Contributions of the synoptic meteorology to the seasonal CCN cycle over the Southern Ocean

• Main Authors: Alinejadtabrizi, Tahereh, Huang, Yi, Lang, Francisco, Siems, Steven, Manton, Michael, Ackermann, Luis, Keywood, Melita, Humphries, Ruhi, Krummel, Paul, Williams, Alastair, Ayers, Greg
• Format: Text
• Language: English
• Published: 2024
• Subjects: Antarc*; Antarctica; Southern Ocean
• Online Access: https://doi.org/10.5194/egusphere-2024-2397; https://egusphere.copernicus.org/preprints/2024/egusphere-2024-2397/

Cloud Condensation Nuclei (CCN) play a fundamental role in determining the microphysical properties of low-level clouds, crucial for defining the energy budget over the Southern Ocean (SO), a region dominated by low-level clouds. Despite this importance, many aspects of the CCN budget over the SO remains poorly understood including the role of the synoptic meteorology. In this study, we classify the dominant synoptic meteorology over kennaook/Cape Grim Observatory (CGO) and examine its influence on the seasonal variation of the CCN concentration (N CCN ). Our analysis identifies six distinct synoptic regimes: three prevalent in the austral winter, when the subtropical ridge (STR) is strong and centred at lower latitudes, and three in the austral summer, when the STR shifts to higher latitudes. Distinct winter and summer ‘baseline’ regimes contribute to the seasonal cycle in N CCN over the SO with the winter baseline regime characterised by heavier precipitation, a deeper boundary layer and lower N CCN . An analysis of air mass back trajectories, specifically at the free troposphere level, supports this distinction, with wintertime baseline airmasses originating over higher latitudes. Across these two baseline regimes we observe a significant inverse relationship between precipitation and N CCN , underscoring the role of precipitation in reducing N CCN over the SO. Using forward trajectories within this synoptic framework, we examine the transport of continental airmasses over the SO, finding that frontal air masses more frequently reach high latitudes during winter. We conclude that the location of the STR can moderate the advection of air masses between Antarctica and kennaook/Cape Grim.