Miniature marine organisms called phytoplankton, which live in the ocean’s stormy waters, are to blame for the Southern Ocean being one of the cloudiest region on Earth, nearly totally blanketed year round.
A new study of clouds over the remote ocean examined how particles and gases emitted by these creatures enter the atmosphere and become the seeds of clouds. It combines computer modeling with satellite data over the Southern Ocean, the massive sea surrounding Antarctica.
The first large-scale correlation between biological activity in the Southern Ocean and cloud formation, the study is an important first step toward understanding an enduring question in climate modeling- the role of clouds and air particles known as aerosols in global climate change.
The researchers found that cloud droplet numbers nearly double in the summer, boosting the amount of sunlight reflected back to space, by about 4 watts per square meter over the course of the year. And the study estimates how much solar energy that equals over the whole Southern Ocean, for the first time.
“It is a strong effect. But it makes sense because most of the area down there is ocean, with strong winds that kick up a lot of spray and lots of marine microorganisms producing these particles. And continental aerosol sources are mostly so far away that they only have a limited impact. Really the marine aerosols are running the show there.”
The Southern Ocean, whose borders have yet to be settled on by the International Hydrographic Organization, is made up of the southernmost parts of the Atlantic, Pacific and Indian Oceans, and is one of the cloudiest places on Earth. Key to the Southern Hemisphere’s atmospheric and oceanic circulation, the Southern Ocean clouds could also help establish how sensitive Earth is to the build-up of greenhouse gases in its atmosphere.
Studying oceanic aerosols is challenging since they get overwhelmed by man-made pollutants when measured near coastlines. Studying these aerosols in the Southern Ocean has been difficult as well. Satellites cannot tell different kinds of aerosols apart.
Sea salt is one type of aerosol. The ocean also contains marine organisms called phytoplankton that ultimately yield two more kinds of aerosols important to cloud formation, sulfates and organic matter aerosols.
Credit: Daniel McCoy
Earlier studies only examined how cloud droplet numbers correlated with chlorophyll, an easy-to-measure molecule involved in photosynthesis that gives plants their green color, as a simulation of marine life and were unable to determine the individual roles of actual aerosols.
The research team, to fill in the role of different aerosols, used computer models to simulate both organic matter and sulfates, as well as sea salt.
In addition, Burrows, McCoy and colleagues turned to a new set of satellite measurements of cloud droplets. The data set fixes the seasonal issues with the Southern Ocean and covers the latitudes between 35 degrees south and 55 degrees south.
“Satellite data allows us to observe events that occur over the course of months and on a scale of thousands of kilometers in the remotest regions on the planet,” said UW’s McCoy. “It really gives us an unparalleled glimpse of the Earth System’s complexity.”
Analysis of their model suggested that sea salt was the biggest source of aerosols in the ocean, contributing the most aerosols around which cloud droplets formed. And it was also the most uniform, contributing about the same number all year round.
But the organic matter and sulfate aerosols produced more cloud droplets over summer than winter, as expected since the ocean receives more sunlight for organisms to grow in the summer. The sulfates, in addition, had a bigger effect than organic matter.
“The return of light in the summer ignites an amazing flurry of activity in phytoplankton communities across the Southern Ocean. This seasonality leads to an enhancement in cloud brightness when it will be able to reflect the most sunlight,” said UW’s McCoy.
Since it’s more difficult to observe the effects of marine aerosols in other regions of the world, the researchers will be able to use what they’ve learned about the mechanism and strength of the aerosol interactions with clouds to apply to studies in other regions.