Abstract
<strong class="journal-contentHeaderColor">Abstract.</strong> As observed by the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER), the migrating diurnal tide (DW1) in the upper mesosphere and lower thermosphere (MLT) region decreased by <span class="inline-formula">â¼</span>â10â% during El Niño in the Northern Hemisphere (NH) winter (DecemberâJanuaryâFebruary) from 2002 to 2020. According to the multiple linear regression (MLR) analysis, the linear effects of El Niño on the tropical MLT DW1 are significantly negative in both SABER observations and SD-WACCM (the Specified-Dynamics version of the Whole Atmosphere Community Climate Model) simulations. The DW1 response to El Niño in NH winter is much stronger than its annual mean response. As suggested by SD-WACCM simulation, Hough mode (1, 1) dominates the DW1 tidal variation in the tropical MLT region. The consistency between the (1, 1) mode in the tropopause region and the MLT region and the downward phase progression from 15 to 100âkm indicates the direct upward propagation of DW1 from the excitation source in the troposphere. The suppressed DW1 heating rates in the tropical troposphere (averaged over <span class="inline-formula">â¼</span>â0â16âkm and 35<span class="inline-formula"><sup>â</sup></span>âSâ35<span class="inline-formula"><sup>â</sup></span>âN) during El Niño winter contribute to the decreased DW1 tide. To evaluate the effect of the gravity waves (GWs) on the tide, the GW forcing is calculated as the GW drag weighted by the phase relation between DW1 GW drag and DW1 wind. The negative GW forcing in the tropical upper mesosphere would significantly suppress the MLT DW1 tide during El Niño winter. This tideâGW interaction could be a dominant mechanism for DW1 response in the MLT to El Niño. During El Niño winter, the increased ratio of the absolute and planetary vorticity (<span class="inline-formula"><i>R</i></span>) suppresses the waveguide and thus the DW1 amplitude in the subtropical mesosphere. However, the effect of the waveguide might play a secondary role due to its relatively weak response.
Highlights
IntroductionAtmospheric solar tides are global-scale variations in meteorological variables (e.g., density, wind, and temperature) with subharmonic periods of a solar day
Atmospheric solar tides are global-scale variations in meteorological variables with subharmonic periods of a solar day
10°S-10°N at 100 km derived from SABER observations and SD-Whole Atmosphere Community Climate Model (WACCM)
Summary
Atmospheric solar tides are global-scale variations in meteorological variables (e.g., density, wind, and temperature) with subharmonic periods of a solar day. The response of the MLT DW1 tide to ENSO during the winters is revisited in this study based on the DW1 variation extracted from a long-term temperature dataset observed by the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) onboard the TIMED (Mertens et al, 2001, 2004, Rezac et al 2015). The winter (DJF) mean of the DW1 anomalies is calculated Natural forcing, such as the solar cycle (represented by F107), QBO, ENSO, and long-term trends, jointly affect the DW1 tidal amplitude (e.g., Dhadly et al, 2018; Gurubaran et al., 2005; Gurubaran & Rajaram, 1999; Hagan et al, 1999; Lieberman et al, 2007; Liu et al, 2017; Pedatella & Liu, 2012; Sridharan, 2019, 2020; Sridharan et al, 2010; Vincent et al, 1998; Xu et al, 2009).
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