The Madden-Julian Oscillation (MJO)

[Okular]

Jostemikk nevner fenomenet Madden-Julian Oscillation (MJO) i forbindelse med en NOAA-oppdatering på ENSO-tilstanden i det tropiske Stillehav på tråden Velkommen til La Niña-land! for en drøy uke siden. Han sier:

Madden-Julian Oscillationen de nevner er en konvektiv ikke-stedbunden atmosfæresirkulasjon i det tropiske Stillehavet, og jeg må med skam melde at jeg ikke kan særlig mye om denne sirkulasjonen.

Dette basert på følgende meddelelse i NOAA-rapporten:

The atmospheric circulation resembles La Niña, butt his is at least partially due to an active Madden-Julian Oscillation (MJO).

Jeg tenkte jeg, for de interesserte, kunne ta en gjennomgang av dette fenomenet. Dvs., det er ikke jeg som strengt tatt tar gjennomgangen, men Zhang 2005. Bedagelig som jeg er tar jeg utgangspunkt i at alle som bidrar og leser her på forumet er sånn tålelig stødige i det engelske språk, og velger derfor ikke å oversette utdragene jeg har løftet fra artikkelen hans.

Uansett, MJO (Madden-Julian Oscillation) er en slags koblet tropisk sirkulasjons/konveksjonspuls som i hovedsak oppstår i det vestlige Indiahavet og propageres østover til den i stor grad oppløses over det sentrale Stillehavet, men i mindre fullstendig og observerbar form fortsetter hele veien rundt jorda, i løpet av 30-90 dager (i snitt med en hastighet på 5 m/s). Pulsen observeres i fremste rekke gjennom et markant nedbørssenter, men også av distinkte, flyttende vindsystemer. Fenomenet holder altså hus i den tropiske, sonale Walker-sirkulasjonscellen (vest mot øst), hvor det dominerer de sesongsbaserte værmønstrene i tropene. Så dette er så menn ingen liten, sær værhendelse.

Fra norsk wikipedia:

Madden-Julian oscillasjonen (MJO) er en nedbørsanomali på planetær skala som forplanter seg langs ekvatorområdet.

MJO er karakterisert ved en østlig forplantning av store områder med både økt og begrenset tropisk nedbør. Hovedsakelig skjer dette over Indiahavet og Stillehavet. Det unormale nedbørsmønsteret merker man først over vestlige deler av Indiahavet, før det forplanter seg over det varme vannet i vestlige og sentrale områder av det tropiske Stillehavet. Mønsteret ser derimot ut til å forsvinne når det flytter seg over de kjøligere havmassene i det østlige Stillehavet, men dukker så opp igjen i tropiske områder av Atlanterhavet og tilbake til Indiahavet. En våt fase med forsterket konveksjon og økt nedbør er etterfulgt av en tørr fase der konveksjonen blir svekket. Hver syklus varer omtrnt 30–60 dager, og på grunn av det blir MJO også kalt for "30–60 dagersoscillasjonen", "30–60 dagersbølgen" eller "intraperiodisk oscillasjon".

Mer detaljert, fra Zhang 2005:

"The Madden-Julian Oscillation (MJO) is the dominant component of the intraseasonal (30–90 days) variability in the tropical atmosphere. It consists of large[/planetary]-scale coupled patterns in atmospheric circulation and deep [moist] convection, with coherent signals in many other variables, all propagating eastward slowly (5 m/s) through the portion of the Indian and Pacific oceans where the sea surface is warm. It constantly interacts with the underlying ocean and influences many weather and climate systems.
    The MJO is intriguing for many reasons. It influences the variability of rainfall over the Pacific islands, in the monsoon regions of Asia and Australia, along the west coast of North America, in South America, and in Africa. It modulates the genesis of tropical cyclones in the Pacific Ocean and the Caribbean Sea and affects equatorial surface winds in the Atlantic Ocean.
    The dynamics of the MJO involves atmospheric planetary-scale circulations and its interaction with mesoscale convective activities; the MJO also interacts with the ocean and thereby may influence the evolution of El Niño–Southern Oscillation (ENSO); it is very difficult to simulate the MJO correctly by state-of-the-art global weather prediction models and global climate models (GCM). The MJO therefore tests our understanding of how the atmosphere operates in the tropics.
    The MJO might have even helped shape human history. The episodic strong westerly surface winds lasting up to 30 days, a distinct feature of the MJO, might have been the critical weather conditions for the courageous and gifted Polynesian seamen to sail eastward in the trade wind-dominated equatorial Pacific to reach and settle in Polynesia almost 4500 years ago.
    It is important to point out that the MJO is a dominant but not the only component of tropical intraseasonal variations. For example, prominent intraseasonal oscillations exhibit a northward propagation during the Asian summer monsoon.

