To investigate why ERA5 (European Centre for Medium-Range Weather Forecasts Reanalysis v5) does not accurately capture the observed increase in annual precipitation at Macquarie Island during 1979 to 2023, we classify daily synoptic systems using k-means clustering. Find that the increase in mean intensity across all systems is the main contributor to the observed annual precipitation trend and the resulting discrepancy, rather than changes in the frequency. And this increase may also have a substantial impact on the freshwater fluxes over the Southern Ocean.