Tropospheric warming, due to increased greenhouse gas concentrations over the last 150 years, is often termed the “greenhouse effect”. However, there is also a middle atmosphere manifestation of the greenhouse effect: enhanced cooling in the stratosphere and mesosphere. Modelling studies indicate a maximum cooling response in the high-latitude mesosphere. Therefore the ability to use the Hydroxyl layer to measure the temperature in the Antarctic mesosphere, makes the OH spectrometer an ideal instrument for monitoring middle-atmosphere temperatures for studies of climate change.

Some reported observations suggest that pronounced cooling, (up to 7 Kelvin/decade) in excess of model predictions, has already taken place.

Long-term trends in the Hydroxyl layer temperature

Nightly averages for each year measured and for the MSISE-90 model are plotted below.

An association with solar activity is expected. The “10.7cm solar flux” is a measure of the activity of the sun and this index is plotted below over the same period of time.

If there is in fact a link between wintertime temperature (for example) and solar activity, then we should see a relationship between them if we plot a graph of one against the other. Some care is needed, however, in calculating the value of the wintertime temperature.

Mean winter temperatures are calculated from the daily averages between DOY 106 (mid-April) and DOY 258 (mid-September) for each year. Corrected temperatures are derived by adjusting each nightly-average by the mean winter slope to an equivalent 1-Jul temperature. F10.7cm solar flux values are averaged over the same days each year. These averaged values are listed in the following table:

Mean winter temperatures calculated from the daily averages
Year 1990 1994 1995 1996 1997 1998 1999 2000
Winter nights 53 81 138 111 126 143 81 140
OH Temp 214.32 205.93 202.77 204.94 203.36 205.23 209.09 208.12
Error in mean 0.96 1.22 0.63 0.70 0.69 0.55 0.78 0.63
OH corrected 214.18 205.53 202.84 205.02 203.14 205.20 208.94 208.14
Error in mean 0.91 1.10 0.64 0.66 0.72 0.50 0.73 0.65
Correction −0.13 −0.40 0.07 0.08 −0.23 −0.03 −0.15 0.02
F10.7 186.45 82.66 74.20 71.06 75.75 117.03 157.87 178.52
Error in mean 6.91 0.80 0.50 0.34 0.71 1.57 2.49 2.65

Corrected mean winter temperatures are plotted below. The trend between 1990 and 1995 is at 2.24 Ka−1 (degrees Kelvin per year) and the warming trend between 1995 and 2000 is at 1.15 Ka−1.

The winter-mean temperatures have a similar character to the solar activity over the same period of time. In fact, the association suggested above can be calculated and is found to be +0.066 Kelvin/solar-flux-unit (the temperature increases by 0.066 degrees Kelvin when the solar flux unit increases by one) for the Davis observations. The relationship between temperature and solar-flux-unit can be shown by plotting one against the other. This is done below:

The value of +0.066 Kelvin/solar-flux-unit is considerably lower than most values reported by other researchers so far. A larger solar cycle association, coupled with significant long-term cooling of the upper mesosphere, cannot yet be discounted by these data.