The ACE of a season is the sum of the ACEs for each storm and takes into account the number, strength, and duration of all the tropical storms in the season.

As well as being squared ACE can also be cubed, and this version is known as the Power Dissipation Index (PDI).

During the last several decades the number of El Niño events increased, and the number of La Niña events decreased.[48] The question is whether this is a random fluctuation or a normal instance of variation for that phenomenon, or the result of global climate changes towards global warming.

The studies of historical data show that the recent El Niño variation is most likely linked to global warming. For example, one of the most recent results is that even after subtracting the positive influence of decadal variation, shown to be possibly present in the ENSO trend[49], the amplitude of the ENSO variability in the observed data still increases, by as much as 60% in the last 50 years[50].

It is not certain what exact changes will happen to ENSO in the future: different models make different predictions (cf.[51]) It may be that the observed phenomenon of more frequent and stronger El Niño events occurs only in the initial phase of the global warming, and then (e.g., after the lower layers of the ocean get warmer as well), El Niño will become weaker than it was[52]. It may also be that the stabilizing and destabilizing forces influencing the phenomenon will eventually compensate for each other[53]. More research is needed to provide a better answer to that question, but the current results do not completely exclude the possibility of dramatic changes.

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El Niño is credited with suppressing hurricanes and made the 2009 hurricane season the least active in twelve years.[28] El Niño is also associated with increased wave-caused coastal erosion along the United States Pacific Coast.[citation needed]

There is some evidence that El Niño activity is correlated with incidence of red tides off the Pacific coast of California[citation needed].
[edit] Tropical cyclones

Most tropical cyclones form on the side of the subtropical ridge closer to the equator, then move poleward past the ridge axis before recurving into the main belt of the Westerlies.[29] When the subtropical ridge position shifts due to El Niño, so will the preferred tropical cyclone tracks. Areas west of Japan and Korea tend to experience much fewer September-November tropical cyclone impacts during El Niño and neutral years. During El Niño years, the break in the subtropical ridge tends to lie near 130°E, which would favor the Japanese archipelago.[30] During El Niño years, Guam's chance of a tropical cyclone impact is one-third of the long term average.[31] The tropical Atlantic ocean experiences depressed activity due to increased vertical wind shear across the region during El Niño years.[32]