5:15 PM - 7:15 PM
[PEM12-P29] Impact of Increasing CO2 Concentration on Diurnal Tide with Zonal Wavenumber 1: Mesospheric Cooling Effects
Keywords:Atmospheric tides, CO2, Mesosphere and Lower Thermosphere
CO2 concentration has increased since the Industrial Revolution (IPCC AR 6, 2021). According to the World Data Centre for Greenhouse Gases, the global mean CO2 concentration reached 420 ppm in 2023, and this upward trend is expected to continue throughout the century. Rising CO2 levels have led to warming in the troposphere while causing cooling in the stratosphere, mesosphere, thermosphere, and ionosphere.
Numerous studies have examined the thermodynamic impacts of these changes on atmospheric dynamics, particularly in the troposphere. However, the effects on interactions between the lower, middle, and upper atmosphere known as atmospheric vertical coupling remain poorly understood. Since vertical coupling is primarily driven by atmospheric waves (e.g., Rossby waves, gravity waves, and tidal waves), this study explores how increasing CO2 concentration affects atmospheric waves, focusing on the diurnal tidal wave with zonal wavenumber 1 (DW1).
DW1 tides dominate the equatorial mesosphere and lower thermosphere (MLT), with their energy largely confined to lower latitudes. They are generated in the troposphere, primarily due to solar radiation absorption by water vapor, and propagate upward to the lower thermosphere (~110 km). Their propagation is modulated by background wind and temperature variations.
To analyze the impact of increasing CO2 concentration on DW1 tides, we conducted a long-term simulation (2000-2079) using WACCM-X with a horizontal resolution of 1.9x1.7 degrees. CO2 concentration was specified using historical observations until 2014, after which it followed the Representative Concentration Pathway 8.5 (RCP 8.5), meaning CO2 concentration on the surface increased from ~370 ppm in 2000 to ~810 ppm in 2079. Solar cycle activity was also specified using historical observations until 2014, and from 2015 onward, it was simulated by rewinding the solar forcing from 1850.
We derived monthly mean temperature amplitudes of the DW1 (1, 1) mode component, which is the predominant propagating mode in the equatorial MLT, and examined its trends across 18-110 km altitude. Interestingly, amplitudes decreased with increasing CO2 concentration at 70-110 km (e.g., -3.0 x 10-2 +- 2.6 x 10-3 K/year at 90-110 km) but increased at 18-70 km (e.g., 5.0 x 10-4 +- 4.6 x 10-5 K/year at 18-30 km). These findings suggest that increasing CO2 concentration enhances DW1 source activity in the troposphere, primarily through increased solar radiation absorption by water vapor and latent heat. However, DW1 dissipation above ~70 km likely intensifies, outweighing the positive contribution of the source activity.
To further investigate the tidal propagation under the increasing CO2 condition, we compared DW1 (1, 1) amplitudes between two periods: 2000-2019 and 2059-2079. The results indicate a few percent increase in amplitudes at ~40-72 km in 2059-079 compared to 2000-2019, with a vertically consistent ratio. In contrast, amplitudes above ~72 km were lower in 2059-2079, with the ratio decreasing up to ~92 km. Consequently, amplitudes at 92-110 km decreased by 30% in 2059-2079 relative to 2000-2019.
Since tidal dissipation depends on vertical wavelengths, we estimated DW1 vertical wavelengths. Between ~55-82 km, vertical wavelengths in 2059-2079 were ~3-5 km shorter than those in 2000-2019. This reduction could be linked to strong cooling in the ~50-70 km range, which increased the Brunt-Vﾃ､isﾃ､lﾃ､ frequency at ~55-82 km. Such lower-middle mesospheric cooling aligns with previous model and observational studies (Luﾌken et al., 2013; Kishore et al., 2014; Huang et al., 2014). Above ~82 km, vertical wavelengths in 2059-2079 were comparable to or greater than those in 2000-2019. However, effective vertical diffusion due to gravity wave parameterization increased by ~10-30% around the equatorial MLT (~70-100 km). The shorter vertical wavelengths enhance tidal dissipation at ~72-82 km, while increased vertical diffusion further dissipates DW1 tides at ~70-100 km
This presentation will highlight the negative trend of the DW1 (1, 1) mode tide in the MLT region simulated by WACCM-X and discuss the underlying mechanisms driving this
Numerous studies have examined the thermodynamic impacts of these changes on atmospheric dynamics, particularly in the troposphere. However, the effects on interactions between the lower, middle, and upper atmosphere known as atmospheric vertical coupling remain poorly understood. Since vertical coupling is primarily driven by atmospheric waves (e.g., Rossby waves, gravity waves, and tidal waves), this study explores how increasing CO2 concentration affects atmospheric waves, focusing on the diurnal tidal wave with zonal wavenumber 1 (DW1).
DW1 tides dominate the equatorial mesosphere and lower thermosphere (MLT), with their energy largely confined to lower latitudes. They are generated in the troposphere, primarily due to solar radiation absorption by water vapor, and propagate upward to the lower thermosphere (~110 km). Their propagation is modulated by background wind and temperature variations.
To analyze the impact of increasing CO2 concentration on DW1 tides, we conducted a long-term simulation (2000-2079) using WACCM-X with a horizontal resolution of 1.9x1.7 degrees. CO2 concentration was specified using historical observations until 2014, after which it followed the Representative Concentration Pathway 8.5 (RCP 8.5), meaning CO2 concentration on the surface increased from ~370 ppm in 2000 to ~810 ppm in 2079. Solar cycle activity was also specified using historical observations until 2014, and from 2015 onward, it was simulated by rewinding the solar forcing from 1850.
We derived monthly mean temperature amplitudes of the DW1 (1, 1) mode component, which is the predominant propagating mode in the equatorial MLT, and examined its trends across 18-110 km altitude. Interestingly, amplitudes decreased with increasing CO2 concentration at 70-110 km (e.g., -3.0 x 10-2 +- 2.6 x 10-3 K/year at 90-110 km) but increased at 18-70 km (e.g., 5.0 x 10-4 +- 4.6 x 10-5 K/year at 18-30 km). These findings suggest that increasing CO2 concentration enhances DW1 source activity in the troposphere, primarily through increased solar radiation absorption by water vapor and latent heat. However, DW1 dissipation above ~70 km likely intensifies, outweighing the positive contribution of the source activity.
To further investigate the tidal propagation under the increasing CO2 condition, we compared DW1 (1, 1) amplitudes between two periods: 2000-2019 and 2059-2079. The results indicate a few percent increase in amplitudes at ~40-72 km in 2059-079 compared to 2000-2019, with a vertically consistent ratio. In contrast, amplitudes above ~72 km were lower in 2059-2079, with the ratio decreasing up to ~92 km. Consequently, amplitudes at 92-110 km decreased by 30% in 2059-2079 relative to 2000-2019.
Since tidal dissipation depends on vertical wavelengths, we estimated DW1 vertical wavelengths. Between ~55-82 km, vertical wavelengths in 2059-2079 were ~3-5 km shorter than those in 2000-2019. This reduction could be linked to strong cooling in the ~50-70 km range, which increased the Brunt-Vﾃ､isﾃ､lﾃ､ frequency at ~55-82 km. Such lower-middle mesospheric cooling aligns with previous model and observational studies (Luﾌken et al., 2013; Kishore et al., 2014; Huang et al., 2014). Above ~82 km, vertical wavelengths in 2059-2079 were comparable to or greater than those in 2000-2019. However, effective vertical diffusion due to gravity wave parameterization increased by ~10-30% around the equatorial MLT (~70-100 km). The shorter vertical wavelengths enhance tidal dissipation at ~72-82 km, while increased vertical diffusion further dissipates DW1 tides at ~70-100 km
This presentation will highlight the negative trend of the DW1 (1, 1) mode tide in the MLT region simulated by WACCM-X and discuss the underlying mechanisms driving this