Model studies of the influence of varying UV in the middle atmosphere

6.2 Model studies of the influence of varying UV in the middle atmosphere

In Section 2.3 the thermal response of the stratosphere/troposphere to solar variability was shown to be largest near the tropical stratopause with lobes of warming in the sub-tropics in the lower stratosphere and bands of warming through mid-latitudes in the troposphere. One route to understanding this structure is to see if it can be reproduced in atmospheric models.

Figure 35:

Results from GCM simulations of the response of middle atmospheric temperatures to the 11-year solar cycle variability in UV with different models in which the solar-induced ozone response was specified. From Matthes et al. (2003 ).

Figure 36:

Results from GCM simulations of the response of middle atmospheric temperatures to the 11-year solar cycle variability in UV with a coupled chemistry scheme (shadings indicates statistical significance at the 10, 5 and 1% levels) From Haigh et al. (2004 ).

Figures 35 and 36 show zonal mean temperatures from various atmospheric GCMs in which changes to UV irradiance corresponding to the 11-year cycle variability have been imposed. In Figure 35 ozone changes have been prescribed, so that there is no feedback between the solar heating and ozone which might influence temperatures (and also circulation and winds). All these models show a similar temperature response with greatest warming near the stratopause, stretching across most latitudes. Figure 36 shows results from a model with a coupled chemistry scheme so that feedbacks between chemistry, radiation and dynamics are allowed. Here the upper stratospheric warming is more concentrated at low latitudes and there is a suggestion of a vertical structure more reminiscent of those shown in Figure 14 .

A comparison between satellite observations and model predictions for the solar cycle signal in zonal mean ozone concentration is presented in Figure 37 . The models include older 2D studies, which have good representations of photochemical and radiative processes but poor representation of transport, and more recent 3D coupled chemistry-climate simulations. The profiles retrieved from satellite data suggest maximum fractional change in the upper stratosphere, a minimum near 30 km (especially in the tropics) and possibly a second maximum below. All the models, however, predict a peak near 40 km with a monotonically decreasing signal below. The observational data are only available over less than two solar cycles, so there remains some doubt about the statistical robustness of the signals derived from them, and Lee and Smith (2003 ) have suggested that the 30 km minimum may be a mixing of the solar signal with one due to the quasi-biennial oscillation.

Figure 37:

Estimates of percentage increase in ozone concentration from solar cycle minimum to maximum from satellite data (data points and horizontal bars) and a range of theoretical models (curves). The upper panel shows an average over most of the globe while the lower panel shows tropical data. From WMO (2007 ) based on data from Brasseur (1993 ); Haigh (1994 ); Egorova et al. (2004 ); Tourpali et al. (2003 ).

A recent intercomparison of coupled chemistry climate models (Austin, 2007 ) suggests that this may be true for short model runs but finds that the mid-stratospheric minimum can be reproduced in simulations which include time-varying solar irradiance and prescribed sea surface temperatures. These differ from the 3D models in Figure 37 which used fixed solar max/min scenarios with climatological SSTs. Austin (2007 ) argue that the transience in the simulations allows better reproduction of the mean meridional circulation of the stratosphere and thus the transport of lower stratospheric ozone. Nevertheless, another state-of-the-art coupled chemistry climate model, with very high vertical resolution (Schmidt and Brasseur, 2006 ) does reproduce the vertical structure with time-slice (i.e. not transient) runs. The response of stratospheric ozone to solar variability remains an active topic of research.