Research finds marine bacteria and atmospheric rivers may contribute to the formation of ice clouds

Understanding cloud formation in polar regions is essential for discerning the influence of solar radiation on polar ice caps. Existing numerical models, however, struggle to accurately reproduce ice clouds. Now, using real-world observations and climate data, Japanese researchers have discovered that marine bioaerosols carried by currents of warm, moisture-laden air from higher latitudes contribute to cloud formation ice over the high latitudes of the Southern Ocean.

Clouds, made up of tiny water droplets, ice particles, or a mixture of the two, are dynamic components of our planet’s climate system. They play an important role in regulating the amount of sunlight absorbed or reflected at the tops of their clouds. Depending on their composition, clouds form at different altitudes and exert varying effects on the climate. Understanding cloud formation in polar regions vulnerable to climate change is particularly vital. It will provide us with key information to then study their impact on the ice caps.

Although numerical models have greatly improved our ability to simulate cloud formation, they do not accurately account for how aerosol particles, which serve as starting points for the formation of ice crystals in clouds, , influence the process of ice cloud formation. These biases can lead to errors in how these models predict the behavior of ice clouds in the atmosphere.

To improve the accuracy of numerical models in representing cloud formation, Assistant Professor Kazutoshi Sato and Jun Inoue, both of Japan’s National Polar Research Institute, turned to real-world observations and data satellite and climatic data to discover the mechanisms behind ice clouds. formation in the Southern Ocean due to bioaerosols emitted by the oceans.

“Developing knowledge about ice cloud formation associated with marine bioaerosols could help improve the performance of the cloud phase in numerical models,” says Dr. Inoue. Their findings were recently published in the journal Geophysical research letters.

It all started with an expedition to the Southern Ocean surrounding Antarctica between November 2022 and March 2023. On site, researchers observed the formation of ice clouds in the mid-troposphere at temperatures above –10 °C. Simultaneously, they noticed clouds of liquid water in the upper troposphere at temperatures below –20°C. Typically, ice clouds form at colder temperatures. The researchers therefore wanted to understand why these ice clouds appeared at milder temperatures.

Using reverse trajectory analysis, they tracked a stream of warm, humid air originating from southern Africa. Then, using satellite data, the researchers found that the air mass encountered regions with high concentrations of chlorophyll-a, a pigment associated with phytoplankton, as it crossed the Southern Ocean at midlatitudes. They also found that the amount of dimethyl sulfide (DMS) in the air was higher in regions where there were powerful and intense waves in the water.

Why is the presence of the DMS remarkable here?

A sulfur compound often linked to the activity of phytoplankton, DMS is recognized for its role as a nucleus in the formation of liquid water clouds. Its presence in the atmosphere also serves as an indicator of marine bacteria. These bacteria can be released into the atmosphere due to sea spray generated by strong waves. According to the researchers, marine bacteria in this stream of warm, humid air from the mid-latitude Southern Ocean act as ice nucleating particles, contributing to the formation of ice clouds at higher than expected temperatures in high latitude areas of the ocean. Southern Ocean.

“Using a cloud particle sensor probe, we detected ice clouds at high latitude, at temperatures above −10°C, close to a current of warm, moist air coming mid-latitudes. These waterways are often called atmospheric rivers (ARs),” explains Dr. Sato.

“The AR received marine bioaerosols from the mid-latitude ocean under high wave conditions. These bioaerosols reached the ice cloud formation layer. Our observations suggest that these marine bioaerosols, which traveled via the AR, contribute to the formation of ice clouds under relatively high temperature conditions.”

Climate models have had difficulty accurately simulating the formation of ice clouds under higher temperature conditions. The results of this experimental study could allow more precise numerical modeling of climatic conditions, particularly in vulnerable polar areas.