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Christopher Castro
In pursuit of the Southwest's monsoonToward the end of June in the U.S. Southwest, many people begin to watch the skies in the afternoon. They’re eagerly awaiting the first rains of the summer monsoon season, a period of several months that brings much-needed moisture and relief from the scorching heat of the early summer. When he first saw those spectacular thunderclouds and torrential rains in Ajo, Arizona, Christopher Castro was a 15-year-old from Davis, California, who intended to be a lawyer when he grew up. Now he’s a professor in the University of Arizona's Atmospheric Science Department, and he’s working to forecast the summer downpours. "The monsoon phenomenon directly affects about 20 million people in all sorts of ways—agriculture, wildfires, you name it," Castro says. "If you can come up with a better way to forecast the monsoon, that is of significant interest." A very wet seasonThe monsoon season accounts for a significant portion of the year’s precipitation in northern Mexico and the U.S. Southwest, averaging from over 2.5 inches in Phoenix—more than a third of the annual total—to almost 52 inches in Acapulco, about 95% of that city’s yearly rainfall. The dark side of all this water is millions of dollars per year of property damage and sometimes deaths from flooding, especially if the rains are fueled by a tropical storm in the eastern Pacific. Although most of us immediately think of rain when we hear the word monsoon, the term actually means a seasonal shift in winds. In North America, the prevailing winds from the west or north change to southerly and southeasterly. These flows carry moisture inland from the Pacific Ocean, Gulf of California, and Gulf of Mexico. The complex topography of the region means that rainfall is not uniformly distributed, even at the same latitude; mountainous terrain tends to receive the lion’s share, while some unlucky places like Yuma, Arizona, traditionally stay parched.
From computer models to field campaignsWhile he was in graduate school at Colorado State University, Castro used computer modeling to study the effects of Pacific sea surface temperatures on the North American monsoon. He and his colleagues found that cold sea-surface temperatures in the central and eastern equatorial Pacific typically correlate with an early monsoon and heavy rains in the southwestern United States and dry, hot weather in the Great Plains. Warmer sea-surface temperatures, such as are found in El Niño years, tend to have the opposite effect. The article reporting their findings is available online from the Journal of Climate. Castro got a look at the Mexican side of the monsoon in 2004, when he took part in the field campaign of the North American Monsoon Experiment, or NAME, as a rawinsonde (weather balloon instrument) operator in Mochis, Mexico. His bilingual skills were also put to use as a site translator. NAME, sponsored by the U.S. National Oceanic and Atmospheric Administration and Mexico’s weather service, collected data in both countries and over the Pacific with the goal of improving monsoon forecasting. Castro has since been appointed to the NAME Science Working Group. When Castro completed his Ph.D. in 2005, he chose the University of Arizona because of his interest in the local climate, and also as a place where his bilingual ability and cultural sensitivity would be useful. Monsoon forecasting makes newsIn June 2007, Castro’s prediction of an early monsoon season with heavy rains got him on the front page of Phoenix’s newspaper, the Arizona Republic. This prediction wasn’t only based on observations of cool sea-surface temperatures. Castro explains, "In the Southwest, the winter behaves in a totally opposite way than the summer does, so when we have a dry winter it’s typically followed by a wet summer." Arizona had an extremely dry winter in 2006–07, continuing a string of some of the driest winters on record. How did the season play out? "Aside from the fact that the onset date was normal to a bit delayed, everything else that has happened this summer pretty much fits this pattern," Castro says. "It has been wet in the Southwest and dry and hot in the Dakotas and eastern Montana, with temperatures regularly topping one hundred degrees [Fahrenheit] there in August." The path toward better predictionAlong with his teaching duties, Castro has been putting out proposals to continue his research in several directions. Following up on his graduate school computer modeling, he would like to do retrospective simulations of the NAME field campaign year to address questions about the physical mechanisms of rainfall and help in the design of a long-term observing system in the United States and Mexico. Another proposal is to develop a useful regional version of the seasonal forecasts from the National Centers for Environmental Prediction’s global model, a process known as downscaling."If you look, for example, at the maps of the warm season three months out, there’s virtually no skill, particularly for precipitation," he says. "I believe that a regional model will better represent the processes that lead to summer rainfall."
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