According to a study published in Atmospheric Chemistry and Physics, the abundant life in the Southern Ocean contributes to the brightness of the clouds that form there.
The brightness of the clouds is due to their high density of water droplets, which in turn is due to a chain of atmospheric processes that end up being connected with the extraordinary productivity of the phytoplankton of the Southern Ocean.
The study helps us better understand natural cloud formation processes, says Gerald “Jay” Mace, a professor of atmospheric sciences at the University of Utah and lead author of the study.
Clouds are only made up of water droplets and ice crystals. Those droplets form when water vapor condenses around something in the atmosphere, like an aerosol particle, also called a “cloud nuclei.”
“In most situations, the amount of water available to condense into cloud is fixed,” Mace says. “The number of droplets that are then formed from that fixed amount of water vapor depends on the number of aerosol particles that are present.”
Thus, in cases where a region of the atmosphere contains a high number of aerosols, the clouds that form have many cloud condensation nuclei. Cloud droplet density, or the number of droplets per cloud volume, is also high.
It is that droplet density that Mace and his colleagues tried to study in the clouds of the Southern Ocean.
Using satellite data, the researchers examined the properties of clouds in the Southern Ocean during the summers between 2014 and 2019. They focused specifically on a region between Madagascar and New Zealand that had been transited by research vessels and aircraft in the summer of 2017. -2018. “On the ground” data from these fact-finding missions supported the satellite observations.
By studying cloud trends, the researchers worked to determine where the clouds had traveled before reaching the “airspace” around Antarctica. They observed a significant difference between the two cloud groups. Clouds with relatively low droplet densities were highly likely to have migrated from more northerly latitudes, where airborne salt from oceanic water spray is one of the major cloud nuclei.
But clouds with relatively high drop densities were more likely to have originated over the Antarctic continent and to have passed only over the waters of the Southern Ocean. The main difference between the source areas of the two cloud groups was the productivity of plankton in the Southern Ocean.
Growing abundantly in the cold, nutrient-rich Antarctic waters, plankton release sulfate gases as part of their metabolism. In the relatively calm summer air of the Southern Ocean, these gases can give rise to atmospheric chemical reactions that form aerosols.
“The entire circumpolar ocean is highly productive, so there is a massive source of aerosol that finds its way to become cloud droplets,” says Mace. “This aerosol is also transported northward, and the entire Southern Ocean down to the subtropics experiences a seasonal cycle in cloud properties. That seasonal cycle appears to be much greater in the waters surrounding Antarctica, making clouds have a much higher number of droplets and are therefore more reflective in sunlight.”
That reflectivity, also called albedo, is significantly higher in clouds at latitudes closer to Antarctica, south of about 60° S, than in clouds that formed further north, according to the study.