The normally arid coast of Peru and Chile is saturated by heavy rains. In the western Pacific, over Indonesia, atmospheric pressure builds, and an area that normally receives large amounts of rainfall may experience drought conditions. Sea surface temperatures fall to lower than normal values. The easterly trade winds become stronger, and ocean upwelling intensifies along the coast. High pressure builds over the west coast of South America producing arid conditions.
Atmospheric pressure drops over Indonesia enhancing convection and producing heavy rainfall.
This pattern of sea surface temperatures and atmospheric pressure is quite similar to the normal conditions only more intense. ENSO events usually last several months with peak sea surface warming or cooling occurring in December or January. El Ni o events occur once every 2 to 7 years with the average being between 3 and 4 years. There have been fewer La Ni as than El Ni os over the past century. The enhanced convection over the equatorial Pacific produced by El Ni o injects vast quantities of heat and moisture into the tropical atmosphere altering the general circulation pattern globally.