Abstract
Abstract. Cloud–radiation interactions over the Southern Ocean are not well constrained in climate models, in part due to uncertainties in the sources, concentrations, and cloud-forming potential of aerosol in this region. To date, most studies in this region have reported measurements from fixed terrestrial stations or a limited set of instrumentation and often present findings as broad seasonal or latitudinal trends. Here, we present an extensive set of aerosol and meteorological observations obtained during an austral summer cruise across the full width of the Southern Ocean south of Australia. Three episodes of continental-influenced air masses were identified, including an apparent transition between the Ferrel atmospheric cell and the polar cell at approximately 64∘ S, and accompanied by the highest median cloud condensation nuclei (CCN) concentrations, at 252 cm−3. During the other two episodes, synoptic-scale weather patterns diverted air masses across distances greater than 1000 km from the Australian and Antarctic coastlines, respectively, indicating that a large proportion of the Southern Ocean may be periodically influenced by continental air masses. In all three cases, a highly cloud-active accumulation mode dominated the size distribution, with up to 93 % of the total number concentration activating as CCN. Frequent cyclonic weather conditions were observed at high latitudes and the associated strong wind speeds led to predictions of high concentrations of sea spray aerosol. However, these modelled concentrations were not achieved due to increased aerosol scavenging rates from precipitation and convective transport into the free troposphere, which decoupled the air mass from the sea spray flux at the ocean surface. CCN concentrations were more strongly impacted by high concentrations of large-diameter Aitken mode aerosol in air masses which passed over regions of elevated marine biological productivity, potentially contributing up to 56 % of the cloud condensation nuclei concentration. Weather systems were vital for aerosol growth in biologically influenced air masses and in their absence ultrafine aerosol diameters were less than 30 nm. These results demonstrate that air mass meteorological history must be considered when modelling sea spray concentrations and highlight the potential importance of sub-grid-scale variability when modelling atmospheric conditions in the remote Southern Ocean.
Highlights
Aerosols have an important role in radiative forcing both through direct absorption and scattering of incident solar radiation and through their indirect effects on cloud formation, structure, and lifetime (Haywood and Boucher, 2000; Albrecht, 1989)
Median wind speeds observed at the ship were 12.0 m s−1 (IQR: 9.5–13.9 m s−1) and frequent synoptic-scale weather systems were observed passing the ship throughout the voyage, at latitudes south of 50◦ S
We have presented the first comprehensive study of aerosol properties targeting the full latitudinal width of the Southern Ocean south of Australia
Summary
Aerosols have an important role in radiative forcing both through direct absorption and scattering of incident solar radiation and through their indirect effects on cloud formation, structure, and lifetime (Haywood and Boucher, 2000; Albrecht, 1989). They have been identified as the largest source of uncertainty in the global radiation budget (Myhre et al, 2013). Oceans cover 70 % of the Earth’s surface, and aerosols within the marine environment are of particular importance for climate models. Alroe et al.: Marine productivity and synoptic meteorology drive summer-time variability radiative forcing estimated for the Southern Ocean, with insufficient reflected shortwave radiation occurring behind cold fronts and during the austral summer (Williams et al, 2013; Protat et al, 2017)
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