Trends of mesospheric gravity waves at northern middle latitudes during summer

Abstract

[1] Recent investigations of the seasonal variation of the activity of gravity waves in the mesosphere/lower thermosphere (MLT) at middle and high latitudes suggest a semiannual variation with maxima during winter and summer and minima during the equinoxes. It is generally assumed that this annual cycle is determined by filtering processes due to the background winds in the stratosphere and lower mesosphere. On the other side, long-term observations of mesospheric winds at Juliusruh (55°N, 13°E) since 1990 indicate a stable increase of westward directed winds below 80 km (negative trends) during summer, as, e.g., clearly evident in monthly means in July. Here, we are studying how these long-term changes of winds are related to trends of the activity of gravity waves (GW) with periods between 3–6 hours. Our results show that the observed zonal wind trend at about 75 km during July goes along with an enhancement of the GW activity at altitudes above 80 km. Indeed, also the year-to-year variation of maxima of the observed westward directed winds at altitudes near 75 km and the GW activity at about 80 km are significantly correlated. Our results stimulate the further study of long-term wind changes and corresponding gravity wave trends.