At this point planetary waves may no longer penetrate into the stratosphere ). Hence further upward transfer of energy is completely blocked and a very rapid easterly acceleration and the polar warming occur at this critical level, which must then move downward until eventually the warming and zonal wind reversal affect the entire polar stratosphere. There exists a link between sudden stratospheric warmings and the quasi-biennial oscillation: If the QBO is in its easterly phase, the atmospheric waveguide is modified in such a way that upward-propagating Rossby waves are focused on the polar vortex, intensifying their interaction with the mean flow. Thus, there exists a statistically significant imbalance between the frequency of sudden stratospheric warmings if these events are grouped according to the QBO phase (easterly or westerly). Although sudden stratospheric warmings are mainly forced by planetary scale waves which propagate up from the lower atmosphere, there is also a subsequent return effect of sudden stratospheric warmings on surface weather. Following a sudden stratospheric warming, the high altitude westerly winds reverse and are replaced by easterlies. The easterly winds progress down through the atmosphere, often leading to a weakening of the tropospheric westerly winds, resulting in dramatic reductions in temperature in Northern Europe. This process can take a few days to a few weeks to occur.