DRI NEWS — Scientists at the Desert Research Institute are developing and testing new ideas that could ultimately help predict the kinds of extreme summer weather that has occurred across the U.S., such as the all-time high temperatures recorded in numerous Western cities in June, Arizona’s deadly wildfires and last year’s drought in the Midwest.
These new ideas, outlined in a new website (http://www.dri.edu/monsoon ) launched this week by DRI faculty, focus on a greater understanding of the North American monsoon (NAM) – a large-scale weather pattern that affects the American Southwest and Great Basin and has a strong impact on summer rainfall patterns and amounts over North America. The NAM supplies about 60 to 80-percent, 45-percent and 35-percent of the annual precipitation for northwestern Mexico, New Mexico and Arizona, respectively.
“Although regional climate models have succeeded in reproducing some features of the NAM, its onset, strength and regional extent are not well predicted,” said David Mitchell, an associate research professor in DRI’s Division of Atmospheric Sciences. “A physical understanding of key processes governing its life-cycle remains elusive.”
In a recent intercomparison of regional climate models, the lowest skill for predicting summer rainfall exists over the NAM region.
Mitchell adds that a physical understanding of the NAM is also needed to predict the NAM’s response to climate change and guide improvements in regional and global scale modeling of the NAM and its remote impacts on the summer circulation and precipitation patterns over North America.
Mitchell and his colleagues at DRI propose a new partial mechanistic understanding of the NAM by incorporating local and planetary scale processes that relates the Gulf of California and tropical eastern Pacific sea surface temperatures to the timing, amount and extent of NAM rainfall.
On the local scale, measurements and modeling demonstrate that relatively heavy summer precipitation in Arizona generally begins within several days after northern Gulf of California sea surface temperatures exceed 29 degrees Celsius. This rainfall is related to an inversion found approximately 50 to 200 meters above the surface of the Gulf of California. The inversion restricts moisture in the marine boundary layer from mixing with free air in the Earth’s troposphere for most of the year.
“The inversion weakens with increasing sea surface temperatures and ultimately disappears once the surface temperature exceeds 29 degrees Celsius,” Mitchell explains, “resulting in a deep, moist layer that can be transported by southerly winds inland to produce strong thunderstorms.”
This local scale mechanism may describe key factors affecting the Arizona monsoon low-level moisture flux. This appears to be the main low-level moisture source for NAM rainfall in Arizona, although the eastern Pacific Ocean can provide low-level moisture for summer thunderstorms in California and the Great Basin.
On the larger, planetary scale, new analyses of long-term trends in sea surface temperature, outgoing thermal radiation and high pressure centers over the NAM region support the hypothesis that sea surface temperatures greater than 27.5 degrees Celsius along western Mexico’s coastline are generally required for widespread deep convection to initiate in the NAM region.
As the warmer Pacific sea surface temperatures propagate northward up the Mexican coastline during spring and early summer months, deep convection initiates and advances the monsoon anticyclone (high pressure center) north during June and July. This evolution of the monsoon anticyclone brings mid-level tropical moisture into the NAM region and produces the thunderstorms that the NAM is well-known for with above average rainfall and lightning.
For more information on the new North American monsoon theory, corresponding evidence and links for tracking the evolution of the monsoon please visit http://www.dri.edu/monsoon