The Southern Oscillation Index (SOI) compares atmospheric pressure in Tahiti with that of Darwin, Australia.
(1) It is expressed in standard deviations from the norm.
(2) A negative index is associated with an El Nino event.
(3) A strongly negative value of <= -2.0 is sensitive for the heavy rainfall, but is only about 66% specific.
A concurrent increase in Pacific Ocean and Indian Ocean Sea Surface Temperature (SST) anomalies are associated with increased East African rainfall.
(1) The temperatures for each location are taken at specific equatorial regions.
(2) A Pacific Ocean SST value > 3°C in conjunction with an Indian Ocean SST value > 0.5°C is associated with heavy precipitation in Kenya.
The Normalized Difference Vegetation Index (NDVI) is a measure of vegetation growth as seen by satellite.
(1) A value >= 0.2 indicates increased vegetation following heavy rainfall.
(2) An increase in the NDVI precedes the increase in viral activity by 1-2 months.
Interpretation:
• The best prediction is by combined use of the Pacific Ocean SST, Indian Ocean SST, and NDVI, which allows for prediction of outbreaks 2-5 months in advance.
• Interventions to reduce morbidity and mortality include domestic animal vaccination and use of sustained release insecticides.
