Intensifying precipitation over the Southern Ocean challenges reanalysis-based climate estimates – Insights from Macquarie Island’s 45-year record

Abstract. The Southern Ocean (SO) region plays a critical role in the global climate system but remains observationally limited. Macquarie Island (MAC), situated along the SO storm track, provides a unique, high-quality surface precipitation record since 1948. Based on daily synoptic regime classifications from 1979 to 2023, we find that a significant 28 % increase in annual precipitation at MAC is primarily driven by enhanced mean daily precipitation intensity associated with warm air advection, low pressure, and cold air advection regimes, rather than shifts in regime frequency. In contrast, ECMWF reanalysis (ERA5) shows only an 8 % overall increase in annual precipitation, as it insufficiently reflects the increase in mean daily precipitation intensity under these regimes, likely due to its limited representation of atmospheric moisture transport and increasing evaporation. This precipitation discrepancy reveals moisture and energy budget biases in reanalysis over the SO, with important implications for the estimation of SO freshwater fluxes.