Atmospheric blocking duration in the Northern Hemisphere


Atmospheric blocks are persistent mid-latitude large-scale weather systems that block the westerly flow. They strongly impact our societies as they are often associated with extreme weather (cold spells in winter and hot spells in summer) due to their persistence. Indeed, blocks in the Northern Hemisphere can last from 5 days up to 4-5 weeks. Here we aim at identifying dynamical differences between the short blocks that last only five days, and the longer blocks that last at least ten days, to better characterise long blocks. Using reanalysis data, we show that long blocks mostly involve cyclonic Rossby wave breaking, while short blocks are equally associated with cyclonic and anticyclonic wave breaking. This main result is reproduced in several coupled climate models. Three mechanisms, involving positive eddy feedback on the block, might explain this lower number of long anticyclonic blocks.