Angular momentum is fundamental to the structure and variability of the atmosphere and therefore has an important influence on regional weather and climate. Total atmospheric angular momentum is also directly related to the rotation rate of the Earth and, hence, the length of day. However, the long-range predictability of fluctuations in the length of the day and atmospheric angular momentum is unknown. Here we show that fluctuations in atmospheric angular momentum and the length of day are predictable out to more than a year ahead and that this provides an atmospheric source of long-range predictability for surface climate. Using ensemble forecasts from a dynamical climate model, we demonstrate long-range predictability of signals in the atmospheric angular momentum field that propagate slowly and coherently polewards due to wave–mean flow interaction within the atmosphere. These predictable signals are also shown to precede changes in extratropical climate via the North Atlantic Oscillation and the extratropical jet stream. These results extend the lead time for length-of-day predictions, provide a source of long-range predictability from within the atmosphere and provide a link between geodesy and climate prediction.

角动量是大气结构和变率的基础，因此对区域天气和气候具有重要影响。大气总角动量也与地球的自转率直接相关，因此也与一天的长度相关。然而，白天长度和大气角动量波动的长期可预测性尚不清楚。在这里，我们表明大气角动量的波动和白天的长度可以预测到一年多的时间，这为地表气候提供了长期可预测性的大气来源。使用来自动态气候模型的集合预报，我们证明了大气角动量场中信号的长期可预测性，由于大气中的波均流相互作用，这些信号缓慢而连贯地向极地传播。通过北大西洋涛动和温带急流，这些可预测的信号也显示在温带气候变化之前。这些结果延长了日长预测的前置时间，提供了来自大气内部的长期可预测性来源，并提供了大地测量学和气候预测之间的联系。