2.3 Solar signals throughout the atmosphere

The pioneering work of Karin Labitzke demonstrated a solar cycle variation in stratospheric temperatures. An example of her work is presented in Figure 11View Image which shows the annual mean, at a location near Hawaii in the sub-tropical Pacific Ocean, of the geopotential height of the 30 hPa pressure surface. This is a measure of the mean temperature of the atmosphere below about 24 km altitude. It varies in phase with the solar 10.7 cm radio wave flux over three and a half solar cycles with an amplitude suggesting that the lower atmosphere is 0.5 – 1.0 K warmer at solar maximum than at solar minimum. This is a large response but from this figure alone it is not clear whether it applies locally or globally or how the temperature anomaly is distributed in the vertical.

Figure 12View Image 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 11View Image is not confined to that location. The hemispherically-averaged signal, however, has a somewhat smaller amplitude at 0.2 – 0.4 K.

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Figure 12: Time series of the mean temperature (K) of the 750 – 200 hPa layer in the northern hemisphere summer (solid line) and the solar 10.7 cm flux (10–21 Wm–2 Hz–1) (dashed line). From van Loon and Shea (1999).

Figure 13View Image 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 External Linkhttp://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.

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Figure 13: Results from multiple regression analysis of NCEP zonal mean temperatures and zonal winds. From top: mean temperature; linear trend (K over 44 years); ENSO (max-min, K); NAO (max-min, K), solar cycle (max-min, K); volcanic eruption (effect of Mt. Pinatubo eruption, K) From Haigh (2003).

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 14View Image.

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Figure 14: Difference in annual mean temperature between solar maximum and solar minimum derived from observational data. Above: SSU/MSU satellite data (grey shading denotes statistical significance as shown in the legend). Below: ERA reanalysis data for the period 1979 – 2001; light/dark shading denotes 95% and 99% significance. Note the different height ranges in the two panels. From Haigh et al. (2004Jump To The Next Citation Point).

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 15View Image, 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.

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Figure 15: A scatter plot showing for each year values of 30 hPa temperature at the north pole in Jan/Feb (ordinate), solar 10.7 cm flux (abscissa) and phase of the QBO (symbols, triangle for east and square for west). The horizontal and vertical lines, and the E & W labels, have been drawn to indicate regions of the diagram in which certain phases of the QBO predominate. From Labitzke and van Loon (1992Jump To The Next Citation Point).

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 16View Image presents some results from a multiple regression analysis of zonal mean zonal winds, similar to that carried out on temperatures in Figure 13View Image 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.

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Figure 16: Above: Annual and zonal mean zonal wind, u, as a function of latitude and pressure from NCEP Reanalysis. Below: difference in u between solar maximum and solar minimum. From Haigh et al. (2005Jump To The Next Citation Point).

Studies have also indicated an impact of solar variability on the meridional circulation of the lower atmosphere. Figure 17View Image 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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Figure 17: Results from an analysis of NCEP zonal mean vertical (pressure) velocities. The coloured patches indicate the climatological mean at 500 hPa with upwelling (ω < 0) at the equator and sinking in the sub-tropics. The correlation curve shows at solar max positive ω (i.e. weaker ascent) at the equator and negative/positive values on the equatorward/poleward sides of the descending branches, indicating that these have moved polewards. From Gleisner and Thejll (2003).

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