A recent interruption in the 6-year oscillation in length-of-day

SUMMARY Intradecadal variations in the length-of-day ($\Delta$LOD) can reveal changes in angular velocity interpreted as due to Earth’s core. Previous studies have identified periodic oscillations of around 6 and 8 yr. To complement widely used Fourier methods, we investigate the $\Delta$LOD record from 1962 to 2025 in the time domain, seeking smooth variations using cubic B-splines. We analyse in several ways. A penalized least-squares spline fit allows isolation of coherent variations from analysing the first and second derivatives. Alternatively, a smooth curve fit with least-squares splines allows removal of the long-period behaviour of $\Delta$LOD. From this, we fit the residual with a pure cosine-wave of varying period but examine the data fits carefully in case the signal is non-stationary (e.g. from impulsive forcing). All approaches show clear evidence of signals with periods around 5.9—and in the case for the time derivatives—8.5 yr. We find that the pure 5.9-yr oscillation breaks down in 2010, with a one-off peak to peak separation of around 4.7 yr. After 2014, the variation is once again consistent with an approximate 6-yr oscillation. Such a discontinuous, non-stationary effect is not well-characterized by frequency-domain based methods. Seeking to understand this brief interruption of the 6-yr oscillation, we extend the study length using a $\Delta$LOD series from lunar occultation data extending back to 1800, and find it suitable to repeat our spline-based analysis from 1830 onwards. From this, we find the 6-yr oscillation stable throughout the entire 19th and 20th century, with the exception of 1916–1920, where we observe a similar interruption of the 6-yr variation by a single 4-yr oscillation. The 2010 disruption to the 6-yr oscillation is contemporary with changes in geomagnetic secular variation, modelled core surface flow and inner core seismic signature. All of these events suggest a step change in core-processes around 2010.