Observed triggering of tropical convection by a cold surge: implications for MJO initiation



The extratropical influence is an important mechanism in the initiation of the Madden-Julian Oscillation (MJO). Based on analyses of several datasets, this article demonstrates that the MJO case initiated in late January 2008 was strengthened by a preceding cold surge over West Asia when the MJO was at its initiation phase. The cold surge-related northerlies propagated southward along the east coast of Africa. The associated convergence on the edge of the surge enhanced ascending motion in the tropical western Indian Ocean and led to the onset of deep convection to the north of Madagascar. This process helped to amplify the MJO convection rapidly, as shown in the development of the moist static energy anomalies. In this way, the cold surge affected the behaviour of the MJO and modified the timing of its initiation to some extent. These results support previous studies that extratropical factors are important for the initiation of the MJO, and suggest that the subtropical cold surges are likely to strengthen and accelerate the build-up of deep MJO convection at the MJO initiation phase. Copyright © 2012 Royal Meteorological Society