Tropical Intraseasonal Activity
(Madden-Julian Oscillation- MJO)

What are Intraseasonal Oscillations?

How do scientists detect intraseasonal oscillations (also called MJO) and predict their evolution?

Why is predicting intraseasonal oscillations events important?

What is the relationship between tropical intraseasonal oscillations and El Niño/ La Niña?

What are the impacts of intraseasonal oscillations on the U.S.? When do they occur?

What is the typical scenario linking intraseasonal oscillations to heavy precipitation events in the western U.S.?

Do intraseasonal oscillations influence the weather during the summer months?

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What are Intraseasonal Oscillations?

Variability in weather and climate is pervasive. This variability ranges over many time and space scales, from small-scale weather phenomena such as wind gusts, localized thunderstorms, and tornadoes to larger-scale features such as low-pressure and high-pressure weather systems, to even more prolonged features such as droughts and floods, to longer-lived climate phenomenon such as El Niño and La Niña, to even longer decadal trends. In general, the longer time-scale phenomena are often associated with changes in the atmospheric circulation that encompass areas far larger than a particular affected region. At times these persistent circulation features occur simultaneously over vast parts of the hemisphere, or even the globe, and result in abnormal weather, temperature and rainfall patterns in many regions. Scientists have discovered that important aspects of this variability are linked to global-scale phenomena that affect the distribution and intensity of tropical rainfall, thereby influencing the position and intensity of the subtropical high pressure regions and mid-latitude jet streams.

Year-to-year (interannual) variability in tropical rainfall is often related to the occurrence of either the El Niño or La Niña phenomenon in the tropical Pacific. There is also strong decade-to-decade variability in tropical rainfall, which is thought to be an important source of interdecadal trends for the atmospheric circulation and associated weather patterns.

Tropical rainfall also exhibits strong variability on sub-seasonal time scales. These fluctuations in tropical rainfall often go through an entire cycle in 30-60 days, and are referred to as "intraseasonal oscillations". Four other terms that are often used interchangeably to refer to intraseasonal oscillations are "Madden-Julian Oscillation" or "MJO", "30-60 day oscillation", and "30-60 day wave". In this summary we will refer to this phenomenon by "intraseasonal oscillation" or the "MJO".

The MJO is a naturally occurring component of our coupled ocean-atmosphere system. It significantly affects the atmospheric circulation throughout the global Tropics and subtropics, and also strongly affects the wintertime jet stream and atmospheric circulation features over the North Pacific and western North America. As a result, it has an important impact on storminess and temperatures over the U.S. During the summer the MJO has a modulating effect on hurricane activity in both the Pacific and Atlantic basins. Thus, it is very important to monitor and predict MJO activity, since this activity has profound implications for weather and short-term climate variability through the year.

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. Each cycle lasts approximately 30-60 days

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. Thus, they provide important information regarding the regions of ascending and descending motion associated with particular phases of the oscillation over those parts of the tropics where rainfall is generally low or absent.

There is strong year-to-year 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 ENSO cycle. Strong MJO activity is often observed during weak La Niña years or during ENSO-neutral years, while weak or absent MJO activity is typically associated with strong El Niño episodes.

How do scientists detect intraseasonal oscillations (also called MJO) and predict their evolution?

Due to its slowly evolving nature, accurate prediction of the MJO is fundamentally related to our ability to monitor the feature and to assess its relative position and strength. Dynamical models generally do not predict the MJO well, partly because of the inherent difficulties that still remain regarding the correct mathematical treatment of tropical convective (rainfall) processes.

Meteorologists use a variety of data and analysis techniques to monitor, study and predict tropical intraseasonal oscillations and their evolution. Of primary importance is information derived from NOAA=s polar-orbiting and geostationary satellites. Satellite-derived data are used to indicate regions of strong tropical convective activity, and regions in which the convective activity departs substantially from the long-term mean. These departures from normal are a fundamental diagnostic tool that is used directly to monitor and predict the MJO as it propagates around the global tropics.

