What is the southern oscillation index?

Question

it has some thing to do with cyclones

Answer 1

The Southern Oscillation Index measures the surface pressure difference between Tahiti and Darwin, Australia in the Pacific Ocean.  When the index is negative, the trade winds blow in a way such that causes a build up of warmer than normal waters in the Pacific Ocean, otherwise known as El Nino.  When the index is positive, the trade winds blow in such a way that they cause an upwelling of cold water which translates to La Nina.  So the index is negatively correlated to El Nino, that is to say when the index in negative, sea-surface temperatures anomalies are positive (El Nino).