Are Southern Ocean Low-Cloud Droplet Numbers Buffered by Recently Formed Particles Associated with Cyclonic Uplift?

In-situ observations of low clouds and aerosols in the Southern Ocean (SO) are limited due to the remoteness of the region. This area is climatically important for assessing cloud-aerosol interactions in pristine, pre-industrial conditions and understanding the SO cloud influence on the global radiation budget. The Southern Ocean Cloud Radiation Aerosol Transport Experimental Study (SOCRATES) sampled clouds and aerosols across a range of meteorological environments between 45 and 62°S in austral summer 2018. Evidence of recent new particle formation was observed in aircraft sampling in the free troposphere (~6 km) and above cloud (~3 km), contributing to unusually high concentrations of condensation nuclei (>1000/cc of particles larger than 10 nm) compared to other low cloud regions of the world (e.g. SEP, NEP, and NEA). SOCRATES measurements combined with HYSPLIT trajectories and ECMWF ERA5 reanalysis were used to identify cyclonic uplift and cumulus outflow mechanisms contributing to widespread production of new particles throughout the depth of the atmosphere. Below cloud accumulation mode aerosol is well correlated with droplet number concentration (Nd, ~100/cc) but does not depend strongly on wind speed, indicating direct emissions (i.e. sea salt) are not the largest contributor to cloud condensation nuclei. In-cloud particle composition is largely sulfur-based, consistent with the composition of the small particles seen below and particularly above cloud. We propose that the key contributor to the below cloud accumulation mode controlling SO Nd is the large reservoir of small particles observed above cloud (which grow to cloud affecting sizes in and below cloud). This reservoir is hypothesized to lead to a buffering mechanism in the SO, evidenced by the lack of observed ultra-clean layers (Nd≤10/cc) compared to the NEP. Implications for cloud-aerosol interactions in models will be discussed based on comparisons with nudged Community Atmosphere Model (CAM6) hindcasts.