9:45 AM - 10:00 AM
[PEM11-28] DW1 Tidal Enhancements in the equatorial MLT during 2015 El Nino: the relative role of tropsopheric heating and propagation
Keywords:Thermal tide, El Niño, MLT
Thermal tide is caused by atmospheric heating, conveys its momentum vertically, and causes the circulation in the mesosphere and lower thermosphere (MLT). Additionally, thermal tides control a gravity wave and cause instability. The tide with 24 hours period and westward zonal wavenumber 1 (DW1) is predominant in the equatorial MLT, and mainly exited by heating caused by tropospheric water vaper and propagates to the MLT.
Observations (Zhou et al., 2018) have shown that the tidal component DW1 in the equatorial MLT was enhanced in August-October 2015, which was an El Niño year. We investigate this tidal enhancement event by using the 21-year reanalysis-driven model simulation by the Ground-to-topside model of Atmosphere and Ionosphere for Aeronomy (GAIA). This enhancement is reproduced in the model, which shows the (1, 1) Hough mode dominates this tidal enhancement. The heating anomaly also increased, which was probably caused by El Niño, but can explain only 14 % of the enhanced tidal anomaly at 90 km. Additionally, the tidal anomaly did not increase in the heating peak period (February 2016). To investigate the propagation condition for the (1, 1) mode tides, the vertical wavenumber was derived from the (1, 1) mode phase and was one standard deviation smaller than its climatological value between18-60 km during the period. Additionally, the latitudinal zonal wind shear decreased around 20N/S° in 18-30 km. These results suggest that the dissipation and damping of the (1, 1) mode in the middle atmosphere were smaller than its climatological value; therefore, the decrease of the tidal dissipation and damping could cause the remaining 86% of the tidal enhancement at 90 km altitude. This decrease can be mainly explained by the eastward phase quasi-biennial oscillation (QBO) in the lower stratosphere. This study suggests that the QBO and El Niño induce the interannual variation of the DW1 in the equatorial MLT, and the combination can enhance the DW1 by 70%.
Observations (Zhou et al., 2018) have shown that the tidal component DW1 in the equatorial MLT was enhanced in August-October 2015, which was an El Niño year. We investigate this tidal enhancement event by using the 21-year reanalysis-driven model simulation by the Ground-to-topside model of Atmosphere and Ionosphere for Aeronomy (GAIA). This enhancement is reproduced in the model, which shows the (1, 1) Hough mode dominates this tidal enhancement. The heating anomaly also increased, which was probably caused by El Niño, but can explain only 14 % of the enhanced tidal anomaly at 90 km. Additionally, the tidal anomaly did not increase in the heating peak period (February 2016). To investigate the propagation condition for the (1, 1) mode tides, the vertical wavenumber was derived from the (1, 1) mode phase and was one standard deviation smaller than its climatological value between18-60 km during the period. Additionally, the latitudinal zonal wind shear decreased around 20N/S° in 18-30 km. These results suggest that the dissipation and damping of the (1, 1) mode in the middle atmosphere were smaller than its climatological value; therefore, the decrease of the tidal dissipation and damping could cause the remaining 86% of the tidal enhancement at 90 km altitude. This decrease can be mainly explained by the eastward phase quasi-biennial oscillation (QBO) in the lower stratosphere. This study suggests that the QBO and El Niño induce the interannual variation of the DW1 in the equatorial MLT, and the combination can enhance the DW1 by 70%.