M.J.O. (MADDEN-JULIAN OSCILLATION)

M.J.O. DESCRIPTION CAUSES CONSEQUENCES RELATIONSHIP BETWEEN ENSO AND THE MJO THE INDEX

MJO

DESCRIPTION

The Madden-Julian Oscillation (MJO) is a tropical atmospheric phenomenon in the form of a wave that occurs mainly in the Indian and Pacific Ocean which was discovered in 1971 by Roland Madden and Paul Julian scientists.

The MJO wave is an enhanced convection zone that propagates at a speed of 15 to 20 km/h eastward on timescales typically range between 40 and 50 days following the season (40 days in December and 50 days for events in September). While the MJO convection signals usually disappear in Eastern Pacific, these signals in the wind and surface pressure continue to spread further to the East as free waves (decoupled with convection) at faster speeds about 30-35 m/s. In one year the MJO has a frequency from 6 to 12 events. Forecasting the MJO allows seasonal forecasts on tropical and equatorial zones. That can be used for the risk of precipitations...

This oscillation has two phases :

- 1°) an active phase "wet" with atmospheric pressure which is lower than 1013hPa that promotes cloudy development ;

- 2°) an inactive phase with atmospheric pressure rather above 1013hPa that "dries out" atmosphere and "prevents" any significant cloudy development.

The evolution of the MJO wave propagation

Click here to see a page with animations of the MJO.

CAUSES

Convective instability can be supported by the warm sea surface especially in summer and autumn. Air-sea coupling effects depend on the season. The air-sea coupling can vary the amplitude (more or less), the phase velocity (more or less fast) and seasonality (more or less large) of the MJO only when it can produce signals without feedback on the temperature of the ocean.

Radiation can play an essential role. Convective instability of the atmosphere can be changed by a clear sky because of radiation.

Then the water vapor can be important for the MJO for different reasons. In some theories, the MJO propagation speed depends on the average duration of the stability of humidity and its composition according to time. Intra-seasonal fluctuations of water vapor in the troposphere induced by the movement of the MJO could reactivate its rainfall affecting deeply and directly or indirectly modulate the stability of moisture. The atmosphere tends to be more stable in the west of the convection center than in the East partly because the Ekman divergence is related to the western equatorial surface and partly because a dry part follows the active convection.

Kelvin and Rossby waves have a role in the spread of the MJO. A wave caused by a stone thrown into a pond is a gravity wave. The waves are also transmitted through the air in a similar way (but not exactly the same). The Kelvin wave is a special case of gravity wave associated with proximity to the equator, spreading eastward only. Then there is also the Rossby wave in the atmosphere, spreading weastward whose undulating movements of air or oceanic circulation of long wavelengths whose initiation is due to the variation of the Coriolis force depending on latitude.

Observation works by Hendon and Salby (1994) show that the factor of the movement of MJO is a forced response element which has the shape of a wave Rossby-Kelvin associated with convection. The Kelvin waves tend to be strong in the East but weak in the west of the MJO.

Several hypotheses show that the MJO convection would be accompanied by Kelvin waves in West moving together towards the East. The first Kelvin wave is interrupted by a Rossby wave leaving behind it some time the quasi-stationary convection until another Kelvin wave comes in. Then a second Kelvin wave heading eastward until it is again interrupted or slowed by the Rossby wave. These observations, seems to suggest that Kelvin and Rossby waves interacting with each other can play a key role in the mechanism of propagation of the MJO.

This figure illustrates the complexity of the interactions of tropical convection in space time.
The lack of a process model that takes place properly, process could change the heating time
by convection, which would have direct implications for climate simulation. The ocean-atmosphere
interaction presents a rich interaction between relatively fast and slow process, and seems to be an
important point between the time scales. For example, relatively fast westerly gales associated with
Intraseasonal synoptic time scales can déclencher des ondes de Kelvin dans l'océan ce qui influencera
l'évolution des événements ENSO, including the emergence and disintegration.

CONSEQUENCES

Madden and Julian, (1972, 1994) have demonstrated that the MJO is responsible for a large part of the Intraseasonal climate variance observed not only in the tropics, but also in higher latitudes.

The Madden-Julian Oscillation has a significant impact on intra-seasonal distribution of rainfall over two rainy seasons (March to May and October to December) in East Africa (Kenya and northern Tanzania). The years with high amplitude MJO are characterized by earlier onset of the rains. During the boreal summer, the MJO has activity in the Pacific Ocean, stretching over southern Mexico and Central America. The positive phase of the MJO local precipitations are greater by at least 25% than during the negative phase of the MJO.

The MJO would also have an indirect impact on the atmospheric circulation outside the tropics. When a MJO moves eastward it influences atmospheric circulation through the dispersion of Rossby waves which spread into higher latitudes. What cause the Jet stream, the trajectories of tropical and subtropical storms and cyclones and North Pacific Ocean basin of the North Atlantic, affecting monsoons and other characteristics of the regional circulation.

Although intra-seasonal oscillation may have the potential to improve the long-term predictability, its practical realization is hampered by several factors. Then because of its slow natural evolution, the accurate prediction of the MJO is difficult beyond one or two weeks.The problem is that the MJO is not well understood, and several physical mechanisms have been proposed to explain. The MJO is also difficult to predict because it is a phenomenon that occur very irregularly.

RELATIONSHIP BETWEEN ENSO AND THE MJO

The MJO is often a strong relationship with the phase of the ENSO (El Niño and La Niña). The MJO may influence the evolution of the phase of the ENSO cycle because the convection influence the ocean-atmosphere coupling by a varying the surface radiation budget, an evaporation, a wind stress and therefore slow interaction between the ocean and the atmosphere boundary layer. Which allows to determine the timing and / or the magnitude of the El Niño and La Niña. The precipitations and equatorial strong westerly winds gusting propagating eastward as they are associated with the MJO, create the oceanic Kelvin waves which affects the appearance of the El Niño phenomenon, moving warm waters of West Pacific eastward reducing the zonal gradient of the sea surface temperature and thus the intensity of the trade winds. For example, the MJO has been important during the boreal winter 1996-1997 preceding the beginning of the stage of a strong El Niño 1997-98.

Then during the El Niño and La Niña, the MJO is in turn influenced by the anomaly of the SST (sea surface temperature). Because El Niño has a SST warmer than normal in the Eastern Equatorial Pacific and colder than normal in the West. Then reverses to La Niña. During warm El Niño conditions, the anomalies of convection associated with the oscillation seem to penetrate further into the central Pacific. While during La Niña, the MJO is more concentrated to the West with greater activity on the Indian ocean and the Western Pacific. Overall, the MJO tends to be low or absent during the El Niño episodes, while the activity of the MJO is often important during the years with La Niña events.

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