As a major component of Earth’s energy budget, ocean heat content plays a vital role in buffering climate change. The annual cycle is the most prominent feature of OHC change but has always been removed to study variations in Earth’s energy budget. Using the up-to-date and multiple observational datasets, the annual cycle in OHC are explored. Observations show that the robust signal of annual OHC change is significant down to 1000 m depth globally and can reach down to about 1500 m depth in some areas such as the tropical ocean. The potential drivers are also investigated by comparing the annual OHC signal with the corresponding change in top-of-atmosphere radiation, surface heat flux, ocean heat divergence and meridional heat transport. In the upper 1500 m at mid and high latitudes and in the upper 50 m of the tropical ocean, the net sea surface heat flux dominates the OHC annual cycle, while in the tropical ocean below 50 m, wind-driven Ekman heat transport is the main driver. 

Owing to the huge ocean heat capacity, the annual cycle of OHC (range of ~57 ZJ) dominates that of the global energy budget (~34 ZJ), while different products show a standard deviation of 8 ZJ. However, the result shows a significant decrease in the amplitude of OHC annual cycle under global warming and the CMIP6 models predict the decrease will continue, suggesting the decrease  in the amplitude of the global energy budget annual cycle with future climate change.