Diversity of MJO Initiation Regions and Processes

Abstract

The diversity of the Madden–Julian oscillation (MJO) initiation (i.e., initial onset of active convection before moving eastward) regions was explored using a clustering method. Regions favorable for MJO initiations are grouped into four longitude zones: the Atlantic and Africa (AA), the Indian Ocean (IO), the Maritime Continent (MC), and the western Pacific (WP). The region-dependent dominant initiation mechanisms are explored using a composite procedure. The AA initiation is attributed to a circumnavigating process associated with a preceding MJO. As upper-tropospheric westerly anomalies move into the AA region, the associated descending motion leads to suppressed convection over the IO, which further triggers convection onset to its west through anomalous westward moisture advection. The IO initiation arises from the downstream forcing of a preceding suppressed phase of MJO. A delayed air–sea interaction process also plays a role. The MC initiation is triggered by a westward-propagating dry equatorial Rossby wave in the Pacific. The low-level poleward flows associated with the anticyclonic Rossby wave gyres advect high mean moisture, promoting the convection onset over the MC. The WP initiation is triggered by a preceding suppressed phase of MJO that moves eastward, in a way similar to the downstream scenario in the IO. The AA initiation is usually associated with a La Niña–like background sea surface temperature pattern, which favors the decoupling of upper-tropospheric westerly anomalies from the preceding MJO. The MC and WP initiations are more frequent during El Niño, as the relevant meridional moisture gradient is sharper and the eastern Pacific is moister.