The Southern Ocean mixed layer and its boundary fluxes: fine-scale observational progress and future research priorities

Interactions between the upper ocean and air-ice-ocean fluxes in the Southern Ocean play a critical role in global climate by impacting the overturning circulation and oceanic heat and carbon uptake. Remote and challenging conditions have led to sparse observational coverage, while ongoing field programmes often fail to collect sufficient information in the right place or at the time-space scales required to constrain the variability occurring in the coupled ocean-atmosphere system. Only within the last 10 years have we been able to directly observe and assess the role of the fine-scale ocean and rapidly evolving atmospheric marine boundary layer on the upper limb of the Southern Ocean's overturning circulation. This review summarizes advances in mechanistic understanding, arising in part from observational programmes using autonomous platforms, of the fine-scale processes (1-100 km, hours-seasons) influencing the Southern Ocean mixed layer and its variability. We also review progress in observing the ocean interior connections and the coupled interactions between the ocean, atmosphere and cryosphere that moderate air-sea fluxes of heat and carbon. Most examples provided are for the ice-free Southern Ocean, while major challenges remain for observing the ice-covered ocean. We attempt to elucidate contemporary research gaps and ongoing/future efforts needed to address them. This article is part of a discussion meeting issue 'Heat and carbon uptake in the Southern Ocean: the state of the art and future priorities'.

Investigating the Heterogeneous Ice Nucleation of Sea Spray Aerosols Using Prochlorococcus as a Model Source of Marine Organic Matter

Sea spray is the largest aerosol source on Earth. Bubble bursting mechanisms at the ocean surface create smaller film burst and larger jet drop particles. This study quantified the effects of particle chemistry on the depositional ice nucleation efficiency of laboratory-generated sea spray aerosols under the cirrus-relevant conditions. Cultures of Prochlorococcus, the most abundant phytoplankton species in the global ocean, were used as a model source of organic sea spray aerosols. We showed that smaller particles generated from lysed Prochlorococcus cultures are organically enriched and nucleate more effectively than larger particles generated from the same cultures. We then quantified the ice nucleation efficiency of single component organic molecules that mimic Prochlorococcus proteins, lipids, and saccharides. Amylopectin, agarose, and aspartic acid exhibited similar critical ice saturations, fractional activations, and ice nucleation active site number densities to particles generated from Prochlorococcus cultures. These findings indicate that saccharides and proteins with numerous and well-ordered hydrophilic functional groups may determine the ice nucleation abilities of organic sea spray aerosols.