The lead character in the 2011 climate story was La Niña—the cool phase of the El Niño-Southern Oscillation—which chilled the central and eastern tropical Pacific at both the start and the end of the year. These natural cooling events have a long reach: many of the big climate events of 2011, including famine-inducing drought in East Africa, an above-average hurricane season in the Atlantic, and record rainfall in many parts of Australia, are common “side effects” of La Niña.
The La Niña that was underway at the start of 2011 was among the strongest in the historical record. By late spring, waters had warmed to “neutral” conditions. La Niña re-developed more weakly in the fall, and the tropical Pacific chilliness continued into spring of 2012.