Long-term variations and trends in the simulation of the middle atmosphere 1980-2004 by the chemistry-climate model of the Meteorological Research Institute

A middle-atmosphere simulation of the past 25 years (from 1980 to 2004) has been performed with a chemistry-climate model (CCM) of the Meteorological Research Institute (MRI) under observed forcings of sea-surface temperature, greenhouse gases, halogens, volcanic aerosols, and solar irradiance variations. The dynamics module of MRI-CCM is a spectral global model truncated triangularly at a maximum wavenumber of 42 with 68 layers extending from the surface to 0.01 hPa (about 80 km), wherein the vertical spacing is 500m from 100 to 10 hPa. The chemistry-transport module treats 51 species with 124 reactions including heterogeneous reactions. Transport of chemical species is based on a hybrid semi-Lagrangian scheme, which is a flux form in the vertical direction and an ordinary semi-Lagrangian form in the horizontal direction. The MRI-CCM used in this study reproduced a quasi-biennial oscillation (QBO) of about a 20-month period for wind and ozone in the equatorial stratosphere. Multiple linear regression analysis with time lags for volcanic aerosols was performed on the zonal-mean quantities of the simulated result to separate the trend, the QBO, the El Chichon and Mount Pinatubo, the 11-year solar cycle, and the El Nino/Southern Oscillation (ENSO) signals. It is found that MRI-CCM can more or less realistically reproduce observed trends of annual mean temperature and ozone, and those of total ozone in each month. MRI-CCM also reproduced the vertical multi-cell structures of tropical temperature, zonal-wind, and ozone associated with the QBO, and the mid-latitude total ozone QBO in each winter hemisphere. Solar irradiance variations of the 11-year cycle were found to affect radiation alone (not photodissociation) because of an error in making the photolysis lookup table. Nevertheless, though the heights of the maximum temperature (ozone) in the tropics are much higher (lower) than observations, MRI-CCM could reproduce the second maxima of temperature and ozone in the lower stratosphere, demonstrating that the dynamic effect outweighs the photochemical effect there. The ENSO signals of annual mean temperature, zonal wind, and ozone are generally reproduced in the troposphere and below the middle stratosphere. The volcanic signals of temperature increase and ozone decrease are much overestimated for both El Chichon and Mount Pinatubo.