Long-term studies of the summer wind in the mesosphere and lower thermosphere at middle and high latitudes

Abstract

Abstract. Continuous wind measurements using partial reflection radars and specular meteor radars have been carried out for nearly two decades (2004–2022) at middle and high latitudes over Germany (∼54° N) and northern Norway (∼69° N), respectively. They provide crucial data for understanding the long-term behavior of winds in the mesosphere and lower thermosphere. Our investigation mainly focuses on the summer season, characterized by the absence of intense planetary wave activity and relatively stable stratospheric conditions. This work presents the long-term behavior, variability and trends of the maximum velocity of the summer eastward, westward and southward winds. In addition, the geomagnetic influence on the summer zonal and meridional wind is explored at middle and high latitudes. The results show that a westward summer maximum is located around 75 km with velocities of 35–54 m/s, while the eastward wind maximum is observed at ∼97 km with amplitudes of 25–40 m/s. A weaker southward wind peak is found around 86 km ranging from 9–16 m/s. The findings indicate significant trends at middle latitudes in the westward summer maxima with increasing winds over the past decades, while the southward winds show a decreasing trend. On the other hand, only the eastward wind in July has a decreasing trend at high latitudes. Evidence of oscillations around 2–3, 4 and 6 years modulate the maximum velocity of the summer winds. Particularly a periodicity between 10.2–11.3 years found in the westward component is more significant at middle latitudes than at high latitudes, possibly due to solar radiation. Furthermore, stronger geomagnetic activity at high latitudes causes an increase in eastward wind velocity, whereas the opposite effect is observed in zonal jets at middle latitudes. The meridional component appears disturbed during high geomagnetic activity, with a notable decrease in the northward wind strength below approximately 80~km at both latitudes.

Interannual Variability of Winds in the Antarctic Mesosphere and Lower Thermosphere Over Rothera (67°S, 68°W) During 2005–2021 in Meteor Radar Observations and WACCM‐X

“…There have been a few long term trend analyses using observational meteor radar wind data sets. Recently, Jaen et al (2023), studied summertime trends using MF radars and meteor radars in Northern and middle latitudes. At northern high latitudes, they found a weakening of the eastwards winds at 90-105 km of 4.5 ms 1 per decade which is in agreement with the results that we see in Figure 6a, where the hatched blue region at heights of 90-100 km throughout October to March indicate the eastwards winds have a more westwards trend (i.e., are weakening) at a rate of between 3 and 7 ms 1 per decade.…”

“…However, such studies are often limited by the relatively small number of reliable observational data sets available of long enough duration. Despite this difficulty, some observational studies have explored the possible influence of the solar cycle on the winds in the MLT region (e.g., Bremer et al, 1997;Cai et al, 2021;Greisiger et al, 1987;Jacobi et al, 1997;Jaen et al, 2023;Middleton et al, 2002;Wilhelm et al, 2019). However, as noted by Cai et al (2021), results from such studies vary by location and time period used.…”

Long‐Term Variability and Tendencies in Mesosphere and Lower Thermosphere Winds From Meteor Radar Observations Over Esrange (67.9°N, 21.1°E)

“…From the above studies, it is evidenced that the MLT winds are significantly modulated by the solar forcing and lower atmospheric perturbations. Most of the studies on long‐term tendencies in the MLT winds are from equatorial/low‐latitudes (Burrage et al., 1996; Gurubaran & Rajaram, 1999; Kishore Kumar et al., 2014; Rajaram & Gurubaran, 1998; Sridharan et al., 2007, 2010; Venkateswara Rao et al., 2012) along with short‐term variabilities (K. K. Kumar et al., 2007, 2008; Rao et al., 2014), and mid‐latitudes (Jacobi, 2012; Jacobi et al., 2005, 2012, 2015; Namboothiri et al., 1993, 1994; Portnyagin et al., 2006); however the trends and variabilities in the high‐latitude (including Arctic and Antarctic) MLT winds are very limited (Baumgaertner et al., 2005; Dempsey et al., 2021; Dutta & Sridharan, 2023; Hindley et al., 2022; Jaen et al., 2023; Lukianova et al., 2015, 2018; Mitchell et al., 2002; Venkateswara Rao et al., 2015) from various observations and model simulations. Global variabilities in the MLT winds can be found at Ramesh et al.…”