Figure 12 shows the mean summer time temperature of the upper troposphere (between about 2.5 and 10 km altitude) averaged over the whole Northern Hemisphere. Again this parameter varies in phase with the solar 10.7 cm index suggesting that the signal seen at the sub-tropical station of Figure 11 is not confined to that location. The hemispherically-averaged signal, however, has a somewhat smaller amplitude at 0.2 – 0.4 K.
Figure 13 shows the results of a multiple regression analysis of zonal mean temperatures from the NCEP/NCAR reanalysed dataset (acquired from the Climate Diagnostics Center, Boulder, Colorado, U.S.A., at http://www.cdc.noaa.gov/). In this work data for 1978 – 2002 were analysed simultaneously for ten signals: a linear trend, ENSO, NAO, solar activity, volcanic eruptions, QBO and the amplitude and phase of the annual and semi-annual cycles. It is assumed that all these factors are independent and, indeed, the patterns of response for each signal are found to be statistically significant and separable from the other patterns, however it means that any potential solar influence on the NAO (as discussed in Section 2.2) cannot be resolved. There is a linear trend of warming in the troposphere and cooling in the stratosphere, as expected from enhanced concentrations of greenhouse gases; there is a strong ENSO signal in the tropical troposphere but it is also seen in mid-latitudes and throughout the lower stratosphere. The solar response shows largest warming in the tropical stratosphere and in bands of warming, of 0.4 K, throughout the troposphere in mid-latitudes. The bands of warming/cooling near the surface are consistent with the sea surface temperature study of White et al. (1997), (see Section 2.2) which was based on an entirely different dataset. Note that the analysis was carried out independently at each grid-point so that the hemispheric symmetry of the solar signal provides support for the validity of this result.
In the middle atmosphere measurements made from satellites suggest an increase of up to about 1 K in the upper stratosphere at solar maximum and of a few tenths of a degree in the lower stratosphere, but a minimum, possibly even negative, response in between. However, precise values, as well as the position (or existence) of the negative layer, vary between datasets: some examples are given in Figure 14.
The Quasi-Biennial Oscillation (QBO) is a naturally-occurring variation in zonal winds in the tropical lower stratosphere which has a period varying around 26 months. Accepted theory has it that a west phase in the tropical QBO is linked with cold temperatures in the winter polar lower stratosphere, with vertically propagating planetary waves being channelled equatorwards. Karin Labitzke has pointed out, however, that while this relationship holds well during periods of lower solar activity it tends to break down near solar maximum. Figure 15, which contains data for the years 1956 – 1991, shows that warm polar temperatures tend to occur during the east phase of the QBO at solar minimum and west phase at solar maximum. The reason for this is an active area of current research in dynamical meteorology and is discussed below in Section 5.4.
Another important aspect of the Sun’s influence on the stratosphere is in the modulation of ozone concentrations. This is described in Section 5.4.
Figure 16 presents some results from a multiple regression analysis of zonal mean zonal winds, similar to that carried out on temperatures in Figure 13 although only the solar signal is shown here. When the Sun is more active it appears that the mid-latitude jets are weaker and positioned further polewards. This has implications for the positions of the mid-latitude storm tracks and thus provides further evidence for a solar signal in mid-latitude climate.
Studies have also indicated an impact of solar variability on the meridional circulation of the lower atmosphere. Figure 17 suggests that when the Sun is more active the tropical meridional overturning of the atmosphere is somewhat weaker and broader in latitudinal extent.
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