From AMS Glossary
(Also called sudden warming.) A rise in temperature of the stratosphere in the polar region in late winter resulting from enhanced propagation of energy from the troposphere by planetary-scale waves.
Temperatures near 50 mb can increase by as much as 40°C in just a few days.
Andrews, D. F., J. R. Holton, and C. B. Leovy 1987. Atmosphere Dynamics. 259–294.