The impact of gravity wave drag on mesospheric analyses of the 2006 stratospheric major warming

The 2006 Arctic stratospheric sudden warming was an unusual event in which the stratopause disappeared after the peak of the warming, only to reform at very high altitudes above 75 km. The stratopause then descended during February and March 2006, returning to climatological values. This event, which coupled the troposphere, stratosphere, and mesosphere, has been used to assess the quality of the upper stratospheric and mesospheric analyses produced with the Canadian Middle Atmosphere Model's data assimilation system. By comparing with simulations in which Sounding of the Atmosphere Using Broadband Emission Radiometry temperatures were assimilated, mesospheric analyses obtained without assimilating any observations above 45 km were shown to capture the timing and amplitude of the elevated stratopause event. Mesospheric analyses of temperature and winds were sensitive to the use of a nonorographic gravity wave drag scheme, but this sensitivity was reduced when mesospheric temperatures were assimilated. Moreover, a realistic nonorographic gravity wave drag can somewhat compensate for the lack of mesospheric measurements. On the other hand, the descent of mesospheric NOx after the peak of the stratospheric warming was found to be sensitive to the presence of nonorographic gravity wave drag even when mesospheric temperatures were assimilated, suggesting that eddy forcing is not fully constrained by mesospheric observations in our data assimilation system.