The Madden-Julian Oscillation (MJO) is a large-scale wave pattern in the tropical atmosphere that comprises a major fluctuation in tropical weather over time scales of weeks to months. This eastward moving pulse of anomalous variations in rainfall, wind, sea surface temperatures, and clouds in the tropics typically recurs every 30–60 days, creating a pattern that profoundly influences global weather and climate systems, including monsoons, tropical cyclone activity, and El Niño–Southern Oscillation events.
Although understanding this oscillation is crucial for improving weather predictions, how the MJO is triggered remains poorly understood. Outstanding questions include whether MJO events are commonly preceded by an earlier occurrence and, if so, what role the previous cycle plays in initiating a new MJO event; whether the processes that drive the moistening of the troposphere affect MJO behavior; and whether other large planetary waves—such as Rossby waves—can help trigger the next MJO.
To test the importance of these factors in initiating MJOs, Maloney and Wolding used a modified version of the National Center for Atmospheric Research Community Atmospheric Model version 3.1 (CAM3) run in aquaplanet mode. The results show that nearly 80% of the modeled MJOs are preceded by a previous strong MJO, suggesting that interactions with earlier events strongly influence the development of new intraseasonal oscillations. Consistent with previous studies, the team also found that Rossby gyres associated with a previous MJO event help to initiate the next cycle by moistening the atmosphere in the western part of the Eastern Hemisphere ocean warm pool before the new MJO heads east.
The extent to which these model results apply to the real world is uncertain. Although their conclusions are broadly consistent with reported observations, the researchers acknowledge that the details differ because of the model’s idealized aquaplanet setup. They suggest that future experiments should use more realistic parameters, including a more northerly location for the initial MJO convection, more realistic ocean-atmosphere interactions, and more practical land-ocean locations, to assess the robustness of their findings. (Journal of Advances in Modeling Earth Systems (JAMES), doi:10.1002/2015MS000495, 2015)
—Terri Cook, Freelance Writer
Citation: Cook, T. (2016), What triggers intraseasonal oscillations in the Indian Ocean?, Eos, 97, doi:10.1029/2016EO044069. Published on 22 January 2016.