Southwesterners hoping for a strong monsoon could be in for a disappointment. Or not.

This article is the third installment of the Institute of the Environment's Drought Series.

By Stephanie Doster | July 11, 2006

On June 15, the National Oceanic and Atmospheric Administration's Climate Prediction Center (CPC) issued a monthly outlook indicating that rainfall through the summer has an equal chance of being below-average, near-average, or above-average. In other words, the jury is still out on what this year's monsoon will deliver.

Researchers at The University of Arizona and the University of New Mexico say La Niña conditions, like those that developed earlier this year, could signal a stronger monsoon. But, they warn, the forces that control the timing and strength of monsoons are not fully understood.

The monsoon arrived in Tucson on June 28 and in Phoenix on July 2. The average start date is July 3 and July 7, respectively. The monsoon is considered to have started in those cities when the dew point averages 54 degrees or greater in Tucson and 55 degrees or greater in Phoenix for at least three consecutive days. The average onset of the monsoon in New Mexico is around July 3 for the southwest corner of the state and around July 9 for the central Rio Grande valley, including Albuquerque, according to the National Weather Service. The monsoon began in Albuquerque on July 6.

The monsoon is a wind system that reverses its direction seasonally. In the North American Monsoon system, summer winds from the south bring moisture and rainfall to the Southwest.

"Whenever we have a prior winter that has La Niña conditions in the equatorial Pacific-colder-than-normal water-there is a weak tendency for the monsoon to have an earlier onset in southeast Arizona. And with an earlier onset, it means the odds of having a slightly wetter monsoon are a little higher," said Steven Mullen, department head and professor of atmospheric sciences at The University of Arizona. "It's probably the strongest signal anyone has uncovered to date, and it's an extremely weak signal and is open to debate."

Scientists observed relatively weak La Niña conditions earlier in the year.

But even a strong monsoon this summer likely would not alleviate the effects of roughly 10 years of drought conditions, researchers say. Winter rains and snowpack-both of which were scarce this winter and spring-are more important to replenishing aquifers than summer rains. Although the wet 2004-05 winter and spring helped replenish some southwestern reservoirs, some researchers surmise that a recent record-breaking dry spell, combined with a lingering drought, could once again strain water supplies. Also, researchers said, the summertime rains tend to be spotty, with some areas benefiting from a deluge while others remain bone dry.

Given the "equal chances" outlook for precipitation, David Gutzler, professor of earth and planetary sciences at the University of New Mexico, offered a few arguments in May for both a dry and a wet summer.

The case for a dry summer, Gutzler said, would come from two factors: the continuing drought and warming in the north tropical Atlantic Ocean, which, according to some studies, could influence summertime precipitation.

"As long as we're stuck in a prolonged, multi-year drought, a betting person might shave the odds toward dryness. We've had five to six years now of pretty dry conditions. That means we ought to consider that it might continue," Gutzler said. "Secondly, the Atlantic is warm like it was last summer."

If one prefers to be optimistic about rain, Gutzler suggested taking a look at other correlations, including one that he has examined: an inverse relationship between snowpack in the southern Rockies and northern Arizona and New Mexico, and summertime rainfall in the Southwest. That is, years of low snowfall should be followed by strong summer monsoons, and vice-versa.

Gutzler said researchers are trying to better understand the interplay between the monsoon and drought.

"The research just hasn't gotten far enough along to take these different predictors and pieces of the climate system and sort them out in terms of which is most important under which conditions," Gutzler said.

Gutzler is involved with the North American Monsoon Experiment (NAME), an international effort with scientists from the United States, Mexico, Belize, and Costa Rica. The goal of the 8- to 10-year experiment, which began in 2000, is to improve monsoon forecasts. Researchers have spent the past several years collecting data, primarily in Mexico, as part of the project. Several UA faculty members also are involved with NAME.

"This is the exciting scientific phase now because this is when we all are analyzing data sets and putting papers together," Gutzler said. "Over the next two years you are going to see a parade of scientific results based on the special observations taken during NAME."

David Gochis, a scientist with the National Center for Atmospheric Research (NCAR) and one of the principal investigators for NAME, said the project already has revealed interesting information about the monsoon and its rainfall patterns. That information should eventually lead to a more accurate weather forecast in the summer rainy season, and could potentially help people like farmers and ranchers make decisions about managing their land, water and livestock, Gochis said.

"We are gaining a much better understanding and appreciation for how day-to-day changes in the weather are related to the overall monsoon climate system," he said.