Dry Air Intrusion Associated with Rossby Gyres and Its Impact on Eastward Propagation of the Large-Scale MJO Convection

Improving understanding and numerical model predictions of initiation of the Madden-Julian Oscillation (MJO) over the Indian Ocean (IO) was a main objective of the Dynamics of MJO (DYNAMO) field campaign. One of the fundamental questions central to the MJO initiation is what physical and dynamic processes contribute to the eastward propagation of the large-scale MJO convection. Observations from DYNAMO have revealed that multi-scale interactions among convective cloud systems, synoptic and large-scale circulation play an important role in the initiation of MJO events. In particular, dry air intrusion associated with Rossby-like gyres induced by the equatorial convection can penetrate into the equatorial IO. The drier air from higher latitudes is drawn in to the MJO convective envelope from the west by the circulation of the gyres, which leads to "breaks" in convection within the MJO envelope and eventually the abrupt shut-down of convection observed in DYNAMO as the MJO moved eastward. This study seeks to quantify the Rossby gyre activity and related dry air intrusion in both the DYNAMO and other MJO events from 1999-2013. The Rossby gyres are tracked using an objective vorticity tracking method. Their intensity, longevity, and propagation characteristics are determined for suppressed and active convective periods. Dry air intrusion is quantified by the advective tendency of the total precipitable water (TPW) into the Equatorial region. The characteristics of the vorticity maxima and their related dry air intrusion are different during suppressed and active convective periods in the equatorial IO. During the MJO convectively suppressed periods, the vorticity maxima tend to be dominated by meridional shear ("shear vorticity"), and they occur as strips of nearly-stationary vorticity features along the ITCZ. This large-scale pattern tends to limit dry air intrusion into the Equatorial region, allowing the build-up of moisture leading up to convective initiation. In contrast, during the active periods, the vorticity maxima are strengthened by convection, and some develop into westward-propagating Rossby-like gyres. The cyclonic circulation associated with the gyres brings the dry air deep into the equatorial region. These gyres can contribute drying trend up to ~8 mm/day in TPW over the Equatorial IO on the time scale of a few days. This dry air intrusion causes "break" periods in the active MJO and contributes to the abrupt drying on the west side of the MJO convective envelope. The dry air on the west side of main MJO convection and strong westerly winds associated with gyres act as a "propeller" which favors eastward propagation of the MJO convective envelope.