Strong Influence of Eddy Length on Boreal Summertime Extreme Precipitation Projections

Previous research has shown that projected changes in the horizontal eddy length of ascending anomalies likely drive subtropical changes in large-scale ascent during extreme precipitation events (extreme ascent), which in turn strongly influence regional projections of extreme precipitation. Here we present evidence that this eddy length effect extends into the Northern Hemisphere extratropics during the summer season. This is shown by analyzing output from a large ensemble of the Canadian Earth System Model version 2 as well as models participating in the Coupled Model Intercomparison Project phase 5. As found previously, the changes in eddy length are associated with changes in an effective stability quantity that combines dry and moist effects. It is shown that the change of extreme ascent associated with a projected change of eddy length agrees with expectations based on analysis of internal variability of extreme ascent and eddy length during the historical period. Plain Language Summary Anomalous vertical motion of air during extreme precipitation events (i.e., extreme ascent) is a key factor determining the amount of precipitation (i.e., the intensity) of the event. Earlier studies have analyzed climate model simulations of global warming, showing that long-term changes in extreme ascent strongly influence long-term changes in extreme precipitation intensity. Here we present evidence that, over most of the Northern Hemisphere during the summer season, global warming simulations produce long-term increases in the horizontal scale of extreme ascent (i.e., eddy length), which in turn lead to widespread weakening of extreme ascent. This dynamical effect is strong enough to produce long-term decreases of extreme precipitation intensity over widespread regions. While this dynamical effect is apparent in all modern climate models, these models do not agree on the precise regions where long-term decreases of summertime extreme precipitation intensity will occur. This is because warmer temperatures produce widespread increases in atmospheric moisture, which favor more intense extreme precipitation. The details of how this thermodynamic effect combines with the dynamical effect in particular regions is not consistent across models.