Interactions between the Madden-Julian Oscillation and equatorial Rossby waves through high-frequency transient eddies

Past studies have demonstrated that extratropical waves that propagate into the east Pacific basic modulate convection in the ITCZ during boreal winter when upper-level westerly winds are present. Intraseasonal variability such as the Madden-Julian Oscillation (MJO) can influence this equatorward extratropical wave activity by modulating the background state of both the extratropics and the tropics through which the high-frequency waves propagate. The extratropical waves can enhance the ITCZ convection and trigger atmospheric equatorial Rossby (ER) waves that interact with and feed back onto the convective signals on the MJO timescale. This study diagnoses how variability of extratropical wave activity associated with the MJO plays a role in the relationship between the MJO and ER waves.

Wavelet analysis is applied to Outgoing Longwave Radiation (OLR) and meridional wind data at each grid point to deduce power in the ER wave frequency band and in high-frequency extratropical Rossby wave activity in three dimensions. This linear analysis assesses the activity level of fields characterized by different frequencies. The relationship between the MJO and extratropical wave activity is examined by a composite analysis based on MJO phase. Wind fields and extratropical wave characteristics are studied by selecting a geographical location where the relationship between the MJO and the wave activity is strong and applying a lag composite analysis. Finally, the association of the MJO modulation of extratropical waves with ER wave activity is examined.

Results show a statistically significant modulation of ER wave activity throughout the life cycle of the MJO. ER wave activity is generally enhanced (reduced) at the poleward edges of the convectively active (suppressed) region of the MJO. The active region of the 6-25days extratropical Rossby waves shifts meridionally and zonally as the strength and the spatial extent of the subtropical jet and the westerly duct change in association with the MJO. The extratropical waves tend to propagate closer to the equator during the local phase of the MJO characterized by enhanced upper-level westerly winds over the east Pacific when the subtropical jet is extended eastward and shifted northward. Consistent with theoretical predictions, extratropical Rossby waves can only propagate into the tropics where the upper-level westerlies are present. The southwest to northeast-tilted waves observed during that phase act to advect westerly momentum poleward resulting in strengthening of the jet. There is also some consistent pattern between extratropical wave activity and ER wave activity. Over the east Pacific where MJO convection is weak, ER wave activity becomes enhanced during this period of enhanced equatorward propagation of extratropical waves. These results support the idea that the extratropical waves can act as a source of ER waves, and that the MJO modulates this interaction.

This study supports the existence of feedback loops between the MJO, ER, and higher frequency extratropical wave activity. The MJO modulates the background state of both the extratropics and the tropics, which then modulates extratropical waves and ER activity that can then feed back onto the MJO. This circular relationship also varies with seasons and interannual variability such as ENSO.