Modulation of the Extratropical Circulation by Combined Activity of the Madden–Julian Oscillation and Equatorial Rossby Waves during Boreal Winter

Abstract

ime indices of the Madden?Julian oscillation (MJO) are often used to generate empirical forecasts of the global atmospheric circulation. Moist deep convection associated with the MJO initiates eastward-propagating Rossby waves that disperse into the midlatitudes. The background circulation then guides extratropical waves back into the tropics of the eastern Pacific Ocean. Previous works have shown that equatorial Rossby (ER) waves occur following intrusion of extratropical Rossby waves into the tropics. Westward-propagating ER waves and the MJO modulate the total convection. This convection modulates the zonal wind, which influences the location and existence of westerly wind ducts. These wind ducts, in turn, guide extratropical waves into the tropics. This paper demonstrates through a simple composite analysis that a simultaneous assessment of MJO and ER waves yields more information about the extratropical circulation during boreal winter than can be obtained based on either type of disturbance alone, or from a sum of the signals associated with the MJO and ER waves composited separately. This analysis, together with previous results, suggests a feedback loop between the MJO, these waves, and the extratropical circulation. Thus, assessment of the ER wave state during a particular phase of the MJO might yield better empirical prediction of the global atmospheric circulation that follows.