El Nino Update

?EUR??,,????'??In early February the atmosphere finally started to react to the weak El Nino,?EUR??,,????'?? NOAA's Vernon Kousky said.

Was it El Nino or not? By the start of spring the National Weather Service reported that rainfall totals for Los Angeles had reached a 115-year high.

Many media sources have attributed the deluge to the so-called El Nino weather pattern, but experts say it's not that simple. NOAA-National Weather Service meteorologists have closely examined the ocean-atmosphere conditions, which influenced the recent California storms. Unlike the period from late December through early January, the latest period of heavy precipitation was related to two conditions: the ongoing weak, but diminishing, El Ni?????o in the tropical Pacific, and a blocking pattern over western North America.

"No two El Ni?????o's are exactly alike," said Vernon Kousky, lead El Ni?????o forecaster at NOAA Climate Prediction Center. "Weak El Ni?????o conditions have existed in the tropical Pacific since late last summer through mid-January 2005, with little effect on the global atmospheric circulation and precipitation patterns. However, in early February the atmosphere finally started to react to the weak El Ni?????o," he added. Precipitation over the warm waters of the west-central equatorial Pacific increased and became more persistent. By mid-February the Pacific jet stream responded by strengthening and extending eastward into the eastern North Pacific.

Also, a blocking pattern developed along the west coast of North America. Blocking refers to the situation when the normal eastward progression of the jet stream and weather disturbances is obstructed, which can lead to episodes of prolonged extreme weather conditions. The persistent weather extremes can last from several days up to a few weeks, often accompanied by significant temperature and precipitation variation. "These two features combined to give southern California its latest heavy precipitation event," Kousky said.

In contrast, NOAA scientists identified the Madden Julian Oscillation, or MJO, and blocking as likely culprits for January's severe winter weather in the West. The MJO produces alternating periods of El Ni?????o-like and La Ni?????a-like features that can influence precipitation events in the western United States. "The key difference between the two events is that the January's sequence of events lasted nearly three weeks while the most recent one lasted only about a week," said Kousky. "Though El Ni?????o conditions are expected to weaken during the remainder of the winter, NOAA scientists note that persistent convection near the date line may continue to influence the circulation and precipitation patterns over the eastern North Pacific during the next few weeks."