Impact of the lower thermospheric winter-to-summer residual circulation on thermospheric composition

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Monday, June 12, 2017

Plain Language Summary or Abstract: Gravity wave forcing near the mesopause drives a summer-to-winter residual circulation in the mesosphere and a reversed, lower thermospheric winter-to-summer residual circulation. We conducted modeling studies to investigate how this lower thermospheric residual circulation impact thermospheric composition (O/N2).

TIE-GCM simulated effects image
TIE-GCM simulated effects of the lower thermospheric residual circulation on thermospheric composition (O/N2): upwelling of the simulated lower thermospheric residual circulation greatly decreases O/N2 in the winter hemisphere but downwelling in summer slightly increases O/N2 at high latitudes. This reduces the summer-to-winter gradient of O/N2. Consequently, this decrease of the summer-to-winter latitudinal gradient in O/N2 decreases the ionosphere winter anomaly. The NCAR TIE-GCM has a long-standing issue with producing a stronger winter anomaly in the ionosphere at high solar activity and a winter anomaly at low solar activity that was not observed by COSMIC and ionosondes. Therefore, the lower thermospheric residual circulation may be a process that has been largely ignored but is very important to the summer-to-winter latitudinal gradients, as well as annual/semiannual variations in the thermosphere and ionosphere. The figure shows TIE-GCM simulated O/N2 and meridional circulation on January 30, 2007. Quantities shown here are diurnally averaged zonal-mean quantities. The over-plotted wind vectors are the vectors of meridional and vertical winds, with red and blue vectors representing upward and downward vertical winds, respectively. (a): natural logarithm of O/N2 and vectors of meridional and vertical winds from the default run; (b) natural logarithm of O/N2 and vectors of meridional and vertical winds from the residual circulation run; (c) vectors of meridional and vertical winds at the lowest two pressure scale heights (~ 97 – 110 km) from the default run; (d) vectors of meridional and vertical winds at the lowest two pressure scale heights (~ 97 – 110 km) from the residual circulation run.

We found that the upwelling associated with the residual circulation significantly decreases O/N2 in winter and the downwelling in summer slightly increases O/N2. Consequently, the residual circulation reduces the summer-to-winter latitudinal gradient of O/N2, which causes the simulated latitudinal gradient of O/N2 to be more consistent with observations. The smaller summer-to-winter latitudinal gradient of O/N2 would decrease the ionosphere winter anomaly in model simulations, which would bring the simulated winter anomaly into better agreement with ionospheric observations. The lower thermospheric residual circulation may be a process that has been largely ignored but is very important to the summer-to-winter latitudinal gradients, as well as annual/semiannual variations in the thermosphere and ionosphere.

Publication Name: Geophsical Research Letters

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