14 September 2006 Weak El Niño conditions could explain less active Atlantic hurricane season Washington -- Scientists at the U.S. National Oceanic and Atmospheric Administration (NOAA) Climate Prediction Center reported September 12 that El Niño conditions have developed in the tropical Pacific and are likely to continue into early 2007. The term El Niño refers to the large-scale ocean-atmosphere climate phenomenon linked to a periodic warming in sea-surface temperatures across the central and east-central equatorial Pacific. Ocean temperatures increased remarkably in the equatorial Pacific during the last two weeks, according to a NOAA press release. El Niño is the warm phase of the El Niño/Southern Oscillation (ENSO) cycle and is sometimes called a Pacific warm episode. El Niño originally referred to an annual warming of sea surface temperatures along the west coast of tropical South America. The cool phase of ENSO is La Niña, a periodic cooling of surface ocean waters in the eastern tropical Pacific. These conditions affect weather patterns around the world. "Currently, weak El Niño conditions exist,” said Vernon Kousky, NOAA's lead El Niño forecaster, “but there is a potential for this event to strengthen into a moderate event by winter." Some impacts from the developing El Niño already are evident in the pattern of tropical precipitation. During the last 30 days, drier-than-average conditions have been seen across all of Indonesia and Malaysia and most of the Philippines, areas that are usually first to experience ENSO-related impacts. The dryness probably will continue for the rest of 2006. HURRICANE IMPACTS The development of weak El Niño conditions helps explain why this Atlantic hurricane season has been less active than forecasters expected. (See related article.) El Niño typically suppresses hurricane activity by increasing vertical wind shear over the Caribbean Sea region. Vertical wind shear is the magnitude of wind change with height. Hurricanes need low values of vertical wind shear between the surface and the upper atmosphere. High vertical wind shear can disrupt a tropical cyclone trying to form by literally tearing it apart. High wind shear also can weaken or destroy a healthy tropical cyclone by interfering with the organization of deep convection – the transfer of potential energy, for example heat, by wind currents in the cyclone -- around the cyclone center. But at this time El Niño impacts on Atlantic hurricanes are small. "We are still in the peak months of the Atlantic hurricane season,” said Gerry Bell, NOAA's lead seasonal hurricane forecaster, “and conditions remain generally conducive for hurricane formation." Typical El Niño effects are likely to develop over North America during the upcoming winter season. They include warmer-than-average temperatures over western and central Canada, and over the western and northern United States. The full text of the press release is available on the NOAA Web site. (Distributed by the Bureau of International Information Programs, U.S. Department of State. Web site: http://usinfo.state.gov)El Niño Developing in Tropical Pacific Could Continue into 2007