El Nino expected to strengthen through fall

updated 1:38 p.m. ET Sept. 10, 2009

WASHINGTON - A weak El Niño is under way and is expected to strengthen and last into the early months of next year, U.S. government climate researchers said Thursday.

The periodic climate phenomenon is marked by warming of the central Pacific Ocean and changes in wind direction and air pressure that can affect weather around the world.

The presence of an El Niño can help to suppress Atlantic hurricane activity by increasing the vertical wind shear over the Caribbean Sea and tropical Atlantic Ocean. Wind shear, a sudden change in wind direction, tends to interrupt the rising air that helps fuel the storms.

National Weather Service forecasters said Pacific sea surface temperatures are running between 1.3 degree Fahrenheit and 1.8 degree Fahrenheit above normal.

Computer models indicate this could strengthen during coming months, the researchers at the Climate Prediction Center, a division of the weather service, said in a report.

"A majority of the model forecasts ... suggest El Niño will reach at least moderate strength during the Northern Hemisphere fall," they wrote. "Many model forecasts even suggest a strong El Niño during the fall and winter, but current observations and trends indicate that El Niño will most likely peak at moderate strength."

"Expected El Niño impacts during September-November 2009 include enhanced precipitation over the west-central tropical Pacific Ocean and the continuation of drier-than-average conditions over Indonesia," the experts added. "Temperature and precipitation impacts over the United States are typically weak during the Northern Hemisphere summer and early fall, generally strengthening during the late fall and winter."

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