I'm interested in this question because we invariably get persistent headlines year after year about drought. If it's true that the Earth receives roughly the same amount of precipitation every year, then drought is only a matter of distribution, where some places get more than normal, some get normal amounts, and some get lesser amounts of precipitation. On the other hand, if the Earth receives significant variances in precipitation from year to year, then drought is not necessarily a distribution problem and can be a result of a low precipitation period affecting larger areas at once.
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1 Answer
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Rainfall over large areas and drought are very much linked to the El Nino/La Nina, Southern Oscillation Index in the Pacific Ocean and its equivalent in the Indian Ocean, the Indian Ocean Dipole.
During a La Nina, cold surface waters occur in the eastern Pacific Ocean and warm waters occur to the east of Australia. Such conditions tend to bring drought to the Americas and excessive rainfall to the east coast of Australia, particularly the north east.
An El Nino event is the opposite - warm waters in the eastern Pacific Ocean and cooler waters off the east of Australia with rainfall in the Americas and drought in eastern Australia.
The Indian Ocean Dipole (IOD) is the equivalent in the Indian Ocean. With a negative IOD. Cold surface waters occur off east Africa and warm waters to the north west of Australia. This brings drought to east Africa and more rainfall from the north west of Australia to the south east of Australia.
In the latter part of 2021 and the first half of 2022, there was a La Nina event in the Pacific and a negative IOD in the Indian Ocean which has resulted in drought in east Africa and the USA, while eastern Australian has had two or three major flooding events due to excessive rain.
It is unusual to have three successive La Nina events, usually the maximum is two, however a third such event has been identified, as of late August 2022.
