Examining the modulation of tropical cyclones by the MJO using a genesis potential index

Using tropical cyclone Genesis Potential Index (GPI), we explore the modulation of tropical cyclone (TC) activity by the Madden-Julian Oscillation (MJO). The daily GPI is calculated for the period 1974-2006, using the NCEP-NCAR Reanalysis and Reynolds SST data. Composite daily GPI anomalies are constructed for the different phases of the MJO, as defined by the Wheeler & Hendon MJO index. Northern and southern hemisphere composite anomalies are examined for the August -October and January - March seasons. The composite GPI anomalies (conducive to genesis) propagate eastward in tandem with the MJO phases. The anomalies show explicitly the MJO's modulation of the environment for tropical cyclogenesis. This modulation is still clearly present when the GPI anomalies in the regions around TCs are removed from the data before compositing. This shows that the MJO signal in GPI truly represents large-scale variations in the environment, rather than being an artifact due to presence of the TCs themselves.

We will diagnose the contribution of each of the four environmental variables on which the GPI depends to the MJO signal. We do this by producing composites of modified indices in which only one of the variables varies, with the others set to climatology. This allows us to determine the relative importance of the different factors comprising the GPI - potential intensity, vertical shear, low-level absolute vorticity, and midlevel relative humidity - to the MJO influence on genesis frequency.