Madden-Julian oscillation

The Madden-Julian Oscillation (MJO) is an equatorial traveling pattern of anomalous rainfall that is planetary in scale. The mechanism and cause of the MJO is as yet not well-understood and is a subject of ongoing study.

The MJO is characterized by an eastward progression of large regions of both enhanced and suppressed tropical rainfall, observed mainly over the Indian Ocean and Pacific Ocean. The anomalous rainfall is usually first evident over the western Indian Ocean, and remains evident as it propagates over the very warm ocean waters of the western and central tropical Pacific. This pattern of tropical rainfall then generally becomes very nondescript as it moves over the cooler ocean waters of the eastern Pacific but reappears over the tropical Atlantic and Indian Ocean. The wet phase of enhanced convection and precipitation is followed by a dry phase where convection is suppressed. Each cycle lasts approximately 30-60 days.

The MJO is also known as the 30-60 day oscillation, 30-60 day wave, or intraseasonal oscillation.

Atmospheric patterns

There are distinct patterns of lower-level and upper-level atmospheric circulation anomalies which accompany the MJO-related pattern of tropical rainfall. These circulation features extend around the globe and are not confined to only the eastern hemisphere.

There is strong year-to-year (interannual) variability in MJO activity, with long periods of strong activity followed by periods in which the oscillation is weak or absent. This interannual variability of the MJO is partly linked to the El Niño-Southern Oscillation (ENSO) cycle. In the Pacific, strong MJO activity is often observed 6 - 12 months prior to the onset of an El Niño episode, but is virtually absent during an El Niño episode, while MJO activity is typically greater during a La Niña episode. Globally, the interannual variability of the MJO is most determined by atmospheric internal dynamics.

The phase of the MJO is also extremely important for assessing whether conditions are conducive to tropical storm development over the tropical and subtropical North Pacific and North Atlantic ocean basins. For example, MJO-related descending motion over the tropical Atlantic is not favorable for tropical storm development, whereas MJO-related ascending motion over the North Atlantic is quite favorable for tropical storm development. The MJO is monitored routinely by both the USA National Hurricane Center and the USA Climate Prediction Center during the Atlantic hurricane (tropical cyclone) season to aid in anticipating periods of relative activity or inactivity.

North American winter effects

The strongest impacts of intraseasonal variability on the United States occur during the winter months over the western U.S. During the winter this region receives the bulk of its annual precipitation. Storms in this region can last for several days or more and are often accompanied by persistent atmospheric circulation features. Of particular concern are the extreme precipitation events which are linked to flooding. There is strong evidence for a linkage between weather and climate in this region from studies that have related the ENSO to regional precipitation variability. From these studies it is known that extreme precipitation events can occur at all phases of the ENSO cycle, but the largest fraction of these events occur during La Niña episodes and during ENSO-neutral winters.

During La Niña episodes much of the Pacific Northwest experiences increased storminess, increased precipitation and more overall days with measurable precipitation. The risk of flooding in this region increases as the strength of the cold episode decreases due to an increase in extreme precipitation events in the weaker episodes. In the tropical Pacific, winters with weak-to-moderate cold episodes, or ENSO-neutral conditions are often characterized by enhanced 30-60 day MJO activity. A recent example is the winter of 1996-97, which featured heavy flooding in California and in the Pacific Northwest (estimated damage costs of $2.0-3.0 billion at the time of the event) and a very active MJO. Such winters are also characterized by relatively small sea surface temperature anomalies (SSTA) in the tropical Pacific compared to stronger warm and cold episodes. In these winters there is a stronger linkage between the MJO events and extreme west coast precipitation events.

Pineapple Express events

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. However, it is important to differentiate the individual synoptic-scale storms, which generally move west to east, from the overall large-scale pattern which exhibits retrogression.

There is a coherent simultaneous relationship between the longitudinal position of maximum MJO-related rainfall and the location of extreme west coast precipitation events. Extreme events in the Pacific Northwest are accompanied by enhanced precipitation over the western tropical Pacific and the region of Southeast Asia called by meteorologists the Maritime Continent, with suppressed precipitation over the Indian Ocean and the central Pacific. As the region of interest shifts from the Pacific Northwest to California, the region of enhanced tropical precipitation shifts further to the east. For example, extreme rainfall events in southern California are typically accompanied by enhanced precipitation near 170°E. However, it is important to note that the overall linkage between the MJO and extreme west coast precipitation events weakens as the region of interest shifts southward along the west coast of the United States.

It should be noted that there is case-to-case variability in the amplitude and longitudinal extent of the MJO-related precipitation, so this should be viewed as a general relationship only.

Boreal Summer Effects

During the Northern Hemisphere summer season the MJO-related effects on the Indian summer monsoon are well documented. MJO-related effects on the North American summer monsoon also occur, though they are relatively weaker. However, the relative influences of ENSO and the MJO on the summer precipitation regime of North America are not well understood.

MJO-related impacts on the North American summer precipitation patterns are strongly linked to meridional (i.e. north-south) adjustments of the precipitation pattern in the eastern tropical Pacific. A strong relationship between the leading mode of intraseasonal variability of the North American Monsoon System, the MJO and the points of origin of tropical cyclones is also present.

Tropical cyclone influence

Although tropical cyclones occur throughout the boreal warm season (typically May-November) in both the Pacific and the Atlantic basins, in any given year there are periods of enhanced/suppressed activity within the season. There is evidence that the MJO modulates this activity (particularly for the strongest storms) by providing a large-scale environment that is favorable (or unfavorable) for development. The strongest tropical cyclones tend to develop when the MJO favors enhanced precipitation. As the MJO progresses eastward, the favored region for tropical cyclone activity also shifts eastward from the western Pacific to the eastern Pacific and finally to the Atlantic basin. While this relationship appears robust, the MJO is one of many factors that contribute to the development of tropical cyclones. For example, it is well known that sea surface temperatures must be sufficiently warm and vertical wind shear must be sufficiently weak for tropical disturbances to form and persist.

Further reading

External links

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* [http://ams.allenpress.com/amsonline/?request=get-abstract&doi=10.1175%2FJCLI3735.1 Lin et al: Tropical Intraseasonal Variability in 14 IPCC AR4 Climate Models. Part I: Convective Signals; doi: 10.1175/JCLI3735.1]
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* cite web | title=Variation of tropical cyclone activity in the South Indian Ocean: El Niño–Southern Oscillation and Madden-Julian Oscillation effects| work=Journal of Geophysical Research | url=http://www.agu.org/pubs/crossref/2006/2006JD007289.shtml | accessmonthday=Nov. 16 | accessyear=2006