Climatology Of Global Gravity Wave Activity Revealed By SABERr/TIMED Temperature Observations

Global gravity wave (GW) activity and its dissipation in the height range from the low stratosphere up to the low thermosphere (25-115km) are statistically studied by using11-years (January22,2002-December31,2012) SABER/TIMED temperature data. Also the variations of stratopause and the corresponding GW activity in different latitudes are analyzed.In most previous studies, a Kalman filter was used to extract GW fluctuations, and the problem of its cut-off wavenumber6, which is very close to Nyquist wavenumber8of the SABER observations, would underestimate planetary scale fluctuations then overestimate GW activity. In this paper, a new method is proposed to extract realistic GW fluctuations from the temperature profiles, and the squared fluctuations are treated as GW activity. By comparing with previous results, it shows that this new method can obtain more real GW activity in the atmosphere.It is found that GW activities generally increase with height. In different latitudes and heights, GW activities show different seasonal variations. Near the equator (0°-10°), GW activity shows a quasi-biennial variation in the stratosphere (below40km) and a semi-annual variation at the height range between the upper stratosphere and the low thermosphere (40-115km). In low latitudes (20°-30°), GW activity mainly shows a semi-annual variation in the whole height range from the stratosphere to the low thermosphere (25-115km). In middle latitudes (40°-50°), GW activity shows an annual variation below85km and a nearly four-monthly variation with peaks occurring usually in April, August, December in the northern hemisphere and in February, June, October in the southern hemisphere, respectively, above85km. In order to study the GW dissipation in the atmosphere, we calculate the GWgrowth scale height and GW dissipation ratio, which is defined as the ratio of the GW growth scale height to the atmosphere density scale height. The strong GW dissipation in different latitudes mainly concentrates between the stratosphere and the low mesosphere (30-60km), the mesopause (～85km) and the low thermosphere (100-110km) with different dissipation mechanisms. In the stratosphere, the strong GW dissipation might be attributed to the wind filtering effect because the variation of GW dissipation is correlated with that of zonal wind; around the mesopause, the turbulence is a probable cause of GW dissipation; in the low thermosphere, the molecular viscosity stronger than the turbulence is mainly responsible for GW dissipation. Besides, GW energy enhancement can also be observed, which appears much less frequently.In addition, the variations of stratopause are correlated with that of GW activity in low and middle latitudes. In low latitudes (0°-20°), GW activity and the stratopause height show semi-annual variation with peaks in summer and winter, and the stratopause temperature shows semi-annual variation with peaks in spring and autumn; in middle latitudes (30°-50°), all of the above mentioned three show annual variation, and the peaks of GW activity and the stratopause height are in winter while that of the stratopause temperature are in summer. By analyzing correlation between GW activity and the stratopause, there is a positive correlation between GW activity and the stratopause height while a negative correlation between GW activity and the stratopause temperature, both of which show hemispheric asymmetry.Although in the winter (January-March) of boreal polar regions the stratopause height usually is～50km with a temperature of～260K, in late January-early February of2006,2009and2010, the stratopause height jumps to the higher location～80km with a temperature of～230K, which means the elevated stratopause (ES), at the same time GW activity is enhanced at～80km; as time goes on, the stratopause height decreases and its temperature increases until both of them return to their normal levels, meanwhile GW activity becomes weaker above75km and stronger below75km. It is noteworthy that the ES events in2006,2009and2010all occur after the major sudden stratospheric warmings (SSWs) accompanying with the polar vortex splitting; and the ES event in2012occur after the minor SSW, but the zonal wind in the stratosphere reveals and the polar vortex variation is the same as that in2009. However, in other years, including the years with the major SSW accompanying with the polar vortex dissipation (for example2008) and the normal years without SSW (for example2003), no ES events happened. Furthmore, the correlation analyses between GW activity and the ES event indicate the possible contribution of GW activity to the ES event.