It was also a mystery why the effect did not kick in for several weeks after ocean temperatures reached their peak. Now, Jean Dickey and her colleagues at the California Institute of Technology in Pasadena says that the answer is blowing in the wind.
El Nino events warm Pacific surface waters in the tropics, resulting in strong westerly thermal winds. The total Earth system spins with a constant speed, but these winds make the atmosphere spin slightly faster. Due to the conservation of angular momentum the body of Earth then slows to compensate, making the days a little longer.