Mesosphere/lower thermosphere (MLT) zonal winds measured by a VHF meteor radar at Collm, Germany (51.3°N, 13.0°E) during late winter 2015/2016 show very strong westerly winds above about 90 km, but not below that height. This anomaly appears during a very strong El Niño event. The comparison of Niño3 equatorial sea surface temperature index and the Collm MLT wind time series starting in 2004 shows that in January and especially in February zonal winds are positively correlated with the Niño3 index. 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 on an average, and this is also the case with the easterly lower thermospheric jet. The El Niño effect on the meridional wind is weak. The experimental results can be qualitatively reproduced by numerical experiments using the MUAM mechanistic global circulation model with prescribed tropospheric temperatures and latent heat release for El Niño and La Niña conditions. / Der Zonalwind in der oberen Mesosphäre/unteren Thermosphäre über Collm (51.3°N, 13.0°E) in der zweiten Hälfte des Winter 2015/2016 weist eine besonders starke westliche Komponente oberhalb von etwa 90 km auf. Diese Anomalie erfolgte während eines sehr starken El Niño-Ereignisses. Der Vergleich von Collmer Zonalwinden seit 2004 und dem Niño3-index zeigt im Januar und besonders Februar eine positive Korrelation. Diese ist stark oberhalb von 90 km, nimmt aber nach unten hin ab. Dies spiegelt die Tatsache wider, dass während El Niño-Jahren im Mittel der stratosphärische/mesosphärische Westwindjet schwächer ist. Dieses Signal kehrt aber in der oberen Mesosphäre um, so dass der thermosphärische Ostwindjet ebenfalls schwächer ist. Der Effekt auf den meridionalen Wind ist schwächer. Die Beobachtungen können mit Modellexperimenten qualitativ reproduziert werden.

With the help of MERRA-2 (Modern-Era Retrospective Analysis for Research and Application, version 2) reanalysis data of the zonal wind, measurements of the mesopause region wind over Collm by low-frequency observations, and data of the Horizontal Wind Model, a height profile for the zonal wind in January and July was created by linear interpolation over altitude. Subsequently, three different 5-year periods (1980-1984, 1994-1998 und 1999-2003) were selected, and the obtained wind profiles and NRLMSISE (Naval Research Labatory Mass Spectrometer Incoherent Scatter, Extended) temperatures (January and July) were averaged over these time intervals. This allowed to run the whole atmosphere nonlinear spectral gravity wave (GW) routine of Yiğit et al. (2008) with these data as background profiles. We obtained a height profile of the GW momentum flux (MF) over these three 5-year periods. Based on this, the long-term tendencies of GW MF are estimated and in this way a statement about the trend of the GW activity in the mesosphere and lower thermosphere can be made. GW MF increases in magnitude in both summer and winter. The results are in qualitative agreement with trends of GW proxies obtained from the meteor radar measurements at Collm. / Mit Hilfe von MERRA-2 - Reanalysedaten des Zonalwindes, Messungen des Meteorradars in Collm und mit Daten des Horizontal Wind Models wurde durch lineare Interpolation ein Höhenprofil für den Zonalwind für die Monate Januar und Juli erstellt. Anschließend wurden aus den gewonnenen Windprofilen und NRLMSISE (Naval Research Labatory Mass Spectrometer Incoherent Scatter, Extended) Temperatur Reanalysen (Januar und Juli) drei verschiedene 5-Jahreszeiträume gewählt (1980-1984, 1994-1998 und 1999-2003) und über diese gemittelt. Diese Daten wurden als Hintergrund für die nichtlineare spektrale Schwerewellenroutine nach Yiğit et al. (2008) verwendet, wodurch der Schwerewellenfluss über die drei 5-Jahreszeiträume bestimmt werden konnte. Aus diesen Analysen ließ sich nun der Trend zwischen den jeweiligen Zeiträumen berechnen und so eine Aussage über den Trend der Schwerewellenaktivität in der Mesosphäre und unteren Thermosphäre ableiten. Die Ergebnisse zeigen gute Übereinstimmungen zu anderen Forschungsergebnissen bzgl. der Trends der GW Proxies von Meteorradarmessungen in Collm.

