Temperature trends in the midlatitude summer mesosphere

<jats:p>We have performed trend studies in the mesosphere in the period 1961–2009 with Leibniz‐Institute Middle Atmosphere (LIMA) model driven by European Centre for Medium‐Range Weather Forecasts reanalysis below approximately 40 km and adapts temporal variations of CO<jats:sub>2</jats:sub> and O<jats:sub>3</jats:sub> according to observations. Temperatures in the mesosphere/lower thermosphere vary nonuniformly with time, mainly due to the influence of O<jats:sub>3</jats:sub>. Here we analyze the contribution of varying concentrations of CO<jats:sub>2</jats:sub> and O<jats:sub>3</jats:sub> to the temperature trend in the mesosphere. It is important to distinguish between trends on pressure altitudes, <jats:italic>z</jats:italic><jats:sub><jats:italic>p</jats:italic></jats:sub>, and geometrical altitudes, <jats:italic>z</jats:italic><jats:sub>geo</jats:sub>, where the latter includes the effect of shrinking due to cooling at lower heights. For the period 1961–2009, temperature trends on geometrical and pressure altitudes can differ by as much as −0.9 K/dec in the mesosphere. Temperature trends reach approximately −1.3±0.11 K/dec at <jats:italic>z</jats:italic><jats:sub><jats:italic>p</jats:italic></jats:sub>∼60 km and −1.8±0.18 K/dec at <jats:italic>z</jats:italic><jats:sub>geo</jats:sub>∼70 km, respectively. CO<jats:sub>2</jats:sub> is the main driver of these trends in the mesosphere, whereas O<jats:sub>3</jats:sub> contributes approximately one third, both on geometrical and pressure heights. Depending on the time period chosen, linear temperature trends can vary substantially. Altitudes of pressure levels in the mesosphere decrease by up to several hundred meters. We have performed long‐term runs with LIMA applying twentieth century reanalysis dating back to 1871. Again, trends are nonuniform with time. Since the late nineteenth century, temperatures in the mesosphere have dropped by approximately 5–7 K on pressure altitudes and up to 10–12 K on geometrical altitudes.</jats:p>