Contributions of the synoptic meteorology to the seasonal cloud condensation nuclei cycle over the Southern Ocean

Abstract. Cloud condensation nuclei (CCN) play a fundamental role in determining the microphysical properties of low-level clouds that are crucial for defining the energy budget over the Southern Ocean (SO). However, many aspects of the CCN budget over the SO remains poorly understood, including the role of the synoptic meteorology. In this study, we classify six distinct synoptic regimes over the Kennaook / Cape Grim Observatory (CGO) and examine their influence on the seasonal cycle of the CCN concentration (NCCN). Three “winter” regimes are dominant when the subtropical ridge (STR) is strong and centered at lower latitudes, while three “summer” regimes prevail when the STR shifts to higher latitudes. Distinct winter and summer “baseline” synoptic patterns contribute to the seasonal cycle of NCCN, with the winter baseline regime characterized by heavier precipitation (0.10 vs. 0.03 mm h−1), a deeper boundary layer (850 vs. 900 hPa), and lower NCCN (71 vs. 137 cm−3) than the summer one. Across these two baseline regimes, we observe a significant inverse relationship between precipitation and NCCN, underscoring the contribution of precipitation in reducing NCCN over the SO. An analysis of air mass back-trajectories, specifically at the free-troposphere level, supports this seasonal distinction, with wintertime air masses originating more frequently from higher latitudes. The summertime STR is seen as a barrier to Antarctic air masses reaching the latitude of the CGO. Conversely, the summer baseline regime is found to pass more frequently over continental Australia before reaching the CGO, consistent with enhanced radon concentrations.