Literally - the answer is Blowing in the Wind - my friend

El Nino apparently causes such a ferment in the upper atmosphere that the Jetstream wizzes around and by the laws of conservation of momentum the mass of the earth has to slow down to maintain the total angular momentum of the whole. So our day is slowing down by 1mS when El Nino gusts. Now you may think that is incedental but in three years that could be a whole second (probably more like 5 years). Scientists needed to have a leap second only about 10 years ago, and El has been blowing more often since.

And the exact mechanism of an even more significant slowing down eludes me but I read that also in a recent issue of the New Scientist. I will post when I find it - but right now I am running late............