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

Time indices of the Madden–Julian Oscillation (MJO) are often used to generate empirical forecasts of the global circulation. Moist deep convection associated with the MJO initiates eastward propagating Rossby waves that disperse into the mid latitudes. The background circulation then guides some of these waves back into the tropics of the eastern Pacific Ocean during northern hemisphere winter. Intrusion of these extratropical Rossby waves into the tropics can initiate westward propagating equatorial Rossby (ER) waves by disturbing the equatorial waveguide. The westward propagating ER waves later intersect with and modulate eastward propagating convection associated with the MJO. MJO convection modulates the zonal wind, which influences the location and existence of westerly wind ducts. Thus interactions between ER waves and MJO convection create a feedback loop that helps to determine the preferred location of future ER wave formation. We demonstrate through a simple composite analysis that a simultaneous assessment of MJO and ER waves yields more information about the extratropical circulation than can be obtained from either field alone, or from a simple linear combination of the two fields. 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.