Three different downstream fates of the boreal-summer MJOs on their passages over the Maritime Continent

Abstract

The boreal-summer MJOs exhibit three distinctive downstream evolutions on their passages over the Maritime Continent (MC): (i) smoothly transitioning from Indian Ocean (IO) to western Pacific (type-I); (ii) rapidly decayed over the MC (type-II); and (iii) significantly intensified over the MC (type-III). During the life cycles of type-I and -III MJOs, basin-wide overturning circulations connect the convective dipoles between western and eastern north Pacific. For type-II MJO, regional overturning circulations connect the convective dipoles between the IO and Western North Pacific (WNP). For type-I and -III MJOs, the associated convection over eastern IO is able to generate strong Kevin-wave responses and significant positive SST anomalies over the MC and western Pacific. On the other hand, for type-II MJO in the IO, a persistent convection lingers around the WNP. It intensifies westerly monsoon flows, thus preventing the generation of coherent positive SST anomalies ahead of the IO MJO convection. It also blocks the formation of a robust Kelvin-wave response over the MC and western Pacific by delaying the northeastward migrations of the dry phase. The convection of type-I and -III MJOs at eastern IO is able to generate positive boundary-layer moisture anomalies along with positive column-integrated moisture tendencies over the MC and western Pacific, thus allowing them to propagate eastward smoothly. On the other hand, the type-II MJO convection in eastern IO is associated with a very dry troposphere and negative column-integrated moisture tendency over western Pacific, which result in the rapid decay of convection over the MC. These results emphasize the important roles of robust Kelvin-wave responses and positive SST anomalies on the eastward propagation of boreal-summer MJOs over the MC through moistening the troposphere ahead of the convection.