Boreal summer Asian monsoon precipitation is characterized by northward-propagating disturbances on ~30 day timescales. These northward-propagating (NP) disturbances are often linked to the eastward-moving Madden Julian oscillation (MJO), and are frequently interpreted as the equatorial Rossby (ER) wave component excited by equatorial heating, and then modified by basic state easterly shear. In the presence of deep easterly horizontal shear, near-equatorial convection induces positive (negative) vertical shear anomalies to its north (south).

Several mechanisms have been proposed to explain how the north-of-convection positive vorticity anomaly interacts with developing convection and the anomalous and mean state wind and moisture fields, ultimately leading to the observed northward propagation of the monsoonal precipitation: 1) divergence associated with the positive tropospheric vorticity anomaly may be balanced by boundary layer convergence, leading to new convection developing north of existing convection, and 2) tropospheric vorticity and boundary layer moisture may be advected northward, further displacing developing convection from existing convection. Additionally, warm SST anomalies north of deep convection may destabilize the lower troposphere and lower the surface pressure, further aiding the northern advance of monsoon precipitation.

In this study, we evaluate each of these mechanisms in relation to boreal summer OLR in observations and in the super-parameterized Community Climate System Model (v3.0), which produces a highly realistic Asian monsoon. Because model climatology and variability differ somewhat from observations, all time series are normalized, and then composited onto a) 20-100 day filtered and b) ER wave OLR indices. Normalization allows us to evaluate which NP mechanism is most strongly associated with NP. Our results indicate that boundary layer convergence associated with deep vorticity anomalies (mechanism #1 above) is the primary mechanism of northward propagation for the Asian monsoon, although other mechanisms also play a role. SP-CCSM produces a similar distribution of NP mechanisms, but differences in mean state vorticity and boundary layer moisture lead to different patterns of advection of those quantities by the well-simulated wind anomalies. A tentative conclusion from this study is that simulated convection must be able to realistically respond to changes in boundary layer convergence and moisture in order to simulate observed monsoon behavior.