El Nino Pacific Warming Means Mild 2009 Atlantic Hurricane Season

El Nino, the periodic warming of central and eastern tropical Pacific waters, occurs on average every two to five years and typically lasts about 12 months. The most recent previous El Nino occurred in 2006.

Sea surface temperatures along the equatorial Eastern Pacific, as of July 1, are at least one degree above average — a sign of El Nino, according to NOAA's National Weather Service Climate Prediction Center.

NOAA expects this El Nino to continue developing during the next several months, with further strengthening possible. The event is expected to last through winter 2009-2010.

"Advanced climate science allows us to alert industries, governments and emergency managers about the weather conditions El Nino may bring so these can be factored into decision-making and ultimately protect life, property and the economy," said NOAA Administrator Jane Lubchenco.

This NASA image of the the 1997-98 El Nino event shows warmer than normal ocean temperatures in red and cooler than normal temperatures in blue. (Image courtesy NASA/Goddard Space Flight Center)

El Nino's impacts depend on a variety of factors, such as intensity and extent of ocean warming, and the time of year.

Contrary to popular belief, not all effects are negative, Lubchenco said. On the positive side, El Nino can help to suppress Atlantic hurricane activity.

"El Nino can help to suppress Atlantic hurricane activity by increasing the vertical wind shear over the Caribbean Sea and tropical Atlantic Ocean," the Climate Prediction Center said Thursday.

In the United States, it typically brings beneficial winter precipitation to the arid Southwest, less wintry weather across the North, and a reduced risk of Florida wildfires.

El Nino's negative impacts have included damaging winter storms in California and increased storminess across the southern United States. Some past El Ninos also have produced severe flooding and mudslides in Central and South America, and drought in Indonesia.

The Climate Prediction Center says, "Expected El Nino impacts during July-September 2009 include enhanced precipitation over the central and west-central Pacific Ocean, along with the continuation of drier than average conditions over Indonesia." Weaker trade winds are also expected.

These vast rainfall patterns in the tropics are responsible for many of El Nino's global effects on weather patterns.

"Temperature and precipitation impacts over the United States are typically weak during the Northern Hemisphere summer and early fall, and generally strengthen during the late fall and winter," the CPC forecasts.

An El Nino event may significantly diminish ocean productivity off the west coast by limiting weather patterns that cause upwelling, or nutrient circulation in the ocean, Lubchenco said, adding, "These nutrients are the foundation of a vibrant marine food web and could negatively impact food sources for several types of birds, fish and marine mammals."

In its monthly El Nino diagnostics discussion for July, scientists with the Service Climate Prediction Center noted weekly eastern equatorial Pacific sea surface temperatures were at least 1.0 degree Celsius above average at the end of June.

NOAA will continue to monitor the rapidly evolving situation in the tropical Pacific, and will provide more detailed information on possible Atlantic hurricane impacts in its updated Seasonal Hurricane Outlook scheduled for release on August 6, 2009.

Copyright Environment News Service, ENS, 2009. All rights reserved.