Measurements of stratospheric thermal structure and gravity wave activity have been obtained with a Rayleigh lidar in the Canadian High Arctic at Eureka (80 ¿N, 86 ¿W) during five recent winters. The observations reveal that an annual late December warming of the upper stratosphere occurred in the polar vortex core and was sustained through the winter. Increased gravity wave activity was detected in the vortex jet during the warming. That these two phenomena developed in parallel suggests they are related. It is proposed that increased gravity wave momentum deposition above the jet maximum forced flow into the vortex core where it descended and warmed adiabatically. ¿ 1998 American Geophysical Union