A second fundamental data source used to monitor the MJO is the global radiosonde network which provides crucial information regarding the atmospheric winds, temperature, moisture, and pressure at many levels of the atmosphere. These data are taken twice daily, and assimilated by dynamical weather prediction models into formats that are highly efficient for climate analysis and numerical weather prediction. In combination with the satellite-derived rainfall and convection patterns, these observations provide meteorologists with the capability to routinely monitor and assess the MJO and its evolution. It also allows one to better assess the impacts of the MJO activity on features such as the wintertime jet streams, and the large-scale environment within which tropical storms and hurricanes develop over the tropical Atlantic.

There are several diagnostic analyses which allow us to directly monitor the MJO. These analyses are often displayed in time-longitude format so as to reveal the propagation, amplitude and location of the MJO-related features. Typical time-longitude sections include 1) Outgoing Longwave Radiation, which is a satellite-derived measure of tropical convection and rainfall, 2) velocity potential, which is a derived quantity that isolates the divergent component of the wind at upper levels of the atmosphere, 3) upper-level and lower-level wind anomalies and 4) 500-hPa height anomalies to represent the atmospheric responses in midlatitudes.

Why is predicting intraseasonal oscillations events important?

The MJO can have significant impacts on the wintertime atmospheric circulation over the North Pacific and western North America. It is also a contributor to blocking activity (i.e. atmospheric circulation features that persist near the same location for several days or more) and block evolution over the high latitudes of the North Pacific, which is another important component of winter weather patterns over North America. Thus, improved monitoring and understanding of the MJO and its impacts on these circulation features can help meteorologists to better predict their evolution. This improved prediction of features such as blocking activity, etc. is important since the dynamical prediction of block evolution beyond several days remains a large source of uncertainty in numerical models.

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 Hurricane Prediction Center and the Climate Prediction Center during the Atlantic hurricane season to aid in anticipating periods of relative activity or inactivity.

What is the relationship between tropical intraseasonal oscillations and El Niño/ La Niña?

Intraseasonal oscillations often exhibit a strong relationship to the phase of the ENSO cycle. Overall, there tends to be weak or absent MJO activity during moderate or strong El Niño episodes. In contrast, MJO activity is often substantial during ENSO-neutral years and during weak La Niña episodes.

What are the impacts of intraseasonal oscillations on the U.S.? When do they occur?

The strongest impacts of intraseasonal variability on the U.S. 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 El Niño-Southern Oscillation (ENSO) to regional precipitation variability. From these studies it is known that extreme precipitation events can occur at all phases of the El Niño-Southern Oscillation (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 (Fig. 1).

What is the typical scenario linking intraseasonal oscillations to heavy precipitation events in the western U.S.?

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 (Fig. 1). Typical wintertime weather anomalies preceding heavy precipitation events in the Pacific Northwest are as follows:

(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.

(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.

(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.

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 Indonesia (typically centered near 120^oE) 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^oE. 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. A summary of the simultaneous relationship between the location of maximum MJO-related rainfall and heavy rainfall in west coast of the U.S. is as follows:

west coast location                           longitude of maximum MJO-related rainfall

• western Washington:                                           120°E

• northwestern Oregon                                           125°E

• southwestern Oregon                                           130°E

• northwestern California                                         140°E

• north central California                                         150°E

• west central California                                          160°E

• southwestern California                                        165°E

• southern California                                              170°E

 

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.

Do intraseasonal oscillations influence the weather during the summer months?

The North American warm season precipitation regime experiences climate variations on time scales ranging from intraseasonal to decadal. During the summer months low-frequency variability in the tropics is dominated by interannual variations associated with ENSO and by intraseasonal variations such as the MJO. Both of these phenomena feature near-global patterns of anomalous atmospheric circulation that are closely related to variations in precipitation in many regions of the tropics and subtropics. The MJO can have a significant impact on regions that experience rainy seasons both during winter and summer seasons. For example, 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.

Although tropical cyclones occur throughout the NH 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 (unfavorable) for development (Fig. 2). 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, we caution that the MJO is one of many factors that contribute to the development of tropical cyclones. For example, it is well known that SSTs must be sufficiently warm and vertical wind shear must be sufficiently weak for tropical disturbances to form and persist.