Exploring drivers of Cloud Condensation Nuclei in the Indian Sector of the Southern Ocean

Abstract. The Southern Ocean plays a critical role in Earth's climate system, acting as a major sink for heat and carbon dioxide. Clouds over this region strongly influence the regional radiation budget, yet their formation remains poorly understood and represented in climate models. This is partially driven by the lack of in-situ observations, particularly over the undersampled Indian sector. In this study, we report on the latitudinal variability of cloud condensation nuclei (CCN) and examine the meteorological and biological factors that can influence their distribution across this region. CCN concentrations were measured during the 12^th Indian Scientific Expedition to the Southern Ocean during February and March 2025 aboard a research vessel traversing from 20° S to 68° S. CCN concentrations varied across supersaturation levels, with the highest values approximately 500 cm^-3 observed at S = 1.0 % in the polar band (60–70° S) and lower concentrations at lower S levels. CCN concentrations showed a poor correlation with coarse-mode aerosols, suggesting that wind-driven sea spray does not play a major role in controlling the CCN variability in this region. Chlorophyll a, used as a proxy for biological productivity, showed a positive relationship with CCN concentrations. Air masses passing over biologically productive waters during the preceding 24–48 h were associated with higher CCN concentrations, with the strongest relationship observed within the 30–50° S latitude band encompassing the subtropical and subantarctic fronts. This suggests a closer association between marine biological activity and CCN variability in this region. In contrast, the weaker relationship outside this latitude band indicates that physical processes and non-biological sources may exert a greater influence on CCN concentrations in other regions.