E. Linacre and B. Geerts
The length of day is a measure of the speed of the Earth's rotation. We all know that the daylength is 24 hours, however the exact length varies slightly, by a few thousandths of a second.
Reliable measurements since 1820 until 1970 (1) reveal relatively short days on average around 1845, 1905, 1940 and some time after 1970, and longer days before 1820, around 1870 and 1930. In other words, there was an irregular rhythm in daylength. These changes in Earth’s rotation rate are related to momentum changes in the fluid Earth, i.e. currents in the Earth’s core, the oceans, and the atmosphere. Shorter daylengths were associated with a stronger westerly circulation of global winds.
The onset of an El Niño (warm) event implies a net transport of ocean water from the western to the eastern equatorial Pacific, i.e. in the same direction as the Earth’s rotation. Therefore, during the 2-6 months of the onset of an El Niño event, the length of day is longer. In fact, the longer daylengths are used to confirm independent El Niño observations and forecasts.
(1) Lambeck, K. & A. Cazenave, 1976: Long term variations in the length of day and climatic change. Geophys. J. Roy. Astr. Soc., 46, 555-73.