Climate models project a distinct seasonality to future changes in daily
extreme precipitation. In particular, models project that over land in
the extratropical Northern Hemisphere the summer response is
substantially weaker than the winter response in percentage terms. Here
we decompose the projected response into thermodynamic and dynamic
contributions and show that the seasonal contrast arises due to a
negative dynamic contribution in northern summer, and a positive dynamic
contribution and an anomalously strong thermodynamic contribution in
northern winter. The negative dynamic contribution in northern summer is
due to weakened ascent and is strongly correlated with decreases in mean
near-surface relative humidity which tend to inhibit convection.
Finally, we show that the summer-winter contrast is also evident in
observed trends of daily precipitation extremes in northern
midlatitudes, which provides support for the contrast found in
climate-model simulations.