The seasonal variability in the global atmospheric circulation associated with the Madden-Julian Oscillation

The Madden-Julian Oscillation (MJO) is the dominant mode of intraseasonal variability in the tropics. Numerous studies have shown the association of extratropical circulation with the MJO. Previous studies focused mainly on the Northern Hemisphere winter, but the global circulation signal associated with the MJO (and to deep tropical convection in general) varies seasonally in a complex manner as the response depends on the background seasonal state. However, this seasonally varying tropical-extratropical interactions are not yet clearly understood. This study focuses on the seasonal variation in the association between the MJO and the intraseasonal to synoptic timescale extratropical circulation.

A composite analysis of the global atmospheric flow pattern associated with the MJO is first compared between different seasons to examine the anomalous global flow on the MJO timescale. Wavelet analysis is applied to diagnose poleward and equatorward propagating meridional wind signals at each grid point to deduce time-series of Rossby wave power in various time-frequency ranges. This transform of data into its power allows linear analysis on the relationship between the MJO and the extratropical wave activity moving poleward or equatorward at different time frequencies. The correlation between the power of extratropical wave signals in various frequency bands and the indices of the MJO is computed separately for each season. Comparison of the correlation shows the seasonal variability in the preferred frequency bands of the extratropical waves associated with the MJO.

During boreal winter on the MJO timescale, a pair of anticyclones is generated from the divergent flow in upper troposphere eastward and poleward of the convectively active region of the MJO. However, the anomalous anticyclone is weaker and and zonally more narrow in the summer hemisphere. The meridional potential vorticity (PV) gradient is stronger during winter; therefore the anticyclone and its subsequent response is stronger. The amount of variation associated with the MJO is reduced in the extratropics but strengthened in the tropics during the Southern winter. During that time, the PV gradient increases in the Southern Hemisphere but the northward shift of the MJO reduces the anomalous extratropical flow generated by the interaction between the MJO and the jet. Equatorward propagation of extratropical waves is limited by disappearance of the east Pacific westerly duct during the Southern winter in the mean state. In the region where eastward and equatorward propagating waves occur during the Northern winter, those waves tend to propagate westward and equatorward during the Southern winter. Enhanced convection occurs within or ahead of an upper level anticyclone, but the magnitude of the enhancement is much smaller than during the Northern winter, and no apparent generation of ER waves occurs.

Interaction between the tropical and extratropical circulations varies with the seasonally varying background state. Results suggest that the relative location between MJO convection and the extratropical PV field is important for the extratropical signals on the MJO timescale. The seasonal basic state in the extratropics determines the preferred location of the higher-frequency wave activity that is modulated by the MJO. The variability in the relationship between the MJO and the higher frequency extratropical waves may also feed back onto the seasonal variability of the MJO.