If you let go of the opening of a balloon, 'wind' blows out from the higher pressure area inside the balloon to the lower pressure area outside the balloon. The water at the surface of the oceans is warmer than deeper water. The warmer, lighter water 'floats' on top of the colder, denser water. If winds blow in one particular direction for a long period of time, much of the warm surface water will be moved. Air pressure is usually higher on the eastern side of the Pacific Ocean near South America.
Air pressure is usually lower over the western side of the Pacific Ocean near Australia.
This causes the winds over the Pacific Ocean to blow from east to west. The trade winds cause warm water to be blown away from South America's coast toward Australia. The result is a large 'pool' of warm water in the western Pacific Ocean. Because warm water evaporates easily, thus causing more clouds and rainfall, there is usually a lot of rain over the western Pacific Ocean. The trade winds decrease in strength, or even reverse.