In the Basics section you have seen the normal condition in the pacific and also the deviations under El Niño conditions. (see text El Nino and Worksheets 3, 4, 5 in the basics section). In the section more - text El Nino and SOI you get more information. After having gone through those materials, you should be able to answer the following questions.
The seasonal variation of the pressure patterns is called the Southern Oscillation.
Whenever this happens - every 3-6 years - the whole system in the pacific plays up.