Madden-Julian Oscillation Explained

The Madden-Julian Oscillation (MJO) is a tropical weather cycle that moves eastwards around the globe with a life-span of about 30-60 days. Usually, it appears over the western Indian Ocean, before moving east across the Indo-Pacific Maritime Continent and into the warmer waters of the Western Pacific Ocean - a journey which takes about a month - bringing with it heavy rainfall and strong winds. As it moves, it affects meteorological conditions in several different parts of the globe.
Monsoon storm clouds over Pacific
Madden–Julian Oscillation storm clouds over the Pacific. Photo: Wikimedia Commons (CC BY-SA 3.0)

What is the Madden-Julian Oscillation?

The MJO is recognizable by its eastward movement involving large areas of (a) enhanced and (b) suppressed tropical rainfall. A wet phase of increased convection (rainfall) is followed by a dry episode, during which precipitation is suppressed. Each MJO-cycle lasts between 30 and 60 days and it is usually divided into 8 separate phases. (See below.)

The Madden-Julian Oscillation is active in the Indian and western Pacific Oceans (mostly in the Indo-Pacific warm pool), where it triggers variations in atmospheric pressure, wind, sea-surface temperature (SST), cloud formation, and rainfall. It is the main influence behind intra-seasonal (short-term) weather variations in the tropics and subtropics. 1

This climate variability, in turn, can influence the intensity and timing of the Asian and Australian monsoons. It can also amplify the El Niño-Southern Oscillation (ENSO), adding to the impact of an El Niño or La Niña event.

In addition, the MJO affects climate events in the extratropics and polar regions. For example, the frequency and patterns of tropical cyclones, floods, and heatwaves can all vary according to whether the MJO is over the Indian or Pacific Oceans. 2

There is no direct connection between climate change and the MJO. But as global warming intensifies, leading to a more active water cycle, as well as impacts across the tropical hydrosphere – such as marine heatwaves – the combined effect of these two drivers is almost certain to lead to graver consequences across the Indo-Pacific region.

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Discovery

The Madden–Julian oscillation was first described in 1971 by Dr. Roland Madden and Dr. Paul Julian, who were scientists at the American National Center for Atmospheric Research (NCAR). 3

It all started when they noticed regular anomalies in winds between Singapore and Canton Island in the west central equatorial Pacific. They referred to it as the “40-50-day oscillation” due to its time scale. Their discovery remains an important contribution to climate science, notably in the modelling of short- and long-term weather climate variability in the tropics.

Madden–Julian Oscillation: Surface and upper-atmosphere air flow when wet conditions prevail in the western Indian Ocean.
Madden–Julian Oscillation: Surface and upper-atmosphere air flow when the enhanced convective phase is located in the central and northern Indian Ocean and the suppressed convective phase is located over the west-central Pacific Ocean. The entire system moves eastward over time, returning to its point of origin usually within 60 days. Image Credit: NOAA Climate.gov. Drawing by Fiona Martin.

Characteristics of the MJO

Its key characteristic is that, unlike the standing dipole patterns of other weather cycles, such as ENSO or the Indian Ocean Dipole (IOD), the MJO is a travelling pattern arising from close interactions between atmospheric circulation and convection. 4

The Madden-Julian Oscillation, with its system of very tall convective clouds, develops eastwards, at about 14-29 km/h (9-18 mph), moving through the troposphere above the warm sea surfaces of the Indian and Pacific oceans. Its presence is signalled most clearly by unusual levels of rainfall. It has an average life span from the beginning to end of one event, of roughly 48 days, whereupon it returns to its initial starting point.

The MJO is not confined to the atmosphere – it also affects the ocean, with warm sea surface temperatures leading to higher-than-normal rainfall, and cool sea surface temperatures introducing higher-than-normal rainfall.

Its dipolar weather pattern is driven by higher-than-normal ocean warming. Currents of warm water vapor rise upwards (enhanced convective phase) and circulate in a latitudinal direction before sinking (suppressed convective phase) at the other end of the region.

Diagram showing the phases of the Madden Julian Oscillation.
Madden–Julian Oscillation: Structure and Propagation. A schematic diagram showing the MJO’s size and eastward shift. Phase 1: Enhanced convection (rainfall) develops over Africa and the western Indian Ocean. Phase 2-3: Rainfall moves slowly eastwards into the central Indian Ocean and areas of the Indian subcontinent. Phase 4-5: Rainfall reaches the Maritime Continent (Southeast Asia). Phase 6-8: Rainfall moves further eastward over the western Pacific, eventually fading in the central Pacific. The characteristic MJO pattern of precipitation die outs as it moves over the cooler waters of the eastern Pacific, before materializing once over the Indian Ocean again as the next MJO cycle begins. The period of time for the complete phase shown in the diagram is typically 30-60 days. Image Credit: 5

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Effects of the Madden-Julian Oscillation on Global Weather

The MJO affects several other meteorological systems in the tropics.

• It influences the timing and strength of the West African, Indian and Australian monsoons. 6 The enhanced convection (rainfall) phase of the MJO can trigger the onset of the Monsoon season, while the suppressed convection (dry) phase can delay it.

For example, about 33–80 percent of short-term variability of monsoon rainfall is related to the MJO. 7

• It influences cyclogenesis across the tropics, modulating the numbers and ferocity of extreme weather events such as tropical hurricanes in most Ocean basins. 8

• It contributes to both El Niño and La Niña events in the western and central Pacific. 9

But the MJO is not confined to the tropics. Its meteorological signal is global, with Rossby wave trains (caused by the rotation of the Earth) extending out from the tropical core region into the extratropics and high latitudes.

For example, a number of times each year it causes changes in the Jet Stream, which can lead to extreme weather events in America, such as the appearance of Arctic air flow during the winter months in the central and eastern parts of the United States. 10

According to the latest research, atmospheric wave patterns caused by ENSO and the Madden-Julian Oscillation, combine to influence rainfall patterns (sometimes with flooding or drought) in the United States. 11 12

Another recent study has found clear associations between heatwaves and drought in California’s Central Valley and the Madden-Julian Oscillation (MJO). These heat waves are often preceded by convection patterns over the tropical Indian and eastern Pacific oceans, that are linked to MJO phases. 13

There is also some evidence to suggest that the MJO can influence the Southern Annular Mode weather cycle and even trigger Sudden Stratospheric Warming (SSW) events. 14

Effect of Climate Change on Madden-Julian Oscillation

The MJO travels up to 20,000 km (12,500 mi) over the tropical oceans, mostly over the Indo-Pacific warm pool – an expanse of water with SSTs of 28 °C or higher. This pool has been heating up over recent decades, due to the varying effects of global warming on the oceans, caused by the greenhouse effect. 15

In fact, the warm pool has expanded to twice its size – from 22 million square kilometers, to an area of 40 million square kilometers during 1981–2018 – and is projected to warm and expand even further.

Although the total lifespan of MJO is no more than 60 days, the amount of time it spends on average over the Indian Ocean has decreased by 3–4 days (from 19 to 15 days), while its presence over the West Pacific has increased by 5–6 days (from 18 to 23 days). 16

These changes in the life-cycle of the Madden-Julian Oscillation have resulted in increased rainfall over southeast Asia, northern Australia, the west coast of the United States, Ecuador and the Amazon rainforest, as well as Southwest Africa.

References

  1. “Zhang, Chidong (2005).” Rev. Geophys. 43 (2). []
  2. Mysterious engine of the Madden‐Julian Oscillation.” Zhang, C. 2020. []
  3. “Description of global-scale circulation cells in the tropics with a 40-50-day period.” Madden, R.A., and P.R. Julian, 1972. J. Atmos. Sci., 29, 1109-1123. []
  4. “What is the MJO, and why do we care?” []
  5. “Meet the MJO.” Jon Gottschalck. NOAA Climate Prediction Center. (PDF) []
  6. “Influence of Madden–Julian Oscillation on the Intraseasonal Variability of Summer and Winter Monsoon Rainfall in the Philippines.” Gerry Bagtasa. J. Climate (2020) 33 (22): 9581–9594. []
  7. “Global Impacts of the Madden–Julian Oscillation.” C. Zhang, Encyclopedia of Atmospheric Sciences (Second Edition), 2015. []
  8. “The Madden–Julian Oscillation’s Impacts on Worldwide Tropical Cyclone Activity.” Philip J. Klotzbach. J. Climate (2014) 27 (6): 2317–2330. []
  9. “Meet the MJO.” Jon Gottschalck. NOAA Climate Prediction Center. (PDF) 2008. []
  10. “Evaluating the Joint Influence of the Madden‐Julian Oscillation and the Stratospheric Polar Vortex on Weather Patterns in the Northern Hemisphere.” Matthew R. Green, Jason C. Furtado. September 2019 []
  11. “Skillful Wintertime North American Temperature Forecasts out to 4 Weeks Based on the State of ENSO and the MJO.” Nathaniel C. Johnson, et al. Wea. Forecasting (2014) 29 (1): 23–38. []
  12. “How MJO Teleconnections and ENSO Interference Impacts U.S. Precipitation.” Marybeth C. Arcodia, et al. J. Climate (2020) 33 (11): 4621–4640. []
  13. “Evidence of Specific MJO Phase Occurrence with Summertime California Central Valley Extreme Hot Weather.” Lee, YY., Grotjahn, R. Adv. Atmos. Sci. 36, 589–602 (2019). []
  14. Met Office UK. []
  15. “Human-caused Indo-Pacific warm pool expansion.” Weller, E. et al. Science Advances, vol. 2, issue 7. July 2016 []
  16. “Twofold expansion of the Indo-Pacific warm pool warps the MJO life cycle.” Roxy, M.K., et al. Nature 575, 647–651 (2019). []
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