If two young researchers at the Desert Research Institute are right, people in Arizona and northwestern Mexico may one day know in advance how much rainfall will nourish their fields and fill their lakes and reservoirs.

When the Mexican monsoon arrives in Arizona, it brings flash floods, high winds, hail, and an occasional tornado. It also brings 40 percent of the state's annual rainfall-and 60 to 80 percent of northwestern Mexico's yearly rainfall. The monsoon has a profound impact on the region, affecting whether crops thrive or die, businesses expand or contract, tourism grows or shrinks.

While the impact is known, the factors governing the strength and length of the monsoon season are not. Working to discover them are Drs. David Mitchell and Tim Brown. Like all good scientists, they started with a hypothesis: the Mexican monsoon is born in a warm pool of water off southern Mexico.

To test and expand their theory, they took advantage of Brown's impressive ability to find, filter, and make sense of stupendous quantities of data. "I accessed huge global data sets from National Oceanic and Atmospheric Administration (NOAA) and NASA," Brown recalls, with the delight of someone clearly at home in cyberspace. "It was like having the world at my fingertips."

One of the data sets included ocean surface temperatures, that incredibly, date back to 1854! From those records -200 million observations compiled by captains of sailing ships, steamboats, and modern-day vessels-Brown culled recent monthly averages of sea surface temperatures off southern Mexico and in the Gulf of California.

Next, he sifted through satellite data on outgoing long-wave radiation (as a measure of thunderstorm cloud coverage); wind fields from upper-air balloons, ocean buoys, and ships; and precipitation measured across northern Mexico. Then, he used all those numbers to create visual images he and Mitchell could analyze to discover how the air, sea and land interact in the region. Because they were looking at pictures no one had drawn before, they saw things that no one had seen before.

Mitchell describes what they saw: "In May, the sun's heat forms a large pool of warm water off southern Mexico. Some mechanism, perhaps a coastal current or solar heating, causes the warmest water to migrate up the coast, beginning in late May or June. As the tongue of warm water moves up the coast at a rate of 800 miles a month, it appears to steer the prevailing winds southwest to southeast.

In July, the winds are aligned with sea surface temperature gradients, picking up the maximum amount of moisture from the warmest water, and blowing parallel to the coast up the Gulf of California. Surface winds and low pressure may draw the moisture-laden air inland, where it explodes in thunderstorms over the deserts of northern Mexico and the U.S. Southwest."

From this picture, they theorize the temperature and strength of the coastal current may regulate atmospheric processes, which in turn may determine the strength of the monsoon season.

"The sea surface temperature and the direction and force of the winds are critical," Mitchell explains. "Wind blowing over warmer water picks up more moisture, contributing to stronger thunderstorms."

Mitchell and Brown's theory about the Mexican monsoon has piqued the interest of other scientists, including those researching drought conditions in the U.S. Midwest. (At a recent confer-rence on climate change, three poster presentations showed evidence correlating a strong Mexican monsoon with drier Midwest summers.) "Other studies have looked at monsoons as mainly meteorological phenomena," Brown explains. "Our approach is broader, looking at how air, sea and land interact."

Donations to DRI's Annual Fund campaign helped sponsor their initial research. Now the pair hopes national scientific agencies will fund proposals to further pursue their unique approach.

If those funds come through, it may mean that one day, people in Arizona and northwestern Mexico will look to the warm pool off southern Mexico for rainfall predictions instead of the Farmer's Almanac.
-Cindy Kimball