Using a mechanistic global circulation model, we analysed the trends of solar tides in the middle atmosphere. Forced by monthly mean assimilation of reanalysis data in the lower atmosphere and monthly adjusted CO2 and ozone distributions, the simulations represent a time period from January 1980 to May 2019. The time series of monthly mean wind and temperature amplitudes of all tidal components have been extracted from these data. Trend analyses by linear regression show prevailing negative trends in July and October for all tides and for all latitudes in the mesosphere and lower thermosphere. In April and January, however, trends are positive or negative, depending on the tidal component. Furthermore, the data set has been examined on possible trend changes via a statistical trend algorithm. A large part of those break points for the zonal wind amplitudes were found from 1985 to 1988 and from 2012 to 2015 for the investigated months January and April. Therefore, a clear relation between changes in the atmospheric ozone concentration and trends of the amplitudes of solar tides is not evident for the presented variables. / Unter Verwendung eines mechanistischen globalen Zirkulationsmodells wurden Trends von solaren Gezeiten in der mittleren Atmosphäre analysiert. Die Simulationen, die in den unteren Atmosphärenschichten mit monatlich gemittelten Reanalysedaten angetrieben wurden sowie mit angepassten CO2 und Ozonverteilungen, decken einen Zeitraum von Januar 1980 bis Mai 2018 ab. Aus diesen Daten wurden Zeitreihen für Monatsmittel in den Amplituden des Windes und der Temperatur für alle vier Gezeiten herausgefiltert. Die über lineare Regression gewonnen Trends ergeben -global betrachtet in der Mesosphäre und unteren Thermosphäre - vorwiegend negativsignifikante Trends im Juli und Oktober. Im April und Januar können jedoch je nach Gezeit und Parameter positive wie auch negative Trends vorkommen. Weiterhin wurden die Datenreihen auf mögliche Trendänderungen mit Hilfe eines statistischen Algorithmus untersucht. Ein Großteil dieser Trend-Wendepunkte in den Zonalwindamplituden liegen für die untersuchten Monate Januar und April in den Jahren von 1985 bis 1988 und von 2012 bis 2015. Eine direkte Verbindung zwischen Änderungen in der atmosphärischen Ozonkonzentration und Trends in den Amplituden solarer Gezeiten lassen sich in den hier behandelten Größen daher nicht ableiten.

The Madden-Julian Oscillation (MJO), a tropical phenomenon that exists on the time scale of 30-90 days, commonly initiates over the Indian Ocean and slowly propagates into the western Pacific as a series of convective events, which have time scales on the order of hours or days. These events and the overall MJO convective envelope may interact with convectively coupled waves such as Kelvin waves that propagate more rapidly eastward with time scales of 3-5 days. Radar and sounding data collected during the DYNAMO/AMIE/CINDY2011 field campaign from October 2011 to February 2012 in the central Indian Ocean are used to study the interaction between Kelvin waves and the MJO in terms of atmospheric and cloud properties. The focus is on characterizing the precipitation characteristics, convective cloud spectrum, and atmospheric profiles of Kelvin waves during the active and suppressed phases of the MJO to gain insight on MJO initiation. Characteristics of waves identified using different satellite thresholds and filtering methods are compared. Composites of the radar and sounding observations are calculated for a total of ten Kelvin waves and three MJO events that occurred during the field campaign. Analyzed radar products include convective-stratiform classification of rain rate, rain area, and echo-top heights, as well as cloud boundaries. Sounding data includes profiles of wind speed and direction and relative humidity. Kelvin waves that occur during the suppressed MJO are convectively weaker than Kelvin waves during the active MJO, but display previously documented structure of low-level convergence and a moist atmosphere prior to the wave passage. During the active MJO, Kelvin waves have stronger convective and stratiform rain, and the entire event is longer, suggesting a slower moving wave. The Kelvin wave vertical structure is somewhat overwhelmed by the convective envelope associated with the MJO. When the MJO is developing, the Kelvin wave displays a moisture-rich environment after the passage, providing deep tropospheric moisture that is postulated to be important for the onset of the MJO. The convective cloud population prior to MJO initiation shows increased moisture and a population of low- to mid-level clouds. The moisture precedes shallow convection, which develops into the deep convection of the MJO, supporting the discharge-recharge theory of MJO initiation. Additionally, enhanced moisture after the passage of the pre-MJO Kelvin wave could also support the frictional Kelvin-Rossby wave-CISK theory of MJO initiation. With a better understanding of the interaction between the initiation of the MJO and Kelvin waves, the relationships between the environment and the onset of the convection of the MJO can be improved.