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.

Change in turbopause altitude at 52 and 70° N

“…Overall change in the stratosphere is proposed as the origin of the observed strengthening of the Brewer-Dobson circulation during the last 35 years at least (Fu et al, 2015). Closer to the 70 • N, 19 • E (Tromsø) observations, Hoffmann et al (2011) report increases in gravity wave activity at 55 • N, 13 • E during summer, including at 88 km. Although not co-located, the increasing gravity wave flux, with waves breaking at the summer high-latitude mesopause, would similarly increase turbulence intensity and support the change reported here.…”

Neutral atmosphere temperature trends and variability at 90 km, 70 °N, 19 °E, 2003–2014

“…However, there are large regional differences regarding trends in GW activity. Hoffmann et al (2011) find an increasing GW activity in the mesosphere in summer for selected locations, but Jacobi (2014) finds larger GW amplitudes during solar maximum and relates this to a stronger mesospheric jet during solar maximum, both for winter and summer. Since we have not conducted any gravity wave trend assessment in this study, we cannot conclude that GW activity is responsible for the negative temperature trend, but we cannot rule out its role either.…”

Global Dynamics of the MLT

“…Multi-year observations from radar measurements in middle and high latitudes (e.g., Dowdy et al 2007;Hoffmann et al 2011) indicate that the summer zonal wind changes sign from easterly (from the east) to westerly around 90-95 km. The average winds during winter are westerly and do not change to easterly within the measurement range of the radars (up to 95-100 km).…”