Balanced Dynamics in the Madden-Julian Oscillation

Balanced dynamics in the tropics prescribes the response of the thermodynamic environment to potential vorticity anomalies. Specifically, a positive potential vorticity anomaly will induce tropospheric cooling below the anomaly with a warming above, resulting in a more stable environment. The changes in the thermodynamic environment have a profound effect on the development of convection: the convective mass flux becomes more bottom-heavy which concentrates convergence of moist air at low levels. This in turn results in stronger precipitation rates. In contrast, the opposite-signed temperature dipole anomaly produced by a negative potential vorticity anomaly in the mid-troposphere may completely inhibit the development of convection, since the warmer lower-tropospheric air does not contribute to positive buoyancy for parcels originating in the boundary layer.

Observational and numerical data suggest that the development of a positive mid-level vorticity anomaly produces a dipole temperature anomaly that is highly conducive for the early development of tropical cyclones. Recent work by Raymond et al (2015) suggests that the balanced dynamics response of the thermodynamic environment to vorticity may be relevant to a broader class of tropical convection than just the case of cyclogenesis.

In this work, we analyze data from the 2011-2012 DYNAMO field program--Dynamics of the Madden-Julian Oscillation (MJO)--to determine the extent to which balanced dynamics plays a role in the development of MJO convection. During the DYNAMO period, there were three significant instances of convective organization that resembled MJO events. The vorticity signature associated with each of these events differed, which provides the opportunity to evaluate the extent to which balanced dynamics influences individual MJO events. Preliminary diagnostics suggest that balanced dynamics does play a role in the development and decay of the organized convection in DYNAMO.