climate conditions, the Tucson AMA expects to see a 15 percent groundwater overdraft in the year 2025. A ten-year drought comparable to that experienced in the 1950s could increase the overdraft by another 10 percent. In raw numbers, this would translate into an additional 3.78 million acre feet of groundwater for the Phoenix AMA over ten years, and an additional 380,000 acre feet for the Tucson AMA. These are significant amounts of water, especially given current infrastructure and water policy considerations in the two metroplexes.

Even moderately drier than normal conditions could have serious effects in some sectors. For example, CLIMAS researchers have documented that ranching operations in the Southwest are highly sensitive to climatic conditions and have already been affected by dry conditions of the past couple of winters. Forest fire management is another area crucially influenced by climatic conditions and trends.

Of course, exactly how extended dry conditions may affect specific individuals in the region will depend on where they live, what they do, and most especially what natural resources they depend upon. At this point, CLIMAS recommends keeping a watchful eye on the forecasts and on local conditions.

It’s not too early to begin thinking about what an extended dry period would mean. The region has experienced considerable growth and change in the past several decades, a time when conditions have been relatively wet. Assuming that these conditions will continue into the indefinite future flies in the face of everything we have learned about the ancient and recent climate history of the Southwest. We all should begin thinking about what the impacts of extended dry conditions might be, what viable alternatives exist for coping with various stresses that might arise, and what kinds of drought and other contingency plans exist—or should be developed now.

For more information contact:

Dan Ferguson
CLIMAS Program Manager
Phone: (520) 622-8918
Email: dferg@email.arizona.edu