The most basic observed features of the MJO are illustrated in [the figure below]. In the equatorial Indian and western Pacific oceans an MJO event features a largescale, eastward moving center of strong deep convection and precipitation ("active phase"), flanked to both east and west by regions of weak deep convection and precipitation ("inactive" or "suppressed phases"). The two phases of the MJO are connected by overturning zonal circulations that extend vertically through the entire troposphere. In the lower troposphere (below 10 km, typically at the 1.5km or 850hPa level) and near the surface, anomalously strong westerly winds exist in and to the west of the large-scale convective center with anomalous easterly winds to the east. The zonal winds reverse the directions in the upper troposphere (above 10 km, typically at the 13km or 200hPa level). This close association between the large-scale circulation and convective center, commonly referred to as the coupling between the two, is central to the MJO dynamics. This coupled pattern propagates eastward at an averaged speed of 5 m/s.


Longitude-height schematic diagram along the equator illustrating the fundamental large-scale features of the Madden-Julian Oscillation (MJO) through its life cycle (from top to bottom). Cloud symbols represent the convective center, arrows indicate the zonal circulation, and curves above and below the circulation represent perturbations in the upper tropospheric and sea level pressure.

So prominent a phenomenon is the MJO that its existence can be discerned from observations without any filtering, especially in precipitation [(figure below)]. The spectral peaks of the MJO at 30–90 days in precipitation and zonal wind at 850hPa are clearly distinguished from the lower-frequency peaks.


Longitude-time plots of daily (a) zonal wind (2.5° x 2.5°, m/s) at 850hPa (roughly 1.5km above sea level) from the NCEP/NCAR Reanalysis and (b) precipitation (1° x 1°, mm/d) from the GPCP combined data set for June 2000 to May 2001, both averaged over 10N–10S. The white straight lines mark identified MJO events, with a slope corresponding to an eastward propagation speed of 5 m/s. Notice that each MJO event may propagate eastward at a slightly different speed. The faster eastward moving (15 m/s) signals with shorter periods (5–10 days) (examples marked with black dashed lines) are of convectively coupled Kelvin waves and should not be mistaken for the MJO. The westward moving synoptic [10²–10³ km] signals (examples marked with white arrows) are likely of Rossby or mixed Rossby-gravity waves.

The dominant period of the MJO spreads over a range of roughly 30–100 days. Its power peak is highly variable within this range. [This reflects] a fundamental but often neglected nature of the MJO. Although referred to as an "oscillation," the MJO by no means oscillates regularly. It is highly episodic or discrete. The range of its local period (30–100 days) suggests the interval between two consecutive events is irregular and their propagation speeds may vary.
    The typical zonal extent of an MJO event, measured by its regions of positive and negative anomalies in cloud covers, is roughly 12,000–20,000 km. Only one fully developed MJO event exists in the tropics at a given time. Occasionally, two weak convective centers of the MJO with weak circulations may coexist, one being just initiated in the Indian Ocean and the other decaying in the central Pacific. The zonal scale of the convective component is much less than that of the circulation because of the nature of atmospheric response to localized heating. [...] Meanwhile, the zonal extent of active phase of the MJO is much smaller than that of inactive phase. The MJO is therefore more an isolated or discrete pulse-like event than a sinusoidal wave.
    The slow eastward propagation at an averaged speed of 5 m/s is one of the most fundamental features that distinguishes the MJO from other phenomena in the tropical atmosphere, especially convectively coupled Kelvin waves, which propagate eastward at greater speeds of 15–17 m/s. The phase speed of the MJO varies slightly among individual events and during different stages of the life cycle of a given event. While the convective signals of the MJO normally vanish in the eastern Pacific, its signals in wind and surface pressure continue to propagate farther east as free (uncoupled with convection) waves at much higher speeds of about 30–35 m/s. A continuous, global circumferential propagation of the MJO along the equator exists only in its upper level fields as atmospheric responses to the convective perturbations.
    Both Kelvin and [equatorial] Rossby wave structures have been considered dynamically essential to the MJO.
    Immediately ahead (to the east) of the convective center are low-level convergence, ascending motions, and positive anomalies in humidity; low-level divergence, descending motions, and negative anomalies in humidity occur immediately behind (to the west). Such zonal asymmetry provides favorable large-scale conditions for the development of new convective systems east of the existing ones and discourages such development to the west, resulting in the eastward propagation of the convective center.
    The apparent eastward propagation of the large-scale convective center of the MJO is due to consecutive development of new convective systems, each on average slightly to the east of the previous one.
    MJO signals in convection are normally confined to the Indian and western Pacific oceans, because convective instability can be sustained only over the warm sea surface known as the "warm pool" [WPWP +]. MJO signals in some other fields can be detected in the rest of the tropics. The effect of the warm sea surface in determining the location of the MJO can be further illustrated by two examples. One is the zonal displacement of the MJO in concert with ENSO. The other is the MJO signal in the eastern Pacific north of the equatorial cold tongue and adjacent to the Central American coast, which is nontrivial only in boreal summer when the sea surface temperature there is sufficiently high.
    The convective component of the MJO over the Maritime Continent [Indonesia +] is generally much weaker than over the surrounding oceans. Possible explanations for this are the following: (1) The strong diurnal cycle in convection due to diurnal heating over land tends to compete with the MJO for moisture and energy. (2) Topography interferes with lowlevel moisture convergence believed to be crucial to the MJO. (3) Surface evaporation, another possible crucial factor for the MJO, is severely reduced over land. These possible explanations may also be applied to tropical South America, where deep convection in local summer is almost as strong as in the western Pacific but MJO signals are intriguingly weak.

