Nonlinear forcing mechanisms of the migrating terdiurnal solar tide and their impact on the zonal mean circulation

Friederike Lilienthal,Christoph Jacobi

doi:10.5194/angeo-37-943-2019

Friederike Lilienthal, Christoph Jacobi

Open Access

https://doi.org/10.5194/angeo-37-943-2019

Copy DOI

Journal: Annales Geophysicae	Publication Date: Oct 10, 2019
Citations: 12	License type: CC BY 4.0

Affiliation: Max Planck Institute for Meteorology

Abstract

Abstract. We investigate the forcing mechanisms of the terdiurnal solar tide in the middle atmosphere using a mechanistic global circulation model. In order to quantify their individual contributions, we perform several model experiments and separate each forcing mechanism by switching off the remaining sources. We find that the primary excitation is owing to the terdiurnal component of solar radiation absorption in the troposphere and stratosphere. Secondary sources are nonlinear tide–tide interactions and gravity wave–tide interactions. Thus, although the solar heating clearly dominates the terdiurnal forcing in our simulations, we find that nonlinear tidal and gravity wave interactions contribute in certain seasons and at certain altitudes. By slightly enhancing the different excitation sources, we test the sensitivity of the background circulation to these changes of the dynamics. As a result, the increase of terdiurnal gravity wave drag can strongly affect the middle and upper atmosphere dynamics, including an irregular change of the terdiurnal amplitude, a weakening of neutral winds in the thermosphere, and a significant temperature change in the thermosphere, depending on the strength of the forcing. On the contrary, the influence of nonlinear tidal interactions on the middle atmosphere background dynamics is rather small.

Highlights

The middle atmosphere dynamics are mainly determined by waves that are excited in the troposphere or stratosphere and propagate to the upper atmosphere
To extend the work of Lilienthal et al (2018) and in order to further investigate the nonlinear mechanisms of TDT forcing, we present model simulations, which are each restricted to only one terdiurnal forcing mechanism, i.e., either the solar heating absorption, or nonlinear tidal interactions, or gravity waves (GWs)–tide interactions
Note that the amplitudes of the REF simulation are generally smaller than those observed. This is due to the fact that Middle and Upper Atmosphere Model (MUAM) tends to underestimate tides in general, which is frequently seen in other models (Smith, 2012; Pokhotelov et al, 2018; Lilienthal et al, 2018)

Summary

Introduction

The middle atmosphere dynamics are mainly determined by waves that are excited in the troposphere or stratosphere and propagate to the upper atmosphere (see, e.g., reviews by Forbes, 1995; Yigit and Medvedev, 2015). The authors partly focused on different latitudes and altitudes that cannot be compared The majority of these publications agrees that the direct solar forcing is the most dominant, not the only, excitation mechanism of the TDT (Akmaev, 2001; Smith and Ortland, 2001; Du and Ward, 2010; Lilienthal et al, 2018). Lilienthal et al (2018) found that the solar forcing is the primary excitation mechanism, but nonlinear tide–tide interactions and GW–tide interactions play a role They analyzed the phase relations of differently forced TDTs and found destructive interferences between them.

Model description and experimental setup

Zonal wind amplitude distributions for different forcing mechanisms

Impact of different forcing mechanisms on tidal amplitudes and background

Conclusions