ENSO and Walker Circulation EI Nino is defined by prolonged differences in tropical Pacific Ocean SST when compared with the average value.The accepted definition is a warming or cooling of at least 0.5 C averaged over the east-central tropical Pacific Ocean. Typically,this anomaly happens at irregular intervals of 2-7 years and lasts nine months to two years. The Southern Oscillation is the atmospheric component of EI Nino.This component is an oscillation in surface air pressure between the tropical eastern and the western Pacific Ocean waters.The strength of the Southern Oscillation is measured by the Southern Oscillation Index(SOl).The SOl is computed from fluctuations in the surface air pressure difference between Tahiti and Darwin,Australia. 授课教师：张洋 9授课教师：张洋 9 ENSO and Walker Circulation n El Niño is defined by prolonged differences in tropical Pacific Ocean SST when compared with the average value. The accepted definition is a warming or cooling of at least 0.5 °C averaged over the east-central tropical Pacific Ocean. Typically, this anomaly happens at irregular intervals of 2–7 years and lasts nine months to two years. n The Southern Oscillation is the atmospheric component of El Niño. This component is an oscillation in surface air pressure between the tropical eastern and the western Pacific Ocean waters. The strength of the Southern Oscillation is measured by the Southern Oscillation Index (SOI). The SOI is computed from fluctuations in the surface air pressure difference between Tahiti and Darwin, Australia