Controls on the pristine aerosol population over the Southern Ocean (SO) are critical for constraining the strength of global aerosol indirect forcing. The 2018 SOCRATES aircraft campaign sampled summertime clouds and aerosols across varied SO synoptic conditions during repeated southerly transects (140-160 degE, 45-62 degS). The SO free troposphere (3-6 km) is characterized by frequent recent particle formation (RPF) events contributing to large (>1000 cm-3) and widespread concentrations of condensation nuclei (diameters >11 nm), consistently higher than other low cloud regions (e.g. North East Pacific). A SO synoptic uplift mechanism responsible for producing free tropospheric Aitken-mode particles (11-100 nm) is identified using SOCRATES measurements, air-mass trajectories, and reanalysis. SO cloud droplet number concentrations (Nd~100 cm-3) are controlled by sub-cloud cloud condensation nuclei (CCN, 100-1000 nm), but direct emissions (i.e. sea spray) are not the largest contributor to summertime CCN. Sub-micron aerosol number concentration in, above, and sub-cloud is dominated by sulfur-based particles, consistent with growth of free tropospheric Aitken particles with DMS-oxidation product volatility signatures. We propose a hypothesis for SO cloud-aerosol interactions: above-cloud Aitken mode particles grow in and sub-cloud to dominate sub-cloud summertime CCN, acting to buffer clouds and aerosols against precipitation removal and leading to persistently high summertime SO Nd. Aerosol and cloud prediction capabilities in nudged hindcasts from the Community Atmosphere Model (CAM6) are tested. CAM6 under-predicts Nd but matches observed CCN despite sea-spray dominance in the model, indicating incomplete representations of biological aerosol production mechanisms and associated summertime aerosol-cloud interactions.