The Southern Oscillation Index (SOI) is an index that is based on the difference in air pressure between Darwin and Tahiti. The SOI is closely related to the El Nino/La Nina climate phenomenon. A consistently negative SOI often indicates the presence of an El Nino. An El Nino is defined as a prolonged warming of the eastern tropical Pacific Ocean, which leads to a weakening or even a reversal in the trade winds across the Pacific Ocean. Subsequently, this tends to reduce rainfall in eastern and northern Australia. Major El Nino events in the past have included 1997/98 and 1982/83.
A consistently positive SOI often indicates the presence of a La Nina. A La Nina occurs when sea surface temperatures to the north of Australia are warmer than normal and trade winds across the Pacific Ocean strengthen. This is the opposite of an El Nino, and subsequently, the results are also opposite. Rainfall probabilities across eastern and northern Australia are higher than normal. Recent strong La Nina events have included 1998/99 and 1988/89.
To calculate the SOI, the formula used is:
SOI = 10 x ((average Tahiti MSLP for month) - (average Darwin MSLP for month) - long term average difference for month) / long term standard deviation of difference for month
However, the website makes it easy for you by providing a graph of the current SOI, updated weekly. When interpreting this graph, remember that it is sustained trends you are looking for. One month of positive SOI does not indicate a La Nina; six months might.