3.8.3.3 Eastern North Pacific

Tropical cyclone activity (both frequency and intensity) in this region is related especially to SSTs, the phase of ENSO and the phase of the QBO in the tropical lower stratosphere. Above-normal tropical cyclone activity during El Niño years and the lowest activity typically associated with La Niña years is the opposite of the North Atlantic Basin (Landsea et al., 1998). Tropical cyclones tend to attain a higher intensity when the QBO is in its westerly phase at 30 hPa in the tropical lower stratosphere. A well-defined peak in the seasonal ACE occurred in early 1990s, with the largest annual value in 1992 (Figure 3.40), but values are unreliable prior to 1970 in the pre-satellite era. In general, seasonal hurricane activity, including the ACE index, has been below average since 1995, with the exception of the El Niño year of 1997, and is inversely related to the observed increase in activity in the North Atlantic basin over the same time period. This pattern is associated with the AMO (Levinson, 2005) and ENSO. Nevertheless, there has been an increase in category 4 and 5 storms (Webster et al., 2005).

3.8.3.4 Indian Ocean

The North Indian Ocean tropical cyclone season extends from May to December, with peaks in activity during May to June and November when the monsoon trough lies over tropical waters in the basin. Tropical cyclones are usually short-lived and weak, quickly moving into the sub-continent. Tropical storm activity in the northern Indian Ocean has been near normal in recent years (Figure 3.40).

The tropical cyclone season in the South Indian Ocean is normally active from December through April and thus the data are summarised by season in Figure 3.40, rather than by calendar year. The basin extends from the African coastline, where tropical cyclones affect Madagascar, Mozambique and the Mascarene Islands, including Mauritius, to 110°E (tropical cyclones east of 110°E are included in the Australian summary), and from the equator southward, although most cyclones develop south of 10°S. The intensity of tropical cyclones in the South Indian Ocean is reduced during El Niño events (Figure 3.40; Levinson, 2005). Lack of historical record keeping severely hinders trend analysis.