Dr Gerald Mace | Cloud Dynamics Over the Southern Ocean: Unravelling Nature’s Marine Cloud Brightening

In the pristine waters of the Southern Ocean surrounding Antarctica, scientists have discovered fascinating patterns in cloud formation that could have major implications for understanding Earth’s climate. Recent research conducted by Dr Gerald Mace from the University of Utah and colleagues reveals how air masses passing over the Antarctic continent naturally boost cloud brightness through a complex chain of chemical and physical processes. This natural phenomenon may hold important clues for improving climate models and predicting future climate change.

Nature’s Cloud Factory

The Southern Ocean is one of the most remote and pristine regions on Earth, making it an ideal natural laboratory for studying how clouds formed before human industrial activity began altering Earth’s atmosphere. This vast expanse of water encircling Antarctica experiences some of the planet’s strongest winds and stormiest conditions, yet it’s the region’s clouds that have captured scientists’ attention.

These clouds play a crucial role in Earth’s climate by reflecting sunlight back to space, but climate models have struggled to accurately simulate their properties. Understanding the mismatch between models and observations has become increasingly urgent as scientists work to improve predictions of future climate change.

Dr Gerald Mace and an international team of researchers have been investigating an intriguing pattern: clouds near Antarctica’s coast tend to be brighter and more reflective than those further north over the open ocean. This brightness comes from having more numerous but smaller water droplets packed into the clouds – a property that makes them more effective at reflecting sunlight.

Following the Air’s Journey

To understand what creates these especially bright clouds, Dr Mace and his colleagues tracked air masses as they moved across the Antarctic continent and over the Southern Ocean. They combined multiple types of observations, including data from satellites, research ships, and atmospheric measurements, to build a comprehensive picture of how the clouds evolve.

The team’s analysis revealed that air masses which had recently spent time over the Antarctic ice sheets produced clouds with particularly high numbers of droplets. This effect was especially pronounced when the air had travelled over Antarctica’s high-altitude ice domes, where temperatures are extremely cold and the sun’s rays are intense during the summer months.

These conditions, the researchers surmised, create an ideal environment for forming new particles that can later serve as seeds for cloud droplets. When this particle-rich air descends from the Antarctic plateau and moves out over the ocean, it produces clouds with markedly different properties from those formed in air masses that haven’t passed over the continent.

The Chemistry Behind the Clouds

The process begins in the biologically productive waters near Antarctica’s coast, where tiny marine organisms flourish during the summer months. These organisms release a chemical called dimethyl sulphide (DMS) into the air – a process that has been occurring in Earth’s oceans for millions of years. When this DMS-rich air rises and passes over Antarctica’s ice sheets, it undergoes a remarkable transformation.

Research at Australia’s CSIRO research organisation has examined the complex chemistry involved in this process. This work shows that over the ice sheets, where there are very few existing particles in the air and intense sunlight during summer, chemical reactions convert the DMS into sulfuric acid vapour. This vapour can then form completely new particles through a process called nucleation, which eventually become the seeds for cloud droplets.

This natural particle formation process proves particularly efficient because the air over Antarctica’s ice sheets is exceptionally clean – any existing particles have usually been removed by precipitation before the air reaches the continent. The newly formed particles, therefore, have little competition as they grow large enough to serve as cloud condensation nuclei, the essential seeds around which cloud droplets form.

Antarctica from space. Elements of this image furnished by NASA.

Advanced Measurement Techniques

The research team employed several innovative approaches to study how these clouds evolve over time. They deployed advanced instruments on research vessels traversing the Southern Ocean, including sophisticated radar and lidar systems that can probe cloud properties in detail. These shipboard measurements were combined with data from multiple satellite passes over the same air masses to track changes in cloud properties over time.

One of the team’s most significant findings came from tracking how quickly cloud properties can change as air masses move away from Antarctica. When air that had recently been over Antarctica moved out over the ocean, it initially formed clouds with very high numbers of droplets – sometimes exceeding 200 per cubic centimetre. However, over just a few days, this number would typically decrease to between 50 and 100 droplets per cubic centimetre as the air mass continued northward.

