21 September 2016

Uncertainties in Projecting Future Changes in Atmospheric Rivers and Their Impacts on Heavy Precipitation over Europe

Present and future multi-model ensemble climate simulations project substantial increases in the frequency of atmospheric rivers over Western Europe.

Science

This study investigated the future changes in the frequency of landfalling atmospheric rivers in Western Europe.  An analysis of a multi-model ensemble of climate simulations shows substantial increases of atmospheric river frequency under a climate warming scenario. And changes in the amount of moisture and winds are both contributing to these increases.

Impact

Large increases of atmospheric river days are projected under climate warming in Western Europe, and the increased atmospheric rivers are projected to induce more extreme precipitation in future. This study identifies a possible causal relationship between model biases in simulating the westerly wind and the changes in atmospheric river frequency in a warmer climate that may be used to provide more robust projections of the latter.

Summary

In a study conducted by Department of Energy researchers at Pacific Northwest National Laboratory, scientists investigated the North Atlantic atmospheric rivers (ARs) making landfall over western Europe in the present and future climate scenarios. A multi-model ensemble of climate simulations from the Coupled Model Intercomparison Project Phase 5 (CMIP5) captured the seasonal and spatial variations of historical landfalling AR days. They found that the large inter-model variability of AR days correlates strongly with the inter-model spread of the westerly jet position. In the future under a high greenhouse gas emissions scenario, the models project the AR frequency to increase significantly by the end of this century. The increase in moisture with warmer temperatures plays a dominant role in the future increase of AR days. However, wind changes associated with the midlatitude jet shifts result in an increase in AR days poleward and a decrease in AR days equatorward in all seasons. As the model-projected jet shifts were strongly correlated with the simulated historical jet position in North Atlantic, they also correlated changes in AR days in Western Europe with the historical jet position. Hence, the observed jet position may provide an important constraint on model projected changes in ARs making landfall in Western Europe. Compared to the present, the research shows that both total and extreme precipitation induced by ARs in the future contribute more to the seasonal mean and extreme precipitation, primarily because of the increase in AR frequency.

Contact
Ruby Leung
Pacific Northwest National Laboratory (PNNL)