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Southern Oscillation Index (or SOI): a graph of air pressure comparisons across the Pacific.
Considered together with the El Nino/La Nina (or ENSO) it
affects weather in countries around the Pacific and as far afield as Africa.
el nino = warm currents along Peru:cooler West Pacific/drier weather in Australia.
la nina = fish rich cool water upwellings along Peru:warmer West Pacific/wetter weather in Australia.
This is the most powerful climate force on earth and also involves trade winds & ocean currents, giving rise to droughts, floods, cyclones & typhoons. In turn causing disasters ranging from famines & disease epidemics to coral bleaching.