Reply on RC3

Chengyun Yang

doi:10.5194/acp-2021-884-ac3

Abstract

Abstract. 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 â¼â10â% during ElÂ NiÃ±o in the Northern Hemisphere (NH) winter (DecemberâJanuaryâFebruary) from 2002 to 2020. According to the multiple linear regression (MLR) analysis, the linear effects of ElÂ NiÃ±o 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 ElÂ NiÃ±o 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 â¼â0â16âkm and 35ââSâ35ââN) during ElÂ NiÃ±o 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 ElÂ NiÃ±o winter. This tideâGW interaction could be a dominant mechanism for DW1 response in the MLT to ElÂ NiÃ±o. During ElÂ NiÃ±o winter, the increased ratio of the absolute and planetary vorticity (R) 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

Introduction

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).

Results

Tidal forcing and propagation

Effect of background wind

Effect of gravity wave forcing

Summary