Quantifying the influence of the stratosphere on the mesosphere and lower thermasphere

Abstract

In this thesis, the influence of the major modes of stratospheric variability - the seasonal cycle and sudden stratospheric warmings (SSWs) in the extratropics, and the quasi-biennial oscillation (QBO) in the equatorial region - on the mesosphere and lower thermosphere is quantified. Using meteor radar observations obtained throughout 2013 over Trondheim, Norway (63 N, 10 E), the seasonal cycle of high-frequency gravity wave (GW) momentum flux and its divergence is determined. Eastward (westward) GW forcing at mesopause heights is observed in winter (summer), and it is shown that the asymmetry in the wind field underlying the mesopause region can be used as a quantitative proxy for the seasonal variability of GW forcing at mesopause heights. In January 2013, a major SSW occurred during which the zonal wind over Trondheim reversed from eastward to westward from the surface up to 100 km. Six days prior to the January 2013 major SSW, when winds in the stratosphere began to weaken, the meteor radar derived GW forcing turned eastward, reaching peak values of +145 60 msð€€€1dayð€€€1. Post-SSW, when enhanced eastward winds were observed below 85 km, the GW forcing turned westward reaching a minimum of -240 70 msð€€€1dayð€€€1 around 18 days after the SSW onset. The evolution of the GW forcing derived by the Whole Atmosphere Community Climate Model with specified dynamics was shown to compare well with the observations. The global middle atmosphere temperature response to the 2013 major SSW is presented using Microwave Limb Sounder satellite data. The eastward mesopause GW forcing observed over Trondheim during the 2013 major SSW is associated with a mesospheric cooling about 10 km below mesopause heights at the same location. In addition, a global pattern of temperature perturbations is observed in the mesosphere, with equatorial warming and a warm anomaly overlaying a cold perturbation that propagates downward in time in the summer hemisphere. These results provide direct observational evidence for the interhemispheric coupling mechanism demonstrated in modelling studies. Focussing on the equatorial region, the interannual variability in mesospheric/lower thermospheric (MLT) zonal mean winds is presented in relation to stratospheric QBO (SQBO). A mesospheric QBO with a period of 27.5 months and a magnitude of 4.1 0.7 msð€€€1 was found to be out-of-phase with the SQBO measured at 15-20 hPa, and in-phase compared to 70 hPa. This observed phase-relation is shown to be consistent with the selective filtering of a symmetrical spectrum of upward propagating GWs by the SQBO winds. As a product of this thesis, a method to routinely determine GW momentum flux and forcing from the Trondheim meteor radar data was developed. This technique has been used to show that during the fall equinox, when a temporary enhancement of planetary wave activity in the MLT is observed coincident with increased poleward flow and temperatures, the net GW forcing at these altitudes ceases. In the absence of GW forcing, it is concluded that planetary wave driving temporarily becomes the dominant forcing of the MLT region.