Michael Tjernström, Professor, MISU

Atmospheric rivers and the onset of Arctic melt

ARTofMELT (Atmospheric rivers and the onset of Arctic melt) centers on an expedition with the Swedish research icebreaker Oden to the central Arctic Ocean in May and June 2023. The purpose is to study in detail when the sea-ice surface of the Arctic Ocean starts to melt in spring, an under-observed phase in the Arctic annual cycle. Specifically we will explore the impacts of so-called “atmospheric rivers”; geographically limited episodic inflows of moist and warm air from south. We ask ourselves if these events play a role in the timing of the melt onset. This date is important for the annual sea-ice melt-back that has increased substantially over the last several decades, as a consequence of global warming. On site in the Arctic Ocean we will use advanced weather forecasts to guide us to optimal locations. The nature of the problem necessitates an expedition when the sea ice is at maximum, challenging the capacity of icebreaker Oden and the logistics. When successful we will bring back unique observations of the processes that contribute to determining the time for when the summer melt starts.

Thorsten Mauritsen, Associate Professor, MISU

The Tropical Precipitation Tipping-Points (TPTP) project

Proposed tipping-points that would lead to the tropical rain forest systems involve positive feedback processes: initial tree loss leads to less evapotranspiration and precipitation, longer and warmer dry seasons, more fires and thereby further tree losses. However, looking back in time, the Amazon rain forest has been surprisingly resilient to even very large climate changes and has persisted despite higher temperatures and carbon dioxide levels. This motivates the TPTP project to investigate if other negative feedback mechanisms, which are not included in the tipping-point hypothesis, may exist.

In particular the TPTP project aims to understand the atmospheric component of the feedback loop. Focus is on soil-moisture-precipitation feedback, convective cloud dynamics embedded in a large-scale context of internal climate variability, the Hadley and Walker circulations and global energetic constraints. We will approach the problem using observations combined with both the global cloud resolving model ICON and contemporary climate models in order to explain and understand the observed decline of the Amazon rain forest. This project would be the first time that a global cloud resolving model is applied to the problem of the Amazon rain forest system tipping-points under global climate change and human exploitation.