AGU: Geophysical Research Letters

 

Editors' Highlight

A study in angular momentum: Why and how does El Niño slow the spin of the Earth?

During an El Niño event, the air around the equator is heated and expands upward, energizing the westerly winds (or jet streams) and increasing the atmosphere's angular momentum. Because the angular momentum of the atmosphere-Earth system must be conserved, the solid Earth slows down, increasing the length of the day by roughly a thousandth of a second. However, scientists studying the solid Earth observe a lag of about a couple of months between the peak of an El Niño event and the strongest braking of the Earth's rotation. Dickey et al. (2007) sought to understand the reasons behind this lag. Through analysis of atmospheric, oceanic, and geodetic data spanning the years between 1979 and 2004, the authors found that thermal winds rising from the contrast between tropical and subtropical air temperatures tend to maximize a couple months after the peak of an El Niño event, as the heated air spreads outward from the equator. The authors showed that these winds drive the observed change in the length of day and account for the studied lag.

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