The variability of Madden-Julian oscillation convection and the role of the Western Hemisphere circulation

The convective initiation of the Madden-Julian Oscillation (MJO) over the Indian basin is often associated with eastward-propagating intraseasonal circulation approaching from the Western Hemisphere (WH), which is suggested to influence the onset of MJO convection. This dissertation first documents the event-to-event variability in the characteristics of MJO convection, which shows a geographical discrepancy in the relative roles of moist and dry dynamics on MJO convection. This dissertation then examines the structural evolution and dynamics of the intraseasonal upper-tropospheric circumnavigating circulation over the WH and its subsequent impact on MJO convection over the Indian basin, where MJO convective characteristics are highly associated with circumnavigating signals.;Most MJO convective initiation events occur concurrently with upper-tropospheric easterly wind anomaly over the WH and negative equatorial atmospheric angular momentum (AAM) anomaly, which subsequently develop stronger convection over the Indian basin than the events with westerly wind or positive AAM anomalies. Previous studies often refer to the circumnavigating circulation over the WH as a Kelvin wave radiated from MJO convection over the warm pool. However, both the structure and driving mechanisms of intraseasonal zonal wind over the east Pacific basin are inconsistent with a theoretical Kelvin wave. The initial development of the easterly wind anomaly is associated with subtropical and midlatitude Rossby gyres. The easterly wind anomaly develops simultaneously with negative equatorial AAM anomaly because they share source phenomena that transport momentum while driving dynamics are different.;As the easterly wind anomaly propagates farther eastward, it constructs a Kelvin wave structure. The equatorward intrusion of midlatitude wave train triggers convection over South America, leading to the generation of an upper-tropospheric trough. The generated upper-tropospheric trough couples with easterly wind anomalies to construct the Kelvin wave, which then induces upper-tropospheric divergence above the region of initiating convection over Indian basin, enhancing lower-tropospheric updraft and convective development. Equatorial negative AAM anomaly can strengthen and zonally extend the upper-tropospheric easterly wind anomaly over the WH, further enhancing the development of MJO convection over the Indian basin. This dissertation demonstrates that extratropical circulation interacts with circumnavigating circulation over the WH, which together influence MJO convection over the Indian basin.