Sudden stratospheric warming (SSW) is an atmospheric phenomenon that occurs when polar stratospheric temperatures suddenly rise by several degrees (sometimes as much as 50 °C (90 °F)) over the course of a few days. SSW's occur high in the stratosphere, are often associated with Rossby waves and Polar Vortex breakdown and come in varying magnitudes. SSW events are significantly more common in the northern hemisphere than the southern hemisphere.
History SSW's were discovered by Richard Scherhag, a German Meteorologist who worked at the Free University of Berlin. Starting in 1951, Scherhag launched radiosondes from Berlin's Tempelhof Airport to research temperature behavior in the upper stratosphere. However, on January 26th, 1952, Scherhag noticed that the upper stratosphere was beginning to warm at an abnormal rate. The warming continued for four days, by which time the upper stratosphere had warmed 33 °C. Scherhag reported this phenomenon in a journal later that year, expressing considerable uncertainty regarding its cause and nature. Throughout the 1950s, radiosonde coverage improved substantially worldwide, allowing major meteorological organizations all over the world to analyze the stratosphere for the first time. This improvement in coverage detected similar SSWs in 1957 and 1958, making SSWs a focus for research groups of the time. Over the next decade, these groups discovered several characteristics of and raised awareness for SSWs, leading the World Meteorological Organization to establish of the STRATWARM warning system, which launched more soundings and issued alerts on the magnitude and location of SSW events, in 1964. These groups also began to classifying SSW events based on their time of occurrence and overall magnitude, leading to the classification categories that most scientists use today. SSW documentation and understanding has improved substantially since the modern satellite era began in 1979. Modern satellites make stratospheric measurements with accuracy and consistency that radiosondes never could, allowing for the creation of an SSW database and breakthroughs in SSW research. The WMO also developed STRATALERT, an international stratospheric monitoring program that monitors stratospheric conditions and documents SSW events.
Classification and description
SSW events are categorized by a sudden and abrupt increase in stratospheric temperatures over a short period of time, usually within a few days. SSW events occur in the winter, when winds in the stratosphere are normally westerly. SSW events disrupt the normal westerly wind flow of the upper troposphere, and, depending on severity, can sometimes reverse it entirely. As a result, some of the warmed air can intrude into the troposphere, which often disrupts the Polar Vortex. SSW events usually affect polar areas only, although the strongest SSW events can come down to 60 degrees north. Almost all SSW events take place in the northern hemisphere, which normally sees one SSW every 2-3 years. There was one major SSW event in the southern hemisphere in 2002, but no other major SSW events have been detected in the southern hemisphere since then. Because SSWs come in all magnitudes and sizes, it has led meteorologists to classify SSW events in two main categories: major and minor, based on how much they change temperature patterns and disrupt the polar vortex circulation. Sometimes, if an SSW event permanently reverses stratospheric winds to the east, which is the typical stratospheric wind flow for the summer, it is called a final warming.
Major A major SSW event is the strongest type of SSW event. These events are usually strong enough to entirely reverse the westerly flow that is common in the stratosphere during winter. These events influence temperatures as far south as 60 degrees north and are usually strong enough to completely disrupt the polar vortex, often splitting it into smaller vortices or displacing it entirely from its normal location. In order to qualify as a major SSW event, the event must completely reverse the westerly flow and come down to 60 degrees north.
Minor A minor SSW event is the weaker type of SSW event. These events are usually not strong enough to entirely reverse the westerly flow that is common in the stratosphere during winter, but instead add a noticeable easterly component to the westerly flow. Minor SSW events are usually not strong enough to break down the polar vortex and affect temperatures as far south as 60 degrees N.
Final A final SSW event is any SSW event that permanently reverses the stratospheric wind flow from westerly to easterly for the summer. Normally, stratospheric winds are westerly in the winter and easterly in the summer, so any late-winter SSW event that permanently causes this is called a final warming. Final warmings are usually major SSW events.
… excerpt ends here. Continue reading the full article.
