Self-Similarity in Organized Tropical Convection

Tropical convection organized over a wide range of spatial and temporal scales, from Mesoscale Convective Systems (MCS) all the way up to the hemispheric phenomenon known as the Madden-Julian Oscillation (MJO). It is well-known that the convective envelope of the MJO, rather than being a slowly migrating band of enhanced mean cloudiness, is instead comprised of a host of higher frequency disturbances propagating both eastward and westward. We examined the large scale vertical structure of MJO circulation and moisture variability in reanalysis and radiosonde data. It was found that the observed vertical structure of the MJO also has much in common with other equatorial waves, although distinct differences are apparent in between the Indian and Pacific sectors. Over the western Indian Ocean, where the MJO is initially amplifying, the vertical structure of temperature is similar to convectively coupled Kelvin and inertio-gravity waves, with warmth at 250 hPa and separate cold perturbations at the surface and in the lower troposphere coincident with deepest convection. At the other extreme, in the central Pacific the lower tropospheric cool layer is missing at the expense of a much higher amplitude warm signal, which still has its peak at 250 hPa and now leads the convection by several days. The origin of the differences in these signals could be the result of differences in: (1) the basic state; (2) the horizontal scale and propagation characteristics of the diabatic heat sources and sinks within the waves; (3) the vertical structure of the heating profile due to variations in the morphology of convection within the wave; (4) the mechanisms of convective coupling; and (5) air-sea interaction effects, among other possibilities. Despite the large difference in scales between the MJO and the various waves that comprise its convective envelope, the MJO is still characterized by shallow convection which gives way to deep convection then stratiform precipitation as it propagates through a region. This progression is similar to that seen on the mesoscale and in other equatorial waves. This scale invariance is an intriguing feature of the coupling between convection and the large scale in the tropics, and may be crucial to its successful simulation in models.