Enhanced spring warming in a Mediterranean mountain by atmospheric circulation.

We analyzed trends of air temperature across the Cévennes National Park in Southern France, a mid-altitude coastal mountain experiencing a rapid spread of forests at the expense of rangelands and submitted to Mediterranean Sea influences and so, impacted by local and regional processes of climate change. Since 1980, April to June warming trend reached a maximum temperature increase of + 0.124 °C year and uniform whatever the altitude. Minimum temperature increased by + 0.058 °C year at 500 m altitude and + 0.089 °C year at 1500 m. Concomitantly, forest cover is increasing by + 0.51% year. Using an intrinsic biophysical mechanism model, we demonstrated that, at monthly scale, the forest surface is 1.7-3.1 °C cooler than that of nearby grasslands. As a result, the decrease in albedo corresponding to the conversion from grasslands to dense forests, translates into a cooling of maximum air temperatures of 0.023 °C year which contributes to slow down the warming rate enhancement. Spring warming trends co-varied with negative WeMO phases associated with a low in the Gulf of Cádiz and an anticyclone in Central Europe. An east to west pressure gradient increases atmospheric humidity leading to a strong water vapor feedback, enhancing the forcing of thermal long wave radiations and hence the rise in temperature.