Based on 1979-2012 TCtrack datasets from the JTWC, TC tracks in South Indian Ocean were interpolatedto grid boxes of 2.5° longitude by 2.5° latitude for attaining TC occurrencefrequency datasets. This paper analyzes the effect of IOD on tropicalcyclone activity in the South Indian Ocean. Tropical cyclone activities overthe South Indian Ocean are closely related to IOD SST anomalies. Due to the effectof IOD (Positive minus Negative), the difference between the positive andnegative IOD (Normal) years of TC genesis showed the east-west dipole patternof TC genesis. TCs increase in the western half of the SIO (west of 70°E) and decrease in the eastern half of the SIO (eastof 70°E). While for the differencebetween Negative IOD and Normal of TC genesis showed also the east-west dipolepattern of TC genesis but with TCs increase in the east of 95°E of SIO and decrease in the west of 95°E of SIO.
The IOD-TC genesis relation is supported byevident decrease in OLR in the central west (southeast) tropical SIO during Positive(Negative) IOD periods. Tropical cyclones formation is also affected by largescale circulation changes. During the Positive IOD periods, there is anticyclonic(cyclonic) circulation anomalies in the eastern part of the South Indian Oceanand cyclonic (anticyclonic) anomalies in the central west of South Indian Ocean.Also there is cyclonic vorticity anomaly, large relative humidity, and rising/ascendingmotion in the same position that both contribute to the formation of tropicalcyclones in the central west of SIO and in the Mozambique Channel while in theeastern South Indian Ocean there is low relative humidity, anticyclonicvorticity anomaly, sinking motion and large vertical zonal wind shear that inhibittropical cyclone formation and vice versa during Negative IOD period.
Thereforepositive IOD event influences more TCs genesis in the central west of SIO andin the Mozambique Channel while Negative IOD event influences more TCs genesisin the South east of SIO. The spatial differences in TC paths between Positive IOD and Negative IOD indicate a significant decrease in TC activity in the southeastern part of Madagascar (45°-55°E), with a slight increase in TC activity in the mid-latitudes of the SIO (55°-70°E) and a decrease in TC activity in the eastern part of the SIO (75°-105°E) .This may be due to the fact that the vertical wind shear in the eastern part of Madagascar and in the eastern part of the SIO is relatively strong, with relatively low relative humidity and a sinking movement at 500 hPa, resulting in a decrease in the number of TC paths here. While the mid-latitudes in the SIO have strong convective activity, weak vertical wind shear and large relative humidity, leading to an increase in the number of tropical cyclone paths here. During positive IOD period, southwesterly winds in the southeast of Madagascar may hinder further westward spread of tropical cyclones. Subsequently, the tropical cyclone was guided by anomalous northwesterly winds between two large-scale vortices, then weakened in the midlatitude of the central South Indian Ocean.
There are more TCs in the western half of South Indian Ocean (west of 75°E) during Positive IOD periods and more TCs in the south east of South Indian Ocean during Negative IOD periods.