The Hiccup – A Dynamical Vertical Coupling Process during the Fall Transition of the Northern Hemisphere

Abstract:

Sudden stratospheric warmings (SSWs) are the most prominent and dramatic vertical coupling process in the middle atmosphere occurring during winter months. However, during the fall transition another vertical coupling process occurs, approximately every second year, which we refer to as the Hiccup. The Hiccup acts like a “mini SSW” and is argued to be caused by the sudden onset of planetary wave (PW) activity in the stratosphere as the background winds become westerly. The average characteristics of zonal wind, temperature and PW activity between 65° and 75°N during the Hiccup are considered using the nudged CMAM30 simulation, representing 30 years of historical data. Additionally, the mesospheric zonal wind results are compared to radar observations in Andenes (69°N, 16°E) for the years 2000 – 2013. The average Hiccup is characterized by a weakening of the zonal wind from the troposphere up to the mesosphere and by a warming in the stratosphere and a cooling in the mesosphere, i.e. similar to a SSW but with a much smaller magnitude. A comparison of the Hiccup and SSW shows similarities and differences between both coupling processes.