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ISPE Faculty Spotlight
Christopher Castro: Demystifying the monsoon
August 3, 2007
By Stephanie Doster
Christopher Castro
It is one of the Southwest's most anticipated and embraced—yet seemingly unpredictable—events of the year.
It's the North American monsoon, the replenishing and refreshing thunderstorm season that supplies Tucson with up to 50 percent of its annual precipitation and quenches the desert's palpable thirst each summer.
But the monsoon is as variable as it is welcome, spurring Christopher Castro to explore the dynamics behind it and, in the process, dismantle the rainy season's reputation as unpredictable. An assistant professor of atmospheric sciences at The University of Arizona, Castro has produced a framework for understanding the onset of the monsoon from year to year and determining its initial potential payload—a lot or a little bit of rain. The results of his research, to be published in the August 2007 volume of the Journal of Climate, shed new light on the predictability of summer climate on seasonal timescales and provide a foundation for improved seasonal forecasting.
The monsoon largely drives the summer climate in the Southwest and northern Mexico from July into September. The day it begins—its onset date—is important to water resource planning, farming and ranching, emergency management, and a number of other sectors that are closely tied to rainfall. Typically, the earlier the onset, the wetter the season will be, Castro said.
"Knowing that information is very important, especially in Mexico, where agriculture and residents are dependent on monsoon rainfall for their water supplies and subsistence. That's a big deal. That area of the world is very vulnerable," he said.
Castro's work at UA stems from his days at Colorado State University, where he completed his PhD dissertation under Roger Pielke Sr., an expert in mesoscale atmospheric modeling. Using a regional climate model, Castro crunched 53 years of climate data, from 1950 through 2002, to generate a detailed picture of the average summer climate conditions and variability of the contiguous United States and Mexico. Using such a record to investigate North American summer climate had never been done before, Castro said.
A regional climate model essentially is a smaller-scale version of an existing global climate model that offers a big picture view about global climate processes, like large-scale circulation patterns. Castro showed that use of a regional climate model improved the representation of summer climate because, unlike the global model, it provides a much better representation of warm season rainfall in areas like the southwestern and central U.S. "When you look at the rainfall products from these [global] models or analyses, they don't get a monsoon," Castro said.
Next, Castro studied the interannual variability in the regional climate model results. The study confirmed that variability in summer climate fundamentally is tied to Pacific Ocean sea surface temperatures. Processes associated with cooler Pacific sea surface temperatures accelerate the monsoon's onset; warmer sea surface temperatures tend to point to a delayed start.
"The model not only gets the climatology right, it gets the interannual variability right, too," Castro said. "It's a big deal because some scientists say there is no predictability in summer climate on seasonal timescales. This shows that that probably isn't correct."
The study also indicated that climate change may be forcing some summer rainfall changes.
"One of the most important, and surprising, findings is that Mexico may be currently experiencing the effects of anthropogenic climate change more acutely than Arizona," Castro said. The model findings suggest that rainfall has decreased in Sonora and Sinaloa over the past twenty years, and similar trends exist in observations. Those findings could have profound implications for that region, which is economically and socially vulnerable to rainfall deficits.
Castro has applied for a grant to explore applying his work to forecasting what the weather may do over a season. He also wants to pursue research into how climate change will likely influence summer rainfall.
"There are still many major questions and uncertainties," he said.
Castro also participates in the North American Monsoon Experiment (NAME). NAME is an extensive and ongoing research project designed to investigate the summer monsoon through a network of observing stations in northwestern Mexico and the southwestern United States. From June through September 2004, scientists collected data in the region to get a detailed climate picture of a single season. With that stage of the experiment complete, the observing network was largely dismantled, but Castro would like to use a regional model to retrospectively simulate the 2004 season. Such work would help to determine where observing points would be useful in a sustained climate and weather network.
"That's important because we can use that information to design a long-term observing system that would be a benefit for the U.S. and Mexico," Castro said. "Doing that is in the national interest of both countries. What goes on down there in terms of weather data is important to getting the forecast right for five to ten million people in the U.S."