This decrease occurs through various processes, including cloud droplets colliding and combining into larger drops that can fall as drizzle, gradually removing some of the particles that serve as cloud seeds. However, the team found that even after several days over the ocean, clouds formed in Antarctic-influenced air masses still tended to have more droplets than those in air that hadn’t recently passed over the continent.

Seasonal Patterns and Long-term Observations

The research team’s analysis of five years of satellite data revealed strong seasonal patterns in cloud properties over the Southern Ocean. During the austral summer (December through February), when solar radiation is most intense over Antarctica, the contrast between clouds formed in Antarctic-influenced air masses and those formed in maritime air is most pronounced.

This seasonal cycle aligns with patterns in biological activity in the ocean, suggesting a complex interplay between marine life, atmospheric chemistry, and cloud formation. The team found that chlorophyll concentrations in the ocean, which indicate the presence of phytoplankton that produce DMS, peak about a month before the highest cloud droplet numbers are observed.

Technical Challenges and Innovations

Studying clouds in the Southern Ocean presents numerous technical challenges that Dr Mace and his colleagues had to overcome. The remote location, harsh conditions, and vast areas involved required innovative approaches to data collection and analysis. The researchers developed new methods for combining different types of observations, including ship-based measurements, satellite data, and atmospheric tracking models.

The team made particular use of the MODIS (Moderate Resolution Imaging Spectroradiometer) instruments aboard NASA’s Terra and Aqua satellites, which provide detailed measurements of cloud properties. They developed sophisticated algorithms to track air masses and match them with satellite observations, allowing them to follow the evolution of cloud properties over time.

Antarctic ice sheet

Implications for Climate Science

This research has significant implications for understanding Earth’s climate system. The Southern Ocean region has been identified as an area where climate models show significant biases – they tend to predict too much sunlight reaching the ocean surface compared to observations. This suggests the models aren’t correctly representing the clouds in this region.

The team’s findings help explain why the models might be struggling. The process of particle formation over Antarctica and subsequent cloud brightening is quite complex and happens at scales smaller than most climate models can directly simulate. Understanding these processes better should help improve how they are represented in climate models.

The research is particularly significant because the Southern Ocean is one of the few places left on Earth where scientists can study cloud formation in conditions similar to those that existed in pre-industrial times. This provides valuable insights into how clouds behaved before human activities began adding large amounts of particles to the atmosphere.

Impact on Global Climate Understanding

The team’s discoveries extend beyond regional implications. The Southern Ocean plays a crucial role in Earth’s climate system, absorbing vast amounts of heat and carbon dioxide from the atmosphere. The clouds over this region help regulate how much solar energy reaches the ocean surface, affecting atmospheric and oceanic circulation patterns that influence climate worldwide.

Understanding these natural cloud brightening processes helps scientists better evaluate proposed climate intervention strategies that might attempt to artificially enhance cloud reflectivity. The team’s research shows that such processes are complex and interconnected, highlighting the importance of thoroughly understanding natural systems before considering any artificial modifications.

New Avenues for Research

The team are particularly interested in understanding more about the chemical processes occurring over Antarctica’s ice sheets and how they might change as the climate warms. Future research plans include direct measurements of particle formation over the Antarctic plateau, despite the challenging conditions and remote location. Dr Mace and his team are now planning several follow-up studies to address remaining questions. They hope to better understand how changes in sea ice extent might affect these processes, as diminishing sea ice could alter both the biological production of DMS and the paths that air masses take over the region.

This research represents a significant step forward in understanding a complex natural process that has important implications for Earth’s climate. By combining multiple types of observations and focusing on following air masses through their journey over Antarctica and the Southern Ocean, Dr Mace and his colleagues have helped unravel one of the many intricate ways in which our planet’s systems work together to regulate its climate.