Anatomy of the MJO in Upper Tropospheric Circulation

Abstract:

The upper tropospheric atmospheric circulation signal associated with the Southern summer Madden Julian Oscillation (MJO) has been described as having Kelvin wave structure over the Western Hemisphere, with a forced Kelvin wave ridge to the east of the deep convection and forced Rossby wave ridges straddling the equator to the west. This summary is misleading. Patterns east of the Dateline are more strongly associated with eastward advected Rossby waves that originate from extratropical Rossby waves that break into the tropics, and the signal becomes more Kelvin wave like as it approaches Africa.

Reversal of background zonal wind from westerly to easterly across Africa transmits Kelvin wave signal eastward to the Indian basin, but not the Rossby waves. MJO convection can emerge from these Kelvin waves coming from the west or from Kelvin wave signal generated over the Indian Ocean due to extratropical waves breaking there. Regardless, the upper tropospheric Kelvin wave diverges mass over the Indian Ocean, resulting upward motion cools the deep troposphere and ultimately enhances convection there due to reduced convective inhibition. Thus, over the equatorial Indian Ocean, the MJO is associated with a Kelvin wave ridge to the east and a Kelvin wave trough to the west. The Rossby gyre ridges are centered poleward of this pronounced Kelvin wave trough west of the convection.

This pattern appears equivalent between 2-6 m/s MJO signals, 12-30 m/s Kelvin wave signals, and 6-12 m/s intermediate disturbances. All of these three bands are occupied by signals fully consistent with the pressure-wind relationships of Kelvin waves in the upper troposphere proximate to the equator over the Indian Ocean, but also include Rossby gyres farther poleward. Vertical cross sections show that the slower and more strongly convectively coupled the upper tropospheric Kelvin wave becomes, the more the lower tropospheric pattern conforms to classical MJO signals, with a trough anomaly extending westward from the low-level easterly wind anomalies into the region of deep convection and low-level westerly wind. This presentation highlights this general story of MJO structure by using simple regression maps and real-time multivariate MJO (RMM)-index based composites.