Preprints
https://doi.org/10.5194/angeo-2021-51
https://doi.org/10.5194/angeo-2021-51

  13 Sep 2021

13 Sep 2021

Review status: this preprint is currently under review for the journal ANGEO.

Long-term studies of MLT summer length definitions based on mean zonal wind features observed for more than one solar cycle at mid- and high-latitudes in the northern hemisphere

Juliana Jaen1, Toralf Renkwitz1, Jorge L. Chau1, Maosheng He1, Peter Hoffmann1,, Yosuke Yamazaki2, Christoph Jacobi3, Masaki Tsutsumi4, Vivien Matthias5, and Chris Hall6, Juliana Jaen et al.
  • 1Leibniz-Institute of Atmospheric Physics at the University of Rostock, Schloss-Strasse 6, 18225 Kühlungsborn, Germany
  • 2GFZ German Research Centre for Geosciences, Potsdam, Germany
  • 3Institute Meteorology, Leipzig University, Stephanstr. 3, 04103 Leipzig, Germany
  • 4National Institute of Polar Research, Tokyo, Japan
  • 5Institute for Solar-Terrestrial Physics, German Aerospace Center (DLR), Neustrelitz, Germany
  • 6Tromsø Geophysical Observatory, The Arctic University of Norway, Tromsø, Norway
  • deceased, Peter Hoffmann on 29 October 2020 and Chris Hall on 9 August 2021

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 PRR and MLS. Under both definitions, the summer begins around April and ends around mid-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 oscillations (QBO), El Niño-southern oscillation (ENSO), major sudden stratospheric warming events). We complement our work with an extended time series of 31 years at mid-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.

Juliana Jaen et al.

Status: open (until 25 Oct 2021)

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Juliana Jaen et al.

Juliana Jaen et al.

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Short summary
To study long-term trends in the mesosphere and lower thermosphere (70–100 km) we established two summer length definitions and analyzed the variability over the years (2004–2020). After the analysis, we found significant trends in the summer beginning of one definition. Furthermore, we were able to extend one of the time series up to 31 years, and we obtained evidence of non-uniform trends and periodicities similar to those known for quasi-biennial oscillation and El Niño-southern oscillation.