The Madden‐Julian Oscillation (MJO) is a tropical atmospheric phenomenon. It usually starts over the Indian Ocean, moves eastward across the Indo-Pacific Maritime Continent and into the Pacific Ocean over about a month, bringing heavy rainfall and stiff winds. As it moves eastward, it influences weather and climate phenomena in many parts of the world. A recent article in Reviews of Geophysics explores four different theories that seek to identify the mechanisms for the MJO. Here, one of the authors gives an overview of what we know and still don’t know about the MJO and its impacts.
What influences does the MJO have globally?
The MJO affects many weather and climate events globally, not only in the tropics but also in the extratropics and polar regions. It influences the physical characteristics of the atmosphere and oceans, as well as chemical and biological processes on Earth (Zhang, 2013). For example, frequencies of tropical cyclones, tornados, flood, and heat waves can all change depending on whether the MJO is located over the Indian or Pacific Oceans.
What have been some of the major advances in our understanding of the MJO?
Since it was first documented by Madden and Julian [1971, 1972], the MJO has attracted many scientists to study it, but a complete understanding remains a holy grail in tropical atmospheric research (Raymond, 2001). After almost 50 years of research, we know that the MJO is closely related to other tropical perturbations, water vapor plays a prominent role in it, and it actively interacts with the ocean, the extratropics, and the stratosphere.
To what extent is it possible to forecast the MJO and its impacts?
MJO wind can be better predicted (up to 30 days in advance) than MJO rainfall (limited to less than 10 days in advance). Prediction of MJO global impacts is even harder but progress has been made lately. For example, near-surface temperature might be predicted 15 to 30 days in advance in certain regions of North America and during certain parts of the year. Weekly tornado activities in the United States can sometimes be predicted 2 to 5 weeks ahead based on the MJO.
Why is there no consensus on a single theory to explain MJO characteristics and dynamics?
There is not even consensus on what an MJO theory should explain! Because this is still a rapidly growing research area, new ideas keep coming out. The diverse thinking of MJO dynamics is a healthy sign of the field. That said, different ideas gradually converge on the important roles of equatorial waves and moisture in the MJO.
What are some of the unresolved questions where additional research, data, or modeling are needed?
The main unresolved issue is to understand the fundamental dynamics of the MJO. Another area of ongoing investigation is how the Indo-Pacific Maritime Continent (MC) affects the eastward propagation of the MJO. In observations, some MJO events propagate from the Indian Ocean across the MC into the Pacific, while others do not. This is known as the “barrier effect” of the MC on the MJO. The precise reason for this barrier effect is unclear.
Scientists are also working on how to better predict the MJO. Current MJO prediction skills are below estimated prediction limit (predictability) by a large margin. MJO prediction errors should be reduced by improving both prediction models and observations that feed information to the prediction models.
A further challenge is how climate models can be improved so they can reproduce the MJO. Most climate models cannot reproduce the observed statistics of the MJO. This indicates their deficiencies that must be fixed.
Another question being pursued is to better understand the combined effects of the MJO and other semi-oscillatory phenomena that also affect global weather. These other phenomena, including the El Niño – Southern Oscillation, North Atlantic Oscillation, Indian Ocean Dipole, and others, also affect the MJO. Their combined effects on global weather make forecasting beyond two weeks both challenging and rewarding.