Investigating the sources of Southern Ocean and coastal Antarctic ice nucleating particles: the cloud and aerosol impacts over the Southern Ocean (CAISO) project

Abstract

The Southern Ocean is the cloudiest region on Earth, where low-level mixed-phase clouds are a critical control on climate feedbacks. The liquid or ice content within these clouds controls their reflectivity, thus modulating shortwave radiation reaching the surface. Climate models struggle to accurately simulate absorbed shortwave radiation over the Southern Ocean due to limitations in representing these unique mixed-phase clouds. Increased observations of ice nucleating particles (INPs) provide necessary insights to resolve cloud microphysical schemes. Here we present data from Southern Ocean INP observations across six voyages, from 2016 to 2023. The last three voyages are featured as part of the Cloud and Aerosol Impacts over the Southern Ocean (CAISO) project. These new datasets provide insights into Southern Ocean INP concentrations and sources, as well as a latitudinal framework to evaluate them in. Southern Ocean INP concentrations remained low and consistent with previous voyages, ranging from 2 × 10-4 to 2.64 INP/L at -30 °C. The Southern Ocean exhibits clear latitudinal gradients, with higher INP concentrations near low-latitude terrestrial sources and the lowest INP concentrations below 60 °S. We also identify the Antarctic continent as a weak but non-negligible INP source for the high-latitude Southern Ocean. Additionally, we discuss the need to treat various latitudinal regions of the Southern Ocean as independent sectors to better isolate sources and define representative INP spectra. Our findings provide new insights to improve cloud microphysical schemes and resolution of these low-level mixed-phase clouds in climate model.