Madden-julian oscillation

From Glossary of Meteorology

Madden–Julian oscillation

A quasiperiodic oscillation of the near-equatorial troposphere, most noticeable in the zonal wind component in the boundary layer and in the upper troposphere, particularly over the Indian Ocean and the western equatorial Pacific.

This phenomenon is named after the codiscoverers. The oscillation can be detected globally in winds near the tropopause. The period of the oscillation varies between about 30 and 50 days, and appears to represent an eastward-propagating disturbance with the structure of a Kelvin wave with a vertical half-wavelength of the depth of the troposphere, but with a phase speed of only about 8 m s-1, much less than that of an adiabatic Kelvin wave. The disturbance is accompanied by strong fluctuations of deep convection, easily detectable using satellite observations, and is a major contributor to intraseasonal weather variability in equatorial regions from eastern Africa eastward to the central Pacific.