A Dynamic Genesis Potential Index for Understanding how MJO Modulates Tropical Cyclone Genesis

Diagnostic analysis of Madden-Julian Oscillation’s (MJO’s) modulation of tropical cyclone (TC) genesis has been popularly based on Emanuel-Nolan Genesis Potential Index (GPI). Factors such as total vertical wind shear and maximum potential intensity that are critically important for explaining climatological TC genesis have been thought also important for explaining MJO’s modulation of TC genesis. Here we show that these two factors do not represent important physical processes by which the MJO circulation anomalies affect TC genesis. Based on analysis of MJO and TC genesis relationship in Austral summer (November to April) from 1979 to 2014, we found the most effective factor is the low-level relative vorticity, which is largely the meridional shear vorticity associated with the MJO zonal wind anomalies. The 500 hPa vertical motion and the vertical shear of zonal winds are additional complementary factors. A new dynamic GPI (DGPI) is established to quantify MJO’s control of anomalous TC genesis using these three dynamic factors. The DGPI has significant high correlation skill in characterizing TC genesis variability on eight different phases of MJO during November-April (r=0.83). The DGPI has also significant skills in characterizing TCG variability in individual basins including southern Indian Ocean, Australian monsoon region and South Pacific. We have shown that the new DGPI has significantly higher performance than currently used GPI in representing intraseasonal variation patterns of TCG in each phase of the MJO evolution and in each sub-regions of Southern Ocean. Application of the new anomalous DGPI may improve sub-seasonal prediction of TC genesis.