Explaining the primary observed features described [above] tests our understanding of the MJO. Because the Kelvin wave is the only equatorial mode with an eastward propagating, planetary-scale zonal wind field resembling that of the observed MJO, it has been taken as the dynamical backbone of the MJO from day one. However, convectively coupled Kelvin waves propagate eastward at a much faster speed than does the MJO. The reduction in the phase speed of the Kelvin wave by damping is insufficient to bridge the gap. Therefore key questions that must be addressed by any MJO theory are as follows: What are the mechanisms that distinguish the MJO from the convectively coupled Kelvin waves? What processes must take place to supply energy against dissipation selectively to the intraseasonal, planetary-scale, and slowly eastward propagating disturbances known as the MJO?
    Episodic, extraordinarily strong surface westerly winds (up to 10 m/s) in the equatorial western Pacific have long been known to sea-going oceanographers, who named them westerly wind bursts. These WWB can leave significant imprints in the upper ocean. Some WWB, especially the long-lasting ones, are associated with the MJO. Unfortunately, confusion between the concepts of the MJO and WWB is widespread throughout the literature. Oceanographers usually refer to any strong westerly wind events as WWB, no matter how long they last.


(INNVIRKNING PÅ ENSO ...?)

During a warm event of ENSO (El Niño), as the eastern edge of the warm pool extends eastward, so does MJO activity. The MJO in the Pacific appears to be extraordinarily vigorous prior to the peak of an ENSO warm event and anomalously weak after the peak and during a cold event.
    Simultaneous relationship between the level of global MJO activity and sea surface temperature (SST) indices representing ENSO has been found to be very weak. This suggests that globally the interannual variability of the MJO might be driven more by the atmospheric internal dynamics than surface conditions.
    The MJO-ENSO problem consists of the following questions: Does the MJO make any unique contribution irreplaceable by other types of stochastic forcing to the detailed evolution of ENSO warm events when the coupled system is in an unstable dynamic regime and to the sustenance of the ENSO cycle when the coupled system is in a neutral or stable regime? What are the mechanisms by which the MJO affects ENSO? Can ENSO prediction benefit from an inclusion of the MJO in ENSO prediction models?

When stochastic forcing is assumed to be completely random (white) in time and space, it includes all types of high-frequency weather variability, such as tropical cyclones, waves, and westerly wind bursts as well as the MJO. None of them should be more special than others.
    In observations the interannual variability of Kelvin wave forcing by the MJO component of wind stress is much greater than by non-MJO wind stress. The relative importance of the MJO compared to other types of stochastic forcing of ENSO, such as westerly wind bursts independent of the MJO, is a subject under debate and needs to be further scrutinized quantitatively.
    The MJO may affect an ENSO warm event by helping reduce the zonal gradient of SST. Three processes can be involved. [1] Mean SST in the western Pacific can be reduced by net cooling due to the MJO. [2] Zonal current forced by the MJO westerly wind advects eastward the eastern edge of the [WPWP]. [3] Oceanic Kelvin waves forced by the MJO propagate into the eastern Pacific, where they suppress the thermocline, reduce cooling due to the upwelling, and induce warm anomalies at the surface.
    A positive feedback mechanism between the MJO and ENSO has been suggested: Thermal advection by the MJO-forced oceanic Kevin waves results in an eastward expansion of the western Pacific warm pool, which allows the MJO to propagate farther into the central Pacific. A longer zonal fetch of wind forcing would generate a stronger Kelvin wave. This progressive eastward penetration of the MJO is commonly observed during the onset and development stages of ENSO warm events. This possible feedback between the MJO and ENSO SST is manifested by the observed correlation between the local MJO in the Pacific and ENSO SST indices, in contrast to the interannual variability of the global MJO that is independent of ENSO SST. Observed correlation between ENSO and MJO forcing of the Kelvin waves in the equatorial Pacific also suggests a possible role of the MJO in enhancing ENSO warming at its very early stage.
    If forcing the oceanic Kelvin wave is central to the MJO influence on ENSO, then only in the equatorial waveguide can the strength in zonal stress of the MJO determine the efficiency of this influence. This may explain why not all MJO events, even some strong ones, necessarily lead to a warm event of ENSO. Other possible explanations are the following: The susceptibility of ENSO to the influences of the MJO depends on the mean state of the coupled system and on the timing of the MJO relative to the phase of ENSO. ENSO is perhaps influenced more effectively by the seasonal activity of the MJO than by any individual MJO event, as implied by the importance of the seasonal cycle of stochastic forcing."


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Selv syntes jeg denne artikkelen gjorde de fleste ting klare angående hva MJO er og gjør for noe (jeg visste ikke overvettes mye om fenomenet på forhånd, jeg heller). Jeg vet ikke med dere ... Litt i overkant?