The influences of ENSO and the subtropical Indian Ocean Dipole on tropical cyclone trajectories in the southwestern Indian Ocean

El Nino-Southern Oscillation (ENSO) is known to associate with variability of tropical cyclone (TC) trajectories in the southwestern Indian Ocean. However, consideration of ENSO phase alone does not account for all variability of TC tracks within this region. This study demonstrates that the subtropical Indian Ocean Dipole (SIOD) sea-surface temperature anomaly pattern is also significantly associated with variability in southwestern Indian Ocean TC tracks. Hierarchical cluster analysis is employed to group TC trajectories by their initial and final positions. Median monthly values of the Nino-3.4 index and Subtropical Dipole Index corresponding to the life cycles of TCs in each group are compared using non-parametric analysis of variance. The results suggest that both ENSO and SIOD are significantly associated with different types of southwestern Indian Ocean TC trajectories. Furthermore, significant interactions of ENSO and SIOD phases are found to influence certain types of TC tracks using contingency table tests. During simultaneous warm ENSO and negative SIOD phases, TCs moving across the southwestern Indian Ocean tend to follow more southward or southeastward tracks. During neutral or cool ENSO and positive SIOD phases, TCs moving through the southwestern Indian Ocean tend toward more westward trajectories. These findings suggest that use of an SIOD index in addition to an ENSO index could improve intraseasonal to seasonal statistical prediction of southwestern Indian basin TC activity. Copyright (C) 2010 Royal Meteorological Society