How Atmospheric Synoptic Kelvin Waves Influenced the Propagation of the Madden-Julian Oscillation (MJO)

The Madden-Julian Oscillation (MJO) is a large pattern of cloudiness and precipitation that forms over the equatorial Indian Ocean and the adjacent west Pacific and propagates slowly eastward over several weeks. The pattern is an east-west dipole with a region of enhanced cloudiness and rainfall big enough to cover much of the tropical Indian Ocean, accompanied by an equally large region of clear skies and reduced precipitation to the east.

The MJO has been called the "holy grail" of tropical meteorology, as 50-years of research to date has not produced a generally accepted explanation for it.

The MJO is sometimes called a multiscale phenomenon since its rainy half is best described as an envelope of convection in which various forms of convective aggregation occur. The extent to which the smaller convective disturbances within the MJO envelope affect the large-scale behavior of the MJO is an open question. Work conducted here looks specifically at the extent to which the MJO is influenced by synoptic Kelvin waves, wave disturbances centered on the equator which are smaller than the MJO and propagate through its envelope from west to east.

Previous work by the PI shows that the vertical tilt of the Kelvin waves is altered as they move through the convective envelope. The change in tilt may lead to a vertical flux of eddy momentum that contributes to the eastward advance of the MJO. A further mechanism through which Kelvin waves can affect the MJO involves upper-level ridging due to the advection of low potential vorticity air by the Kelvin waves, which could also help move the MJO envelope eastward.

The project uses a combination of statistical techniques combined with examination of the moisture and momentum budgets to assess Kelvin wave impacts on the MJO. Datasets to be used include reanalysis products and satellite-based estimates of convective clouds and rainfall. Once the analysis is completed using observational datasets it is repeated for simulations from the Coupled Model Intercomparison Project (CMIP).

The work has societal relevance due to the importance of the MJO for long-range weather prediction. The MJO influences weather phenomena around the world including the monsoons of Asia, hurricanes in the Gulf of Mexico, and atmospheric rivers making landfall in the western US. The slow propagation of the MJO means that its effects can, at least in principle, be anticipated well in advance.

But the MJO is not well captured in weather and climate models, and work conducted here can directly inform efforts to improve MJO simulation in such models. In particular the work may show the need for better simulation and initialization of Kelvin waves as a key to MJO simulation improvement. The project supports two graduate students, thereby developing the future workforce in this research area.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.