El Niño influence on the mesosphere/lower thermosphere circulation at midlatitudes as seen by a VHF meteor radar at Collm (51.3 ° N, 13 ° E)

Abstract

Abstract. Mesosphere/lower thermosphere (MLT) zonal winds continuously measured by a VHF meteor radar at Collm, Germany (51.3 • N, 13.0 • E) in the height range 82 -97 km from 2004 to date are analyzed with respect to the signature of El Niño. The comparison of Niño3 equatorial SST index and MLT wind time series shows that in January and especially in February zonal winds are positively correlated with the Niño3 index. We note a delay of about one month of the MLT zonal wind effect with respect to equatorial sea surface temperature variability. The signal is strong for the upper altitudes (above 90 km) accessible to the radar observations, but weakens with decreasing height. This reflects the fact that during El Niño years the westerly winter middle atmosphere wind jet is weaker, and this is also the case with the easterly lower thermospheric jet. Owing to the reversal of the absolute El Niño signal from negative to positive with altitude, at the height of the maximum meteor flux, which is around 90 km, the El Niño signal is weak. The experimental results can be qualitatively reproduced by numerical experiments using a mechanistic global circulation model with prescribed tropospheric temperatures and latent heat release for El Niño and La Niña conditions.

Forcing mechanisms of the migrating quarterdiurnal tide

“…Meteor radar measurements of zonal wind QDT amplitudes at 50 • N by Jacobi et al (2017bJacobi et al ( , 2018 show maxima in January and February as well as in April and May, which is analogous to the MUAM simulations. The maxima in autumn, seen in Fig.…”

“…At an altitude of 100 km, amplitudes up to 0.5 K in temperature and 1.5 ms −1 in zonal wind are achieved. Thus, the modeled amplitudes are much smaller than those reported from measurements (e.g., Liu et al, 2015;Azeem et al, 2016;Jacobi et al, 2017b;Guharay et al, 2018), i.e., satellite measurements reveal temperature amplitudes of 5-10 K, depending on season and altitude, whereas radar data suggest wind amplitudes of 2.5-5 ms −1 . In Fig.…”

“…They mainly found maxima during early and late winter at middle latitudes, which coincided with modeled vertical shear QDT maxima of the zonal wind. The QDT has also been observed in radar wind measurements in the MLT region (e.g., Sivjee and Walterscheid, 1994;Smith et al, 2004;Jacobi et al, 2017b;Guharay et al, 2018). Guharay et al (2018) analyzed the variability of the QDT in the MLT over Brazilian low-latitude stations and found QDT wind amplitudes that reach 2 ms −1 with a maximum during late summer and fall.…”

“…The model is unable to produce nonmigrating tides, because it contains no 3-D fields of ozone and water vapor -it only contains zonal means. In contrast to the version by Ermakova et al (2017) and Jacobi et al (2017a), latent heat release is not included here. We used a horizontal resolution of 2.5 • × 5.625 • for the model, which differs from the version of, e.g., Lilienthal et al (2017) and Lilienthal et al (2018), in order to be able to better resolve the meridional structure of the QDT.…”

“…Guharay et al (2018) analyzed the variability of the QDT in the MLT over Brazilian low-latitude stations and found QDT wind amplitudes that reach 2 ms −1 with a maximum during late summer and fall. Jacobi et al (2017b) analyzed MLT (80-100 km) radar data from Collm (51 • N, 13 • E) and Obninsk (55 • N, 37 • E). They found maximum amplitudes in winter with a long-term mean monthly mean zonal amplitude of 7 ms −1 .…”