Clouds & aerosols over the Southern Ocean

Reducing biases in the representation of clouds and aerosols in the NZESM

Clouds have a massive effect on climate. Cloud cover reflects radiation from the sun that would otherwise be absorbed by oceans, raising their temperatures. Cloud cover can also act as a blanket, keeping warmth near the surface.

Despite their significant influence on climate, clouds still represent the largest source of uncertainty in modern climate models. For example, the frequency of clouds over the Southern Ocean is often underestimated, causing models to predict warmer sea surface temperatures than observed, which in turn means the models predict the strength and position of the storm tracks incorrectly. These biases also affect the sensitivity of the model to human-induced climate drivers, such as increasing greenhouse gases.

It is vital to correct these biases, so we can increase our certainty in climate projections.

This project improved our understanding of the chemistry and physics of clouds and aerosols in the Southern Ocean. By combining detailed measurements made during voyages with satellite observations and modelling studies, we were able to pinpoint model biases and identify the underlying errors.

We completed dedicated measurement voyages, which saw researchers travel deep into the Southern Ocean, while making observations from the ship and launching instrumented balloons. We compared the measurements from the voyage with NZESM model output to identify differences and their underlying causes.

The project team also developed new ways of using satellite data – using machine learning to identify different cloud types (not in itself a new strategy, but which has never before been used in model evaluation and improvement). This helps us determine whether the right amount of cloud is represented in the model and also, crucially, whether it’s the right kind of cloud and if it’s in the right place, relative to satellite observations. Results suggest that climate models predict a higher abundance of ice cloud than is found in reality, and we’re now working on understanding the mechanisms that form ice in clouds.

Improving our understanding of clouds and incorporating this into the NZ Earth System Model is critical, as these processes significantly affect New Zealand’s climate and have influences as far away as the tropics.