QBO modulation of gravity wave potential energy and propagation in the Northern-Hemisphere high-latitude stratosphere and mesosphere

Gravity waves (GWs) in the stratosphere and mesosphere (20-100 km) are derived from SABER temperature measurements over 17 years (2002-2018). The QBO (which is represented by the Empirical Orthogonal Functions (EOFs) of equatorial zonal winds) modulation on the GWs is investigated by applying multivariate linear regression and correlation analysis. In the Northern-Hemisphere (NH) high latitude region (50-80 °N), a significant negative response of GW potential energy density per unit mass (GW-Epm) to QBO is found below 65 km, while a positive response is found between 65-90 km. Between 90-100 km, the response is still positive, but weaker. During QBO easterly phase (QBOe), the GW-Epm is ~5% larger below 65 km, while 10-15% smaller between 65-80 km, compared to QBO westerly phase (QBOw).

To investigate the vertical and oblique propagation of GWs under different QBO phases in the NH high latitude region, the Gravity-wave Regional or Global Ray Tracer (GROGRAT) model is employed for ray tracing with MERRA-2 data to set up the background fields. The simulations show that in QBOe, the vertical propagation of GWs within 50-80 °N is more favorable due to the less filtering effect by the background wind, while the oblique propagation of GWs originating from middle latitude (<50°N) is less favorable, compared to QBOw. One possible explanation of the observations is inferred from the ray tracing results as follows. Because the obliquely propagating waves often reach over 65 km in altitude when they propagate from middle to high latitudes, while below 65 km, the GW-Epm is mainly controlled by vertically propagating GWs, a negative correlation with QBO is resulted for GWs below 65 km. Above 65 km, in contrary, obliquely propagating waves contribute to the GW-Epm and lead to a positive correlation with QBO. The combination of the modulation of vertically and obliquely propagating GWs leads to the reversal of QBO signatures in NH high-latitude GW-Epm observed by SABER.