The Southern Ocean is one of the cloudiest regions on earth with a high cloud radiative effect, with a high bias in atmospheric models. However, observations have shown that clouds over the Souther Ocean amplify the warming in this region.

It is also one of the few regions on earth with frequent pristine aerosol conditions where the effect of aerosols on clouds is biggest. The source of Cloud Condensation Nuclei in pristine oceanic regions is expected to be coupled to clouds and appears in the upper troposphere. The HALO-South mission is planned to take place in the Southern Ocean region mainly south-west and south of Christchurch, New Zeeland, from September to October 2025. This will be at the end of the winter and the onset of the spring in the Souther Ocean. The HALO aircraft is equipped with a unique set of instrumentation to investigate aerosol and cloud cycles and its effect on radiative properties of the clouds. The campaign will be embedded in parallel intensive field activities including ground-based measurements from New Zeeland and will be supported by satellite investigations. HALO-South will provide pressing knowledge about the relation between aerosols and clouds in the southern hemisphere from cloud droplet and ice nucleation to the production of cloud and ice active particles partly triggered by increased radiation above clouds and cloud optical properties. These findings will be extrapolated to a larger scale using satellite data and global climate models. The campaign will greatly build on and continue past HALO campaigns with focus on either cloud and aerosol or gas and aerosol properties (CIRRUS-HL, ACRIDICON, CAFE-Pacific, CAFE-EU, CAFE-Brazil, CAFE-Pacific and will also overlap with EMeRGe-EU and EMeRGe-Asia). This campaign will cover the full cycle to study aerosols through clouds and the full cycle of clouds with a particular focus on mixed-phase clouds, as this is where we expect the important microphysical processes affecting aerosols and radiation to take place.