The stratosphere is a layer of the atmosphere that contains just 15% of the atmospheric mass, and is located immediately above the troposphere, where day-to-day weather systems occur. Weather systems are inhibited from penetrating upwards into the stratosphere. Specifically, the much less dense, but more highly stratified, dry and stable stratosphere with its strong winds (see below) cannot support the dynamics that characterize weather systems. Hence weather systems are not directly affected by events in the stratosphere. Indeed, prior to the past decade or so, coarsely resolved stratospheres were considered sufficient for most weather and climate models1, especially given the relatively low mass and density of the stratosphere. Nowadays it is recognized that dynamical links between the stratosphere and troposphere exert a significant downward influence and that this influence affects surface weather.
During the extended winter period, the high-latitude stratosphere is very cold and dominated by a fast-moving westerly circumpolar jet that marks the edge of the polar vortex. When the westerly jet is not too strong, planetary scale waves propagate upwards into the stratosphere2 where they break and dissipate, forming a mid-latitude stratospheric surf zone3 with intense mixing and relatively uniform properties. This wave-breaking decelerates the stratospheric jet (Box 1), and the winds may even reverse and become easterly. These events are accompanied by increases in polar temperatures of tens of degrees in a few days, sometimes to values higher than those in mid-latitudes. In January 2009, for instance, the mid-stratosphere temperature at the North Pole rose from below ？70 °C to above ？10 °C in just four days4. Such events are known as sudden stratospheric warmings, or SSWs5. They occur, on average, approximately six times per decade in the Northern Hemisphere but only one SSW has ever been observed in the Southern Hemisphere5, 6.