Convection–Vorticity Phase Relationship: Revisiting Simple Models of the Boreal Summer Intraseasonal Oscillation

The boreal summer intraseasonal oscillation (BSISO) is characterized by prominent northward propagation over the Indian Ocean and western Pacific and is considered as a distinct mode from its boreal winter counterpart, the Madden Julian Oscillation. The propagation of the BSISO has received increased attention partly due to the oscillations’ critical role in bridging weather and climate, especially its close connection to the variability of the monsoon. Several theories have been proposed to explain the propagation of the BSISO, which receives increased attention in recent years. To be specific, one group of two theories argue that strong vertical shear of the horizontal winds generates barotropic vorticity north of the convective center, which leads to boundary layer convergence and shifts the convection northward. At the heart of these theories lies the phase relationship between convection and barotropic vorticity that the latter leads the former during the northward propagation. However, recent diagnostics of numerical simulation and reanalysis data show that the lag between convection and the barotropic vorticity decreases as the BSISO propagates northward, and convection and barotropic vorticity are largely in phase north of 10°N in the BSISO life cycles. Here, two linear models constructed by previous studies are revisited. In particular, one of the models is able to capture the phase relationship, while the other is not. By contrasting the two models, the following two questions are approached. (1) What are the essential differences between these two theories? (2) What are the key processes that modulate the northward propagation and the convection-vorticity phase relationship?