October 6, 2010 | El Niño and La Niña are counterparts in the El Niño-Southern Oscillation (ENSO), a cyclic warming and cooling of the eastern and central Pacific Ocean that exerts a major influence on global weather patterns. These children of the tropics are not mirror images, however, but display significant differences in spatial structure and seasonal evolution. A new study led by NCAR visiting scientist Yuko Okumura examines the asymmetry of their durations. The findings have important implications for the prediction of ENSO and its global influences.

Okumura and her co-author analyzed two datasets of monthly sea surface temperatures spanning different periods: 1900–2008 and 1982–2008. They found a robust asymmetry between El Niño and La Niña throughout the record, especially during strong ENSO events. Both phases typically begin in late spring or summer. Most El Niños terminate rapidly after peaking in December or January, but many La Niñas persist through the following spring and summer and re-intensify in winter, some even lasting through a third year.

The researchers also looked at how ocean-atmosphere anomalies evolve in association with the asymmetric durations of El Niño and La Niña, identifying a pronounced asymmetry in the surface wind anomalies that develop in late fall over the far western Pacific. La Niña drives strong easterly winds over much of the western Pacific during its developing and mature phases. El Niño exhibits a similar structure with a reversed sign, except that a month prior to its mature phase, the westerly winds begin to weaken in the far western Pacific due to the effects of warming sea surface temperatures in the Indian Ocean.

The influence of the Indian Ocean can be felt more strongly in the western Pacific during El Niño compared to La Niña because of the more eastward location of atmospheric deep convection anomalies in the Pacific.

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