Surface pressure is a fundamental meteorological variable controlling weather systems and providing information on the intensity of weather systems, including tropical cyclones. Pressure observations are required for the long-term simulations of past weather and climate known as "reanalyses". Patterns of large-scale pressure variation are used to construct circulation indices that are closely linked to known variations in global and regional climate.
Philip Jones, Elizabeth Kent
Southern Oscillation Index (SOI)
Figure: The Southern Oscillation Index (SOI) is a standardized index based on the observed sea level pressure differences between Tahiti and Darwin, Australia. The SOI is one measure of the large-scale fluctuations in air pressure occurring between the western and eastern tropical Pacific (i.e., the state of the Southern Oscillation) during El Niño and La Niña episodes. In general, smoothed time series of the SOI correspond very well with changes in ocean temperatures across the eastern tropical Pacific. The negative phase of the SOI represents below-normal air pressure at Tahiti and above-normal air pressure at Darwin. Prolonged periods of negative (positive) SOI values coincide with abnormally warm (cold) ocean waters across the eastern tropical Pacific typical of El Niño (La Niña) episodes.