Modulation of interannual variability of tropical cyclone activity over Southeast Indian Ocean by negative IOD phase

Tropical cyclones (TCs) over Southeast Indian Ocean (SEIO) have the notable interannual variability caused by ENSO and Indian Ocean Dipole (IOD). In the September-November of El Niño and October-November of positive IOD (PIOD), SEIO TCs is far less than its climatology. However, it is hard to separately understand El Niño and PIOD's impact on SEIO TCs due to their similar occurrence time and time scale. Unlike El Niño and PIOD, SEIO TCs is remarkably more than its climatology only in September-November of negative IOD (NIOD) instead of La Niña. Consequently, it is concluded NIOD mainly affects SEIO TCs' increase. Diagnose results suggest the relative humidity (RH) contributes mostly to the TCs' increase, vertical wind shear provides the secondary positive contribution, vorticity term also makes a weak positive contribution and PI term's contribution even may be negligible. The study still uncovers the process of RH change: NIOD reaches its peak period and changes atmosphere circulation to make a positive low-level vorticity anomaly over SEIO. Vorticity anomaly strengthens upward motion. The vertical velocity anomaly and climatogical specific humidity (SH) work together to make vertical advection play a dominant role in SH variation. SH's change mainly reflects in RH variation. Eventually, all of these associates with NIOD lead to more SEIO TCs in September-November and the significance of difference is above 99%.