Long-term trends in Antarctic winter hydroxyl temperatures

Observations of the hydroxyl nightglow emission with a scanning spectrometer at Davis station, Antarctica (68 degrees S, 78 degrees E), have been maintained over each winter season since 1995. Rotational temperatures are derived from the P-branch lines of the OH(6-2) band near lambda 840 nm and are a layer-weighted proxy for kinetic temperatures near 87 km altitude. The current 16 year record allows tentative estimation of the atmospheric response in the mesopause region to solar cycle forcing and the underlying long-term linear temperature trend. Seven years of new data have been added since the last reported trend assessments using these data. A multivariate regression analysis on seasonally detrended winter mean hydroxyl temperatures yields a solar cycle coefficient of 4.8 +/- 1.0 K/100 solar flux units (SFU) and a linear long-term cooling coefficient of -1.2 +/- 0.9 K/decade. These coefficients are consistent within uncertainties for nightly, monthly, and annual mean trend evaluations. A distinct seasonal variation in trend coefficients is found in 30 day sliding window or monthly trend analyses. The largest solar activity response (similar to 7 K/100 SFU) is measured in March, May-June, and September, and there is little or no solar response in April and August. The long-term trend coefficient shows the largest cooling rate (4-5 K/decade) in August-September, through to warming (2-3 K/decade) for the March and May-June periods. Comparisons of trend results are made with other important hydroxyl measurement sites. Variability in the remaining residual temperatures is examined using lag correlation analyses for the influence of planetary waves, the quasi-biennial oscillation, the polar vortex intensity, and the southern annular mode.