Abstract
Abstract. Specular meteor radars (SMRs) and partial reflection radars (PRRs) have been observing mesospheric winds for more than a solar cycle over Germany (∼ 54∘ N) and northern Norway (∼ 69∘ N). This work investigates the mesospheric mean zonal wind and the zonal mean geostrophic zonal wind from the Microwave Limb Sounder (MLS) over these two regions between 2004 and 2020. Our study focuses on the summer when strong planetary waves are absent and the stratospheric and tropospheric conditions are relatively stable. We establish two definitions of the summer length according to the zonal wind reversals: (1) the mesosphere and lower-thermosphere summer length (MLT-SL) using SMR and PRR winds and (2) the mesosphere summer length (M-SL) using the PRR and MLS. Under both definitions, the summer begins around April and ends around middle September. The largest year-to-year variability is found in the summer beginning in both definitions, particularly at high latitudes, possibly due to the influence of the polar vortex. At high latitudes, the year 2004 has a longer summer length compared to the mean value for MLT-SL as well as 2012 for both definitions. The M-SL exhibits an increasing trend over the years, while MLT-SL does not have a well-defined trend. We explore a possible influence of solar activity as well as large-scale atmospheric influences (e.g., quasi-biennial oscillation (QBO), El Niño–Southern Oscillation (ENSO), major sudden stratospheric warming events). We complement our work with an extended time series of 31 years at middle latitudes using only PRR winds. In this case, the summer length shows a breakpoint, suggesting a non-uniform trend, and periods similar to those known for ENSO and QBO.
Highlights
As Earth orbits around the Sun, the duration of the four seasons is well defined at ground level at middle latitudes
We establish two definitions of the summer length according to the zonal wind reversals: (1) the mesosphere and lower-thermosphere summer length (MLTSL) using specular meteor radars (SMRs) and partial reflection radars (PRRs) winds and (2) the mesosphere summer length (M-SL) using the PRR and Microwave Limb Sounder (MLS)
Each time series is treated with a default standard deviation error from the size of the smoothing window plus an extra consideration for the years where the mean zonal wind reversal is difficult to assess due to an unclear transition
Summary
As Earth orbits around the Sun, the duration of the four seasons is well defined at ground level at middle latitudes. Jaen et al.: Long-term studies of MLT summer length defined by the mean zonal wind reversal waves, gravity waves, and tides (e.g., Yigit et al, 2016) Circulation patterns such as the stratospheric quasi-biennial oscillation (QBO, Baldwin et al, 2001) and El Niño–Southern Oscillation (ENSO, Wang and Picaut, 2004) influence the MLT dynamics at middle latitudes. Every year the zonal wind circulation in the MLT displays the final reversal of the wind direction from eastward to westward, in part produced by the wave dissipation generated by gravity wave activity (see Hoffmann et al, 2010; Laskar et al, 2017) In connection with this wind reversal, the mesopause experiences a decrease in temperature resulting in the appearance of ice particles, due to the water vapor present in the atmosphere. Between 80 and 90 km and on nanometer scales, a congregation of ice particles is called noctilucent clouds or polar mesospheric clouds (e.g., Hervig et al, 2001; Baumgarten et al, 2008; Fiedler et al, 2015)
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