Environ. Res. Lett. 6, 034006 (2011)

In recent decades, the correlation between El Niño and the strength of India's summer monsoon has weakened — now a study shows why. A climate phenomenon known as the Indian Ocean dipole interacts with El Niño to moderate the monsoon rainfall.

Caroline Ummenhofer, a climate scientist at the University of New South Wales in Sydney, and her colleagues analysed summer rainfall records for west central India from 1877 to 2006 and looked for the effect of the so-called positive phase of the Indian Ocean dipole, in which sea surface temperatures north and east of Australia are about 0.5 °C cooler than average and those of the surface waters in the western Indian Ocean are slightly above normal.

The researchers found that in years when El Niño appeared alone, average precipitation was reduced by about 2.4 m over the monsoon season. But when the positive phase of the Indian Ocean dipole accompanied El Niño, monsoon rains were normal. This is because the positive phase of the dipole tends to strengthen winds flowing eastwards across the northern Indian Ocean, which counteracts the weakening of the winds typically brought about by El Niño. The team's analyses suggest that this situation has occurred more often in recent decades, weakening El Niño's impact on the Indian monsoon.