NOAA CATCHES A CULPRIT BEHIND WESTERN STORMS
Jan 12, 2005 ó Just as NOAA Weather Service meteorologists exhale after a very grueling two weeks of severe winter weather in the western and central United States, NOAA scientists have identified one of the leading causes. "A leading climate culprit is the Madden-Julian Oscillation or MJO," said Wayne Higgins, lead climate specialist at the NOAA Climate Prediction Center located in Camp Springs, Md. (Click NOAA image for larger view of weather patterns known as the “pineapple express" where a significant amount of the deep tropical moisture traverses the Hawaiian Islands on its way towards the western United States. Click here for high resolution version, which is a large file. Please credit “NOAA.”)
The MJO is a tropical disturbance that influences the patterns of tropical rainfall on time scales of approximately 30-60 days. It produces El Niño-like features that can influence extreme precipitation events in the western United States. Typically, the MJO is most active during El Niño-neutral and weak-El Niño winters.
"Presently, we are in a weak warm-episode (El Niño)," said Vernon Kousky, NOAA's lead El Niño/La Niña scientist. He added, "El Niño conditions are evident in the tropical Pacific and these conditions are expected to continue for the next three months."
During the last half of December, the MJO strengthened as enhanced cloudiness and precipitation over the Indian Ocean shifted eastward. (This activity was not related to the tsunami that occurred late in December.) By early January 2005, the precipitation associated with the MJO extended into the western tropical Pacific.
"As the tropical rainfall associated with the MJO shifted eastward towards the Central tropical Pacific, the jet stream over the North Pacific gradually shifted eastward towards the California coast," said Higgins. "This allowed recent storms to tap a deep-tropical moisture stream that dramatically increased the precipitation over California," Higgins added.
The typical scenario linking the pattern of tropical rainfall associated with the MJO to extreme precipitation events along the West Coast of the United States is shown in a NOAA graphic. This graphic depicts changes in large-scale circulation and moisture patterns that occur over the 10-day period leading up to heavy precipitation events along the West Coast of the United States. "The scenario shown in the schematic is exactly what we observed over the past two weeks," said Higgins. These events can bring up to several days of heavy rain and possible flooding. They are often referred to as "pineapple express" events, so named because a significant amount of the deep tropical moisture traverses the Hawaiian Islands on its way towards the western United States.
NOAA meteorologists use a variety of data and analysis techniques to detect the MJO. Of primary importance is information derived from NOAA's polar-orbiting and geostationary satellites, which are used to identify regions of heavy tropical precipitation. A second fundamental data source is the global radiosonde network, which provides crucial information regarding the atmospheric winds, temperature, moisture and pressure fields at many levels of the atmosphere. These data are taken twice daily and used in dynamical forecast models for weather and climate prediction.
Yet, due to its slowly evolving nature, accurate prediction of the MJO is difficult beyond a week or two. "Good forecasts are related to our ability to monitor the feature and to determine its relative position and strength," Higgins said. Dynamical models generally do not predict the MJO well because of difficulties that remain regarding the correct mathematical treatment of tropical rainfall processes. Because of this, it is difficult to predict whether the MJO will remain active during the remainder of the winter and what influences this might have on USA weather patterns. "It is important to emphasize that these are relatively rare events and that not all MJO events will lead to precipitation extremes in the western United States," said Higgins.
Though the MJO is a naturally occurring component of the coupled ocean-atmosphere system, the basic reasons that explain why it is active in some seasons and not in others are elusive and are the subject of ongoing research.
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