Madden–Julian oscillation

A Hovmöller diagram of 5-day the running mean of outgoing longwave radiation showing the MJO. Time increases from top to bottom in the figure, so contours that are oriented from upper-left to lower-right depict objects moving from west to east.

The Madden–Julian oscillation (MJO) is an equatorial traveling pattern of anomalous rainfall that is planetary in scale. 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 thunderstorm activity is suppressed. Each cycle lasts approximately 30–60 days. Because of this pattern, The MJO is also known as the 30–60 day oscillation, 30–60 day wave, or intraseasonal oscillation.

Behavior

There are distinct patterns of lower-level and upper-level atmospheric circulation anomalies which accompany the MJO-related pattern of enhanced or decreased tropical rainfall across the tropics. These circulation features extend around the globe and are not confined to only the eastern hemisphere. The Madden-Julian oscillation moves eastward at 5 metres per second (11 mph) across the tropics, crossing the Earth's tropics in 30 to 60 days, with the active phase of the MJO tracked using the degree of outgoing longwave radiation which is measured by infrared-sensing geostationary weather satellites. The lower the amount of outgoing longwave radiation, the stronger the thunderstorm complexes, or convection, is within that region.^[1]

Enhanced surface westerly winds occur near the east side of the active convection.^[2] Ocean currents, up to 100 metres (330 ft) in depth from the ocean surface, follow in phase with the east-wind component of the surface winds. In advance, or to the east, of the MJO enhanced activity, winds aloft are westerly. In its its wake, or to the west of the enhanced rainfall area, winds aloft are easterly. These wind changes aloft are due to the divergence present over the active thunderstorms during the enhanced phase. Its direct influence can be tracked poleward as far as 30 degrees latitude from the equator in both northern and southern hemispheres, propagating outward from its orgin near the equator at around 1 degree latitude, or 111 kilometres (69 mi), per day.^[3]

Local effects

Connection to the monsoon

Onset dates and prevailing wind currents of the southwest summer monsoon.

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. 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.

A period of warming sea surface temperatures are found five to ten days prior to a strengthening of MJO-related precipitation across southern Asia. A break in the Asian monsoon, normally during the month of July, has been attributed to the Madden-Julian oscillation, after its enhanced phase moves off to the east of the region into the open tropical Pacific ocean.^[4]

Influence on tropical cyclogenesis

Although tropical cyclones occur throughout the boreal warm season (typically May–November) in both the north Pacific and the north 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. MJO-related descending motion is not favorable for tropical storm development. However, MJO-related ascending motion is a favorable pattern for thunderstorm formation within the tropics, which is quite favorable for tropical storm development. 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.

There is an inverse relationship between tropical cyclone activity in the western north Pacific basin and the north Atlantic basin, however. When one basin is active, the other is normally quiet, and vice versa. The main reason for this appears to be the phase of the Madden-Julian oscillation, or MJO, which is normally in opposite modes between the two basins at any given time.^[5] While this relationship appears robust, the MJO is one of many factors that contribute to the development of tropical cyclones. For example, sea surface temperatures must be sufficiently warm and vertical wind shear must be sufficiently weak for tropical disturbances to form and persist.^[6] 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.^[7]

Downstream effects

Link to the El Nino-Southern oscillation

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. Strong events in the Madden–Julian oscillation over a series of months in the western Pacific can speed the development of an El Niño or La Niña but do not in themselves lead to the onset of a warm or cold ENSO event.^[8] Globally, the interannual variability of the MJO is most determined by atmospheric internal dynamics.^{[clarification needed]}

North American winter precipitation

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. In the tropical Pacific, winters with weak-to-moderate cold, or La Nina, 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 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:^[9]

1. 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.^[9]
2. 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.^[9]
3. 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.^[9]

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.^[9]

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.^[9]

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.^[9]

References

External links

• "Daily Madden–Julian Oscillation Indices". National Weather Service Climate Prediction Center. http://www.cpc.ncep.noaa.gov/products/precip/CWlink/daily_mjo_index/mjo_index.html. Retrieved March 29 2005. 
• "MJO Homepage". Agricultural Production Systems Research Unit. http://www.apsru.gov.au/mjo/. Retrieved July 13 2007. 
• "The influence of intraseasonal variations of tropical convection on sea surface temperatures at the onset of the 1997–98 El Niño". NOAA-CIRES Climate Diagnostics Center Climate Research Spotlight. http://www.cdc.noaa.gov/spotlight/08012001/index.html. Retrieved March 29 2005. 
• Lin et al.: Tropical Intraseasonal Variability in 14 IPCC AR4 Climate Models. Part I: Convective Signals; doi: 10.1175/JCLI3735.1
• "Systematic Variation of Summertime Tropical Cyclone Activity in the Western North Pacific in Relation to the Madden–Julian Oscillation". Journal of Climate. http://ams.allenpress.com/perlserv/?request=get-abstract&doi=10.1175%2F2007JCLI1493.1. Retrieved March 2008. 
• "Variation of tropical cyclone activity in the South Indian Ocean: El Niño–Southern Oscillation and Madden–Julian Oscillation effects". Journal of Geophysical Research. http://www.agu.org/pubs/crossref/2006/2006JD007289.shtml. Retrieved November 16 2006.