The annual cycle of Antarctic sea ice extent is asymmetric. On average, total Antarctic sea ice extent expands (advances) for approximately seven and a half months, from late February to late September, and contracts (retreats) for four and a half months each year. While other factors may influence the size of the extent, previous research suggests that the timing of the growth and retreat stages, also called the phase, is in part due to the influence of the semi-annual oscillation (SAO) of the Antarctic circumpolar trough. Using observed daily sea ice extent, sea level pressure, and winds, over the period 1979 – 2018, we compare the strength and timing of the observed SAO and the related winds with the variability of the observed phase of the annual cycle. We compare the results from the observations with similar variables calculated from the Community Earth System Model version 2 (CESM2) historical runs.  Initial analyses suggest that the timing of retreat of sea ice in the CESM2 occurs consistently later than observed and the SAO calculated from the modelled data is weaker than observed. These results are used to help understand the anomalously rapid sea ice decay which occurred in 2016.