Understanding the Dynamics of Future Changes in Extreme Precipitation Intensity

Climate model projections of extreme precipitation intensity depend heavily on the region: some regions will experience exceptionally strong increases in extreme precipitation intensity, while other regions will experience decreases in extreme precipitation intensity. These regional variations are closely related to regional changes in large-scale ascent during extreme precipitation eventsthat is, extreme ascentbut the drivers of extreme ascent changes remain poorly understood. Using output from a large ensemble of the Canadian Earth System Model version 2, we show that subtropical changes in extreme ascent likely result from changes in the horizontal scale of ascending anomalies, which are in turn associated with changes in vertical stability. Near the equator, changes in the seasonal mean circulation may be an important factor influencing extreme ascent, but this finding is model dependent. Plain Language Summary We are all too familiar with the devastation that extreme precipitation events have caused in many regions, and there is a great need to understand how human activities are influencing such events. The large-scale upward velocity of air during an extreme precipitation event (i.e., extreme ascent) plays a pivotal role in determining the amount of precipitation during the event (i.e., its intensity), but the echanisms driving long-term changes in extreme ascent are poorly understood. Using simulations of climate change in a modern climate model, we show that, near the equator, changes in extreme ascent reflect a shift in the overall statistical distribution of vertical velocity. Elsewhere, however, the story is not so simple, and we show that another key factor influencing extreme ascent is the horizontal scale of the ascending motion. For example, long-term increases in horizontal scale lead to weaker extreme ascent and decreased extreme precipitation intensity in much of the subtropics, opposite to the increased extreme precipitation intensity that is expected over most of the globe. Thus, we explain long-term changes of extreme ascent in terms of well-established theories of atmospheric dynamics and improve our understanding of how these changes result from the long-term increase of surface temperature.