Abstract
Abstract. The long-term trend, solar cycle response, and residual variability in 24 years of hydroxyl nightglow rotational temperatures above Davis research station, Antarctica (68∘ S, 78∘ E) are reported. Hydroxyl rotational temperatures are a layer-weighted proxy for kinetic temperatures near 87 km altitude and have been used for many decades to monitor trends in the mesopause region in response to increasing greenhouse gas emissions. Routine observations of the OH(6-2) band P-branch emission lines using a scanning spectrometer at Davis station have been made continuously over each winter season since 1995. Significant outcomes of this most recent analysis update are the following: (a) a record-low winter-average temperature of 198.3 K is obtained for 2018 (1.7 K below previous low in 2009); (b) a long-term cooling trend of -1.2±0.51 K per decade persists, coupled with a solar cycle response of 4.3±1.02 K per 100 solar flux units; and (c) we find evidence in the residual winter mean temperatures of an oscillation on a quasi-quadrennial (QQO) timescale which is investigated in detail in Part 2 of this work. Our observations and trend analyses are compared with satellite measurements from Aura/MLS version v4.2 level-2 data over the last 14 years, and we find close agreement (a best fit to temperature anomalies) with the 0.00464 hPa pressure level values. The solar cycle response (3.4±2.3 K per 100 sfu), long-term trend (-1.3±1.2 K per decade), and underlying QQO residuals in Aura/MLS are consistent with the Davis observations. Consequently, we extend the Aura/MLS trend analysis to provide a global view of solar response and long-term trend for Southern and Northern Hemisphere winter seasons at the 0.00464 hPa pressure level to compare with other observers and models.
Highlights
Long-term monitoring of basic atmospheric parameters is fundamentally important to understanding natural, periodic, and episodic variability in atmospheric processes; to provide data to verify increasingly sophisticated atmospheric models; and to resolve and quantify perturbations due to global change on decadal to century timescales
We extend the Aura/Microwave Limb Sounder (MLS) trend analysis to provide a global view of solar response and long-term trend for Southern and Northern Hemisphere winter seasons at the 0.00464 hPa pressure level to compare with other observers and models
While the choice is important for comparisons of absolute temperature between observers, it does not affect the trend analysis reported here as the offset is removed by subtracting the climatological mean
Summary
Long-term monitoring of basic atmospheric parameters is fundamentally important to understanding natural, periodic, and episodic variability in atmospheric processes; to provide data to verify increasingly sophisticated atmospheric models; and to resolve and quantify perturbations due to global change on decadal to century timescales. Over long timescales (more than one solar cycle), recent studies using satellite data (Gao et al, 2016; von Savigny, 2015) and OH chemistry-dynamics (OHCD) models have shown that the OH* layer altitude is remarkably insensitive to changes in CO2 concentration or solar cycle variation. This makes these measurements very valuable for monitoring long-term changes in the atmosphere. From the measured temperatures and their nightly, monthly, seasonal, or winter means, temperature anomalies are produced by subtracting the climatological mean or monthly mean (we fit solar cycle and linear trend to the anomalies), residual temperatures have the solar cycle component subtracted (used in the discussion of long-term trends), and detrended temperatures have the long-term linear trend subtracted (used in the discussion about remaining variability)
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