The relationship between the Southern Oscillation Index and tropical cyclone tracks in the eastern North Pacific

Points of origin and downgradation of tropical cyclones in the eastern North Pacific (ENP) east of 160 degrees W are related to the Southern Oscillation Index (SOI) during the hurricane season. All ENP tropical storms and hurricanes from 1966 through 1997 were grouped into three SOI-based categories: those occurring during strong El Nino events (mean SOI < -0.6) strong La Nina events (mean SOI > 0.6), or near zero periods (-0.6 < mean SOI < 0.6). During El Nino storm seasons, ENP tropical cyclones originated approximately 5.7 degrees (617 km) west and downgraded 7.5 degrees (780 km) west of the long-term mean longitudes for the positive SOI group. Near zero group storms also followed more northerly tracks than the negative SOI group storms. However, no significant differences in storm track are evident between the positive SOI and near zero groups, and the track length is not significantly different for any storm group.