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What is the Madden-Julian Oscillation?

The Madden-Julian Oscillation, or MJO for short, is characterised by an eastward spread of large regions of enhanced and suppressed tropical rainfall, which is mainly observed over the Indian Ocean and the Pacific Ocean.

It was discovered in 1971 by Dr Roland Madden and Dr Paul Julian of the American National Centre for Atmospheric Research (NCAR), when they were studying tropical wind and pressure patterns. They observed regular oscillations in winds between Singapore and Canton Island in the west central equatorial Pacific.

How does the MJO work?

An area of enhanced tropical rainfall is first apparent over the western Indian Oceans, which spreads eastwards into the warm waters of the tropical Pacific. This pattern of tropical rainfall tends to lose its identity as it moves over the cooler waters of the eastern Pacific, before reappearing at some point over the Indian Ocean again.

A wet phase of enhanced convection (rainfall) is followed by a dry phase, where thunderstorm activity is suppressed (no rainfall). Each cycle lasts approximately 30-60 days and there are 8 phases.

The MJO phases

  • Phase 1 – Enhanced convection (rainfall) develops over the western Indian Ocean.
  • Phase 2 and 3 – Enhanced convection (rainfall) moves slowly eastwards over Africa, the Indian Ocean and parts of the Indian subcontinent.  
  • Phase 4 and 5 – Enhanced convection (rainfall) has reached the Maritime Continent (Indonesia and West Pacific)
  • Phase 6, 7 and 8 – Enhanced rainfall moves further eastward over the western Pacific, eventually dying out in the central Pacific.
  • The next MJO cycle begins.

Following a region of enhanced convection (rainfall) is a region of suppressed convection (no rainfall). During the MJO cycle there is a ‘dipole’ (a stark contrast) in rainfall anomalies. For example, In phase 6 there is enhanced convection over the western Pacific and suppressed convection over the Indian Ocean. In phase 2, it is the opposite way around.

How does the MJO affect weather around the world?

There are a number of ways in which the MJO influences world weather:

  • The MJO creates favourable conditions for tropical cyclone activity, which makes the MJO important to monitor during the Atlantic hurricane season.
  • The enhanced rainfall phase of the MJO can also bring the onset of the Monsoon seasons around the globe. Conversely, the suppressed convection phase can delay the onset of the Monsoon season.
  • There is evidence that the MJO influences the El Nino Southern Oscillation (ENSO) cycle. It does not cause El Nino or La Nina, but it can contribute to the speed of development and intensity of El Nino and La Nina episodes. The MJO appears to be more active during neutral and weak ENSO years.
  • There is also evidence to suggest that the MJO can influence the onset of a Sudden Stratospheric Warming (SSW) event.

How does the MJO impact UK weather?

  • When the MJO is in its active phase over Indonesia and the West Pacific, it tends to drive a negative North Atlantic Oscillation (NAO) 2 to 3 weeks later.
  • A positive NAO index tends to be preceded by phase 3 and 4 of the MJO, which brings milder and wetter weather across the UK.
  • A negative NAO index tends to be preceded by phase 6 and 7, which influences a ‘blocked’ weather pattern and is often associated with colder and drier weather across the UK.
  • The timescale of the MJO having an influence on North Atlantic weather regimes is usually 10 to 12 days.

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