It says the Pineapple express is caused by an event called the Madden-Julian oscillation.
It reminds me of being tossed to and fro by the waves.
The MJO is also known as the 30-60 day oscillation, 30-60 day wave, or intraseasonal oscillation.
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Pineapple Express events
The Pineapple Express, an MJO effect on North American weather patterns.The typical scenario linking the pattern of tropical rainfall associated with the MJO to extreme precipitation events in the Pacific Northwest features a progressive (i.e. eastward moving) circulation pattern in the tropics and a retrograding (i.e. westward moving) circulation pattern in the midlatitudes of the North Pacific. Typical wintertime weather anomalies preceding heavy precipitation events in the Pacific Northwest are as follows:
7-10 days prior to the heavy precipitation event: Heavy tropical rainfall associated with the MJO shifts eastward from the eastern Indian Ocean to the western tropical Pacific. A moisture plume extends northeastward from the western tropical Pacific towards the general vicinity of the Hawaiian Islands. A strong blocking anticyclone is located in the Gulf of Alaska with a strong polar jet stream around its northern flank.
3-5 days prior to the heavy precipitation event: Heavy tropical rainfall shifts eastward towards the date line and begins to diminish. The associated moisture plume extends further to the northeast, often traversing the Hawaiian Islands. The strong blocking high weakens and shifts westward. A split in the North Pacific jet stream develops, characterized by an increase in the amplitude and areal extent of the upper tropospheric westerly zonal winds on the southern flank of the block and a decrease on its northern flank. The tropical and extratropical circulation patterns begin to "phase", allowing a developing midlatitude trough to tap the moisture plume extending from the deep tropics.
The heavy precipitation event: As the pattern of enhanced tropical rainfall continues to shift further to the east and weaken, the deep tropical moisture plume extends from the subtropical central Pacific into the midlatitude trough now located off the west coast of North America. The jet stream at upper levels extends across the North Pacific with the mean jet position entering North America in the northwestern United States. Deep low pressure located near the Pacific Northwest coast can bring up to several days of heavy rain and possible flooding. These events 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 western North America.
Throughout this evolution, retrogression of the large-scale atmospheric circulation features is observed in the eastern Pacific-North American sector. Many of these events are characterized by the progression of the heaviest precipitation from south to north along the Pacific Northwest coast over a period of several days to more than one week.
http://en.wikipedia.org/wiki/Madden-Julian_oscillationQUOTE
News: Scientists Surf the Seas of Space to Catch an Atmospheric Wave
2/2/06
The eastward movement of the "dry" phase of the Madden-Julian Oscillation across the equatorial Indian and western Pacific Oceans is shown in these plots of atmospheric water vapor superimposed over rainfall data. Image credit: NASA/JPL
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A study by NASA and university scientists is shedding new light on a mysterious, cyclical wave in Earth's atmosphere that at times profoundly affects our planet's weather and climate.
Using satellite data from the Atmospheric Infrared Sounder (Airs) instrument on NASA's Aqua spacecraft, a research team led by Dr. Duane Waliser of NASA's Jet Propulsion Laboratory, Pasadena, Calif., and Dr. Baijun Tian of the California Institute of Technology, also in Pasadena,, set out to study the structure and evolution of the Madden-Julian Oscillation.
This slow, eastward-moving wave of clouds, rainfall and large-scale atmospheric circulation anomalies spans nearly half of Earth's equator, primarily over the Indian Ocean and western Pacific. Because it can strongly influence long-term weather patterns and has far-reaching global effects, scientists want to incorporate it more accurately in the general circulation models used by agencies around the world for weather prediction and climate forecasts.
Ebbing and flowing in cycles of about 40 to 50 days, the Madden-Julian Oscillation triggers showers and thunderstorms as it forces air to rise, and it inhibits clouds and rainfall in its subsiding phase. Its impacts are widespread. It triggers Asian and Australian monsoon wet and dry spells in the Indian and western Pacific oceans. It affects the winter jet stream and atmospheric circulation in the Pacific/North America region, causing anomalies that can lead to extreme rainfall events. It can also change summer rainfall patterns in Mexico and South America and may trigger variations in the El Nino climate phenomenon.
The Airs analysis revealed the oscillation results in an elegant interplay between rainy and clear conditions over the tropical Indian and western Pacific Oceans. Warm and moist conditions near the surface eventually give way to rainy and cloudy conditions aloft. These disturbed conditions are ultimately squelched by the introduction of dry, cool air near the surface, leading to clear skies. The rainy, clear and transitional periods in between them last 10 to 12 days each.
"Details such as these are critical for evaluating weather prediction models and improving our theoretical understanding of the oscillation phenomena," said Waliser. "Temperature and moisture data from sparse weather balloon readings and atmospheric models are simply not sufficient to properly represent many important features of the oscillation. Satellite-based observations such as those from Airs are therefore fundamental to gaining a fuller understanding of the processes at work."
The research team combined 3-D atmospheric moisture and temperature profiles from Airs from September 2002 to January 2005 with rainfall data from the joint NASA/Japan Aerospace Exploration Agency Tropical Rainfall Measurement Mission from January 1998 to February 2005. The rainfall data were used to identify Madden-Julian Oscillation events.
Waliser said that until recently, modeling the wave's hydrological components such as water vapor and clouds, condensation and evaporation processes has been difficult. With the launch of Airs in 2002, however, scientists gained a far greater ability to examine the wave's structure in 3-D and study its evolution over space and time.
"The high-quality Airs water vapor and temperature profiles provide new information on the vertical structure of the Madden-Julian Oscillation," said Tian. "This new information allows us to study the meteorological components that make up the oscillation in a way never before possible."
Airs is a high-spectral resolution infrared instrument that gathers 3-D profiles of atmospheric temperatures, water vapor and trace gases. It is the first in a series of advanced infrared sounders that will provide accurate, detailed atmospheric temperature and moisture observations for weather and climate applications. It views the atmosphere through nearly 2,400 different spectral channels, collecting 144,000 data retrievals daily around the globe.
The Tropical Rainfall Measurement Mission monitors tropical rainfall. The mission is the first space-based platform that uses microwaves along with precipitation radar to probe through clouds to reveal their vertical structure and estimate how much rainfall they are producing. The data are invaluable in areas with no rain gauges, such as the open ocean.
Study results were presented this week at the American Meteorological Society 86th Annual Meeting in Atlanta.

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The eastward movement of the "dry" phase of the Madden-Julian Oscillation across the equatorial Indian and western Pacific Oceans is shown in these plots of atmospheric water vapor superimposed over rainfall data. Image credit: NASA/JPL