Rating System Proposed For El Nino, La Nina

Friday, July 13th 2007, 12:45 pm

By: News On 6

WASHINGTON (AP) _ Hurricanes and tornadoes have popular rating scales that help people understand their power. Now, weather experts are planning a similar way to measure the El Nino phenomena that affect climate worldwide.

The ratings are tentatively expected to begin in the fall, Wayne Higgins, director of the federal Climate Prediction Center said in a telephone interview.

Higgins said his forecasters also are planning watches and advisories, as is currently done with other severe weather.

A watch would be issued when conditions are right for potential development of an El Nino or La Nina within three to six months, he said. An advisory would mean the condition was under way.

In recent years researchers have found that El Nino, a periodic warming of portions of the tropical Pacific Ocean, and its cool opposite, La Nina, can have significant impacts on weather around the planet.

The Climate Prediction Center, a division of the National Oceanic and Atmospheric Administration, monitors these conditions, known collectively as ENSO _ El Nino-Southern Oscillation.

The planned five-point scale would help assess the effects after the fact, he said, much as the Fujita scale is now used following a tornado.

The Saffir-Simpson hurricane scale is also used to assess damage after a hurricane, but it also is often cited as a storm approaches, using estimates based on wind speed to help people understand the imminent danger.

The 1-to-5 rating scales for twisters and hurricanes seem to be well understood, so a similar idea seemed logical for ENSO.

Higgins and researcher Vernon Kousky published a paper proposing the ENSO advisories and rating scale in the journal Weather and Forecasting earlier this year, and Higgins said the response from meteorologists has been enthusiastic.

``We realized that there was a need for something where we could provide users a heads-up on watches for El Nino and La Nina,'' he said. ``For the ENSO cycle we also wanted to develop an index of intensity scale that could be used for similar types of assessment after the fact.''

El Nino was first noticed by Peruvian fishermen _ it affects their catch. Indeed, it was named by those fishermen. El Nino is Spanish for little boy, a reference to the Baby Jesus, since the effects most often become noticeable around Christmastime.

The strength of an El Nino or La Nina varies by the warmth or coolness of the tropical Pacific sea surface, as compared to normal.

The ENSO Intensity Scale varies based on a a three-month temperature index, updated monthly. The scale varies from W1 to W5 for warm episodes and C1 to C5 for cool episodes, allowing climate researchers to develop comparisons with past ENSOs.

El Nino conditions last summer are believed to have moderated what many expected to be a severe hurricane season in the Atlantic. In general, El Nino contributes to more eastern Pacific hurricanes and fewer Atlantic hurricanes while La Nina contributes to fewer eastern Pacific hurricanes and more Atlantic hurricanes.

The ENSO status report issued by CPC on Monday says conditions are currently neutral, but there is a possibility of a change to La Nina within one to three months.

Effects of a La Nina can include:

_Colder than normal air over Alaska and western Canada, which often penetrates into the northern Great Plains and the western United States. The southeastern United States, on the other hand, becomes warmer and drier than normal.

_Decreased rainfall in the tropical Pacific in winter and spring, while rain increases over Indonesia, Malaysia and northern Australia and over the Philippines in summer.

_Wetter than normal conditions over southeastern Africa and northern Brazil during the northern winter season.

_During the northern summer season, the Indian monsoon rainfall tends to be greater than normal, especially in northwest India.

_Drier than normal conditions along the west coast of tropical South America, and at subtropical latitudes of North America such as the Gulf Coast, and South America from southern Brazil to central Argentina.

When the warm-water El Nino occurs the effects can include:

_Added rainfall in the normally arid coastal regions of Ecuador and Peru.

_Increased winter cloudiness and rainfall in the tropical Pacific but reduced rain in Indonesia, Malaysia and northern Australia.

_Drier than normal conditions over southeastern Africa and northern Brazil during the northern winter season.

_During the northern summer season, Indian monsoon rainfall below normal, especially in northwest India where crops are adversely affected.

_Wetter than normal conditions along the west coast of tropical South America and at subtropical latitudes of North America such as the Gulf Coast, and South America from southern Brazil to central Argentina.

_Stronger than usual low pressure systems bringing warm air into western Canada, Alaska and the extreme northern portion of the contiguous United States. Storms also tend to be more vigorous in the Gulf of Mexico and along the southeast coast of the United States resulting in wetter than normal conditions in that region.