Is thermospheric global cooling caused by gravity waves?

Abstract

[1] We analyze ion temperature data near 350 km altitude over the years 1966-2012 to seek explanations for three outstanding questions concerning the long-term cooling observed in the upper thermosphere: (1) Why is the cooling so much larger than expected, (2) why has the cooling lasted so long, and (3) why is the thermospheric density response to the cooling so small? We speculate that gravity waves may cause this cooling and provide answers to these questions. Recent simulations have shown that gravity waves are expected to cool the upper thermosphere by an amount comparable to that observed over our data timeline. A gravity wave proxy formed from the nontidal fluctuations in temperature shows a positive long-term trend throughout its timeline, consistent with the increasing cooling observed. The time scales of the long-term trend and its decadal fluctuations are characteristic of the ocean, not the atmosphere. We suggest that the following scenario may explain these behaviors: (a) the climate regime shift of 1976-1977 launched slow Rossby waves across the oceans which continue to propagate to this day, (b) winds over this increasingly corrugated ocean have launched increasing fluxes of gravity waves into the atmosphere, and (c) these increasing fluxes of gravity waves have propagated to the thermosphere to produce increasing amounts of cooling. The strong thermospheric cooling seen would be expected to produce thermospheric density declines much larger than those observed via satellite drag. These temperature and density results would be compatible if the turbopause were lowered 4 km over the time span of observations.

The importance of geomagnetic field changes versus rising CO2levels for long-term change in the upper atmosphere

“…Perhaps other processes should be considered as well, such as long-term solar variability or climatic changes in the atmosphere below, which could affect the upper atmosphere, for instance, through changes in gravity wave generation and/or propagation conditions (e.g. Oliver et al 2013). To facilitate further comparisons between model predictions and observed trends we will make the output from the simulations presented here, including results for different vertical levels, local/universal times, or different variables, where available, freely available to the community on request (contact the author if interested).…”

Thermospheric mass density: A review

“…Data from the Millstone Hill and Saint-Santin ISRs have been used to infer local temperature trends of −10 to −30 K/decade, which is much larger than suggested by the global density trends (Holt and Zhang, 2008;Donaldson et al, 2010;S.-R. Zhang et al, 2011;S.-R. Zhang and Holt, 2013;Oliver et al, 2014). Possible explanations for this apparent discrepancy are speculated and debated by Walsh and Oliver (2011), Laštovička (2012, and Oliver et al (2013Oliver et al ( , 2014Oliver et al ( , 2015. Emmert (2015) examined the height dependence of orbitderived density trends and inferred an exospheric temperature trend of 1-2 K per decade, confirming that the ISR temperature results are inconsistent with the orbit-derived density trends.…”

Neutral atmosphere temperature trends and variability at 90 km, 70 °N, 19 °E, 2003–2014

“…It has been proposed that GWs may be a major cause of negative temperature trends in the mesosphere and thermosphere (Beig, 2011;Oliver et al, 2013). GWs effectively transport chemical species and heat in the region, and increased GW drag leads to a net effect of cooling above the turbopause (Yigit and Medvedev, 2009).…”