Effects of Southern Ocean phytoplankton and algae on aerosol and liquid clouds

Summer rays of sun
Clouds drift; cool ocean below
Sea ice in full bloom
- Eszter Kovacs, 2025

This thesis investigates how Southern Ocean phytoplankton influence liquid clouds.
Chapter 2 investigates the extent to which four phytoplankton blooms influenced cloud properties using satellite data. Three different definitions of ”bloom” were considered, and cloud properties upwind, over, and downwind of blooms, as well as spatial correlations be tween chlorophyll-a concentration and cloud properties were studied. Cloud droplet number concentration above blooms was increased by 12.6% on average across the four blooms examined.

Chapter 3 examined the trajectories of air masses that ended up in clouds to ascertain source regions of aerosols that contribute to cloud droplet number concentration. Air-masses influenced by Antarctic sea ice were associated with higher cloud droplet number concentration cloud scenes than their marine counterparts. The biogenic influence from phytoplankton could still be seen, with the maximum correlation between chlorophyll-a concentration and cloud droplet number concentration occurring when surface values are considered 5 days before air-masses arrive in a cloud.

In Chapter 4, a new aerosol precursor gas was added to an atmosphere-only model to recreate the high cloud droplet number concentration at southern high latitudes. Emitted from areas of sea ice melt, the gas underwent oxidation, and the resulting gas condensed and nucleated to represent gases which have been theorised to be the ”missing source of aerosol” that contributes to aerosol mass over the Southern Ocean. However, the amount of gas needed to fix the low bias of cloud droplet number concentration in the model led to too-frequent nucleation, showing that the model underestimation of cloud droplet number concentration over the Southern Ocean is not consistent with a lack of new particle formation in the region.