CU researchers eye warmer ocean effect
Monsoons have moved later since the 1970s
Bill Scanlon, Rocky Mountain News
Published April 24, 2007 at midnight
New Mexico and Arizona have been getting wetter in August and September and drier in July in recent decades - and the likely culprit is the warmer Pacific Ocean temperatures.
That's the conclusion of Colorado researchers who studied the monsoons that bring rain to the Southwest, including parts of Colorado, each summer.
Blame more frequent El Niños, the phenomenon that brings warmer Pacific Ocean temperatures during certain years, says lead author Katrina Grantz, of the University of Colorado at Boulder.
The more frequent El Niños likely stem from global warming, according to the researchers.
Since the 1970s, the start, peak and end of the monsoons have moved later into the summer by about 15 days.
The information is vital because farmers and ranchers suffer when there's too little mid- summer rain, or when there are flash floods later in the summer.
Monsoons hit the southwestern United States each summer when the direction of the wind shifts from the west to the south, bringing moist air from the cold Pacific into contact with warmer ground air, said co-author Balaji Rajagopalan, also of CU.
The later monsoons haven't changed the overall rainfall numbers for New Mexico and Arizona, Grantz said. But as the winters grow wetter in the Southwest, the Pacific Northwest tends to have drier winters, which is causing problems in farming and the orchard industry there.
Grantz, a water resources engineer for the University of Colorado's Center for Advanced Decision Support for Water and Environmental Systems (CADSWES), studied monsoon trends from 1948 to 2004.
She wrote the paper, which appears in next month's issue of the American Meteorological Association's Journal of Climate, for her doctoral dissertation in engineering. She expects the information will help water managers with long-term forecasts of rain and stream flows.
Grantz, used RiverWare, a software tool developed by CADSWES and used by 75 water- management agencies in the Southwest. Along with Rajagopalan, she worked on the paper with CU's Martyn Clark and Edith Zagona.
scanlon@RockyMountainNews.com or 303-442-8729
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