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4. Temperature trends

The annual mean model temperature trends are shown in Fig. 6 in comparison with trends determined from Stratospheric Sounding Unit (SSU)/Microwave Sounding Unit (MSU) data (Scaife et al., 2000a) for the slightly shorter period Jan 1980 - Dec 1997. The solar cycle has been removed from the observations (see e.g. Scaife et al., 2000a), but other processes such as the quasi-biennial oscillation and the impacts of aerosol have not been accounted for. In the upper stratosphere the model is broadly in agreement with observations but diverges from them both in the mesosphere and in the middle and lower stratosphere. The temperature trends are to a first approximation independent of latitude. Significant deviations from this occur in the lower stratosphere in southern middle latitudes where the 0.0 and -0.25 contours buckle upwards. The feature is absent in the observations but its cause in the model may be related directly to the ozone trend which also shows an increase in that region. Over Antarctica, where the SSU trends are less reliable, the observations indicate the cooling occurs in the lower stratosphere due to the ozone hole and this is followed by an upper region of warming. Qualitatively, this also occurs in the model, but the influence of the ozone hole does not extend as far north as indicated in the SSU/MSU data.

A quantitative comparison of the globally and annually averaged temperature trends at selected levels (Fig. 7) confirms many of these points. In the figure, comparisons are also made with our previous results from a model without coupled chemistry (Butchart et al., 2000, small circles). Although this simulation covered a different period (1992 to 2051), the changes in GHG amounts and sea conditions were approximately linear throughout the period and changed at virtually the same rate as for the period studied here (1980-2000). Thus the Butchart et al. (2000) simulation provides an important benchmark for model comparisons. The inclusion of (model computed) ozone trends increases model cooling rates significantly throughout the stratosphere, in agreement with Langematz (2000) and Rosier and Shine (2000). This leads to better agreement with observations in the lower stratosphere (indicated by the results for 46 and 100 hPa). In the middle stratosphere, the modelled trends are significantly larger than observed and inclusion of ozone results in poorer agreement with observations. In the upper stratosphere, the previous underprediction of observed trends is replaced by a slight overprediction (within the error bars). In the Figure the quoted error bars (2$\sigma$) are the errors in determining the trend from the sequence of annual temperatures, and in the case of the observations do not include any error due to removal of the solar cycle or instrument drift.

 

Figure 7: Globally and annually averaged temperatures at selected levels. Circles: Results from the climate model simulation of Butchart et al. (2000). Diamonds: Results from the coupled chemistry-climate model (this work). Asterisks: Results from SSU/MSU anomaly data. To convert to absolute values, the SSU/MSU anomaly data were incremented by an amount equal to the chemical model temperature for 1980 + 2K.
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