Rationale for conducting this investigation is based upon the fact that El Ni o events change convective patterns locally over the Pacific Ocean and globally the general atmospheric circulation. When the equatorial trade winds weaken, warm waters build up in the Eastern Pacific. Subsequently, large amounts of water evaporate at the surface. This sequence of events creates a low pressure system, or trough in the Eastern Pacific.
Researchers are uncertain as to what triggers the weakening in the trade winds.
What is agreed upon is that these changes impact the the general atmospheric circulation pattern and can alter climate conditions across the globe. West experiences 6 consecutive months of sea surface temperatures of at least 0.5 deg. C above normal. They further limited the criteria by stating that the series of six consecutive months must begin before September, and must include October, November, and December. They found that an El Ni o typically starts in late summer and lasts